summei14 circuits - americanradiohistory.com 08, 1976 · circuits. contents elektor july/augues...

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uummTur U5/IIJuly/August 1976double issue - SO p

SUMMEI14CIRCUITS

contentselektor july/augues 1976 -1031

contents

About this nue p 709 Audioclass A amplifier re -consideredHefter circuit for quasi-quadrophonyhandbftect organHI -E stereo amplifier for headphonesmotorphonepeak indicatorpia. tunerrain synthesizersimple headphone amplifiersound effects generatorspeech shifterstereo indicatorsupewbootstrap RC oscillatorsymmetrical power amp.three channel mixertremolotriangular wave oscillatorvariable slope filtervariable stereo width mixing stage .

wien bridge oscillatorwind machine7 watt IC audio amplifier

Power unitsbattery chargercarcade current sourcecurrent sourcedissipation dumperquad symmetrical supplyregulated d and 15 V power supplysymmetrical regulated supplyTTLAnsurancevariable regulated supplyvoltage regulator for motorbikes0-30 V/1 A, stabilised

RFaerial boosryraircraft communication receiverOSSC generatorpreset aerial amplifiersample/hold synthesisersimple front-end for VHF FMsingle sideband adapter 70

squelch 64556 adapter 40SSISI exciter with HF compressor 101

bawler band converter 102TV modulator 56wide band frequency doubler 23

eless' bell extender 52200 MHz sample/hold adapter 90

nr Sundries54 a steady hand86 MeV without R and C 5091 aqu.tat1012 battery indicator 22

4 bad -bell 8763 car clock using watch IC 4661 crystal timebase for synchronous clocks 19

6 dark room aid 21

29 digital speed readout for turntables . .

81 driving LEDs from TTL 72

60 electronics lap counter 37

96 electronic voting system 15

98 frequency doubler using 4011 17

89 handy dark room timer 1648 ignition key reminder 4565 infradedphone 9436 kettlestat 8

26 light sensitive astable multivibrator . . 2

1 liquid level indicator 24

32 min.m. temperature indicator 14

34 MMV for Ace 62

82 moisture indicator 97molestation 11

MOS monostable 43n.of,TAP 59

84 one-shot 6661 opto coupler with two LEDs 38

30 perpetual solar clock 42

80 poker 51

99 Porch lighter 33

35 quiz selector 9395 read.out brightness regulator 8528 rev. counter for diesels 2073 Schmitt trigger 58

92 seat belt reminder 39

55 single transistor sawtooth generator . 53

solarstat 16

spike monoflop 44nr tap doorbell 49

75 DPPed coda lock 31

51 ,oucb activated dimmer 77

83 VCO with 74123 97

1006968

Test 11f

audioscope 79acoustic logic probe 13active oscilloscope probe 27linear wale ohmmeter 88microammeter 41

PIP meter 74polarity indicatorsimple pulse generator 18

sine -square -triangle generatortest logic 15

8transistor tester 7

voltage to frequency converter 3

etektor july/august 1976 - 709

about this issue

Our often -quoted motto Is, .Elektoris different'.Well, this issue is even more different.Where a 'normal' issue of Elektor -contains about ten or twelve construc-tion articles, the 'summer circuits' issuecontains more than 100. This meansthat our design staff have to dream upnearly as many new circuits for oneissue as would otherwise suffice for aliole year.iv

d the number of circuits neededt the only problem. The editor

demands that the circuits should be'new', 'original', 'different'. The chiefof design demands that they must work.And the deputy editors for the variouseditions demand that the componentsshould be available.We haven't yet reached the stage whereevery circuit meets all the demands, butnobody can say we didn't try.Admittedly, some of the designs arebased on manufacturer's applicationnotes (for which we thank all con-cerned), so they are not original - butwe have tried to select application noteswhich are of general interest.Admittedly, not all circuits have beentested. However, if any member of ourtechnical staff voiced his doubts abouta design it was either scrapped or builtand tested. The 102 circuits presentedhere were selected from a total of 231.And that is not counting the hundredsof basic ideas that never got past thefirst editorial selection. Furthermore. as

a last resort, we have our technicalqueries service and the 'missing link'.But, if last year's 'summer circuits'issue is anything to go by, the demandson these services should be few and farbetween.Admittedly, not all components usedwill be readily available. This is one ofour major headaches. In our first issuewe stated: 'The availability of com-ponents is always considered, and whennew components are needed everyeffort is made to ensure that they canbe obtained through the normal retailoutlets.''And we meant it - and wemean it.Perhaps it should be clearly stated: it isthe exception rather than the rule thatwe specify components that cannot bemade available - and this is usually theresult of over -optimistic advance infor-mation from the manufacturer.However, the 'normal retail outlets'must buy their components fromdistributors - and the latter are notalways interested. Perhaps they forgetthat if 10% of our readers want tobuild a particular project, this meansnearly 5,000 potential customers inthe U.K Furthermore, most ofthe components used in Elektor havebeen readily available overseas forseveral years. Even 'on the doorstep',so to speak, in Holland. We wouldadvise our readers not to acceptcomments like 'The production of theBC547B was stopped several years ago'

without contacting us.Rest assured, we are still working onthis problem. We have now startedissuing advance information sheets tothe retail trade, listing the componentsto be used in coming issues of Elektor.Retailers who would like to be put onour mailing list can contact ourCanterbury office. We are also con-tacting major supplies in the commonmarket, to see whether they will supplythe U.K. retail trade - and possiblyeven the U.K. reader direct. Somereaders have asked why Elektor doesn'tsupply components when these are notreadily available. However, our basicpolicy has been clearly stated from theoutset: 'Elektor will not sell com-ponents, so that complete editorialindependence is assured'.It is, however, conceivable that if theworst comes to the worst - and inthe interest of editorial independencewhere the choice of components isconcerned - we may decide to act as atemporary go-between for the retailtrade by offering them the necessarycomponents on a non-profit basis. Thiswould, of course, be stopped as soon asthe regular distributors took over.Enough talk about problems. We hopethat we have succeeded in finding`more than 100 circuits' that comesufficiently close to fulfilling all ourrequirements.We wish you pleasant reading - andsoldering!

Tie -Widow lulyhtusesr1070

1 variable stereo width mixing stage2 light sensitive astable multivibrator

I This mixing stage is a modularunit of universal design, so that

several units can be joined in parallel toform a central control for more com-plex stereo systems. In this type of sys-tem, each stereo source is controlled byits own module; the circuit of one mod-ule is indicated inside the dotted line inthe diagram. The total (sum) output sig-nal appeals across Ra (L) and Re. (R);the value of these resistances depends onthe number of input channels.Signal sources with output levels of100 mV or more can be connectedstraight to the module input terminals;lower level sources and those needingspecial processing (microphones,dynamic cartridges) will requireseparate preamplifiers.Some aspects of stereo width controlwere already discussed in the l'reco'articles of earlier Elektor issues (12,p. 416 and 13, p.516).Super -stereo is obtained by closing SI ;the control range can be increased byreducing R6. P2 is used for continuouswidth adjustment between mono andsuper stereo.The value for the two resistances Ra andRe. is found by choosing the neareststandard value to 3900 divided by thenumber of mixing stages. As anexample, if five stages are to be used thevalue should be the nearest standardvalue to

39500777 12

i.e. 820 a In the majority of cases thiscommon resistance will be sufficientlylow to serve as L and R output circuitswithout further buffering.The L and R output levels practicallyequal the sum of the levels at the inputs.

al The circuit of an astable multi -vibrator using a Schmitt trigger

and a single RC time constant has fre-quently been used in Elektor. Figure Ishows how this circuit can be modifiedso that the frequency of oscillation isdependent upon the intensity of lightfalling on a phototransistor.Assuming that CI is initially uncharged,the output of SI will be high. CI willnow charge via RI, D2 and TI at a ratedepending on the leakage current ofTI, which is in turn dependent on thelight level. When the voltage on Cl hasreached the upper threshold voltage ofthe Schmitt trigger the output of SIwill go low and CI will now dischargethrough D I and RI until the lowerthreshold voltage is reached, when theoutput °1St will go high again and thecycle will repeat. With the componentvalues shown the frequency will varybetween about 5 kHz and 10 kHz,depending on the light level incidenton the phototransistor.The disadvantage of this circuit is thatthe duty -cycle of the waveform varieswith the light intensity, since thephototransistor controls only thehalf -cycle of the waveform when theoutput of SI is high. This problem canbe overcome by placing the photo -transistor at the centre of a diodebridge, and a circuit using Iwo CMOSinverters is shown in figure 2. When theoutput of N I O high then current willflow through the phototransistor viaD2 and D3. When the output of NI islow current will flow through thephototransistor via D4 and DI. Withthe capacitor value shown the fre-quency of oscillation will varybetween about 10 Hz and I kHz de-pending on light intensity.

N1,N2 =1/2 x4011

DI... 0,1=1,141413

A circuit using CMOS gates and anal-ogous to figure 1 is shown itt Flom 3.The phototransistor is connected M afeedback loop around NI sod since thephototransistor will be merse-bissedwhen the output of NI is low it controlsonly the half -cycle of the waveformwhen the output of NI is Wahineduty -cycle therefore vatic:sari& lightintensity. This can be cured. thiscircuit by using an LDR (e.g. ORP I

yielder julviaugust 1976 - 711

3 voltage -frequency converter J Borgman

4 motorphone

instead of the phototransistor. Theresistor RI in parallel with the photo-transistor (or LDRI determines therange of variation of frequency andwith the component values given thiswill be between 500 Hz and I kHz. Theleadout configuration of the TIL78is given in figure 4.

3In conjunction with a frequencycounter, this unit can be used as

a digital voltmeter. The output is a TTLcompatible pulse train with a 5 ps pulsewidth. The pulse repetition frequency isdirectly proportional to the input volt-age. The conversion factor is 10 kHz pervolt. In this circuit, due attention waspaid to linearity and zero -point stability.The unit remains linear to about10 kHz. This means that the uppervoltage lim't, without external voltagedividers, is about 10 V.The input i connected, via RI, to theoPamP (IC ), which functions as aninverting integrator. When a voltage isapplied to the input of ICI, its output

drops al a constant rate. This continuesuntil T I is blocked; when this happens,the collector voltage of TI jumps tothe 5 V level, triggering IC2 and pro-ducing a positive -going pulse at the ()-output of this IC.The pulse width is 5µs, determined byR II and C4. During this time T2 isdriven into saturation. switching onFET T3 so that Cl is discharged over aconstant range. This causes the inte-

grator output to rise to its original(positive) value. Then the entire processis repeated. again and again...The repetition frequency of the re-sulting pulse train is directly pro-portional to the input voltage.The value of RI is about 90 k. It can becomposed of a fixed metal film resistorand a stable preset potentiometer, sothat the circuit can be calibrated.It is recommended to use metal filmresistors for RI I and RI6, and poly -carbonate capacitors for CI and C4.With the input short circuited, the fre-quency should be adjusted somewherebetween 0 and 2 Hz by means ofpotentiometer PI.

4 It is almost always impossiblefor a motor cyclist and his

passenger to maintain aural contactwithout dangerous acrobatics. The cir-cuit shown in the figure affords effort-less voice unclad. The microphoneamplifier of post I is formed by anamplifier stage T I followed by a superemitter follower. The DC couplingdetermines the current through RI and

15V

15V

the base -emitter voltage of TI. So whenone party speaks he hears himself, sothat he knows the system is working.Since the two posts are connected inseries, the signal generated in post Itravels through both telephones, sothat this 'intercom' needs only one wirebetween the Iwo posts.The main function of the supply withT7, TO and T9 is noise suppression,whilst at the same time the system ismade short-circuit proof. Via R 13 andD2 an audible indication of thetrafficators is obtained. The interphoneposts can be made so small they can bemounted in the crash helmets. Thesupply is mounted on the machine.The microphones am crystal types.

5 sine -square -triangle generator

Unlike the more usual type ofagj function generator, in which the

sinusoidal output is derived by shapinga triangular waveform, the basis of thiscircuit is a Wien -bridge oscillator, whichprovides a sinusoidal output. The squareand triangular waveforms are thenderived from this.The Wien -bridge oscillator is builtaround CMOS NAND -gates NI to N4,and amplitude stabilization is providedby TI, DI and D2. These diodes should,if possible, bee matched pair, forminimum distortion. The frequencyadjustment potentiometer PI shouldalso be a good quality stereo poten-tiometer with the tracks matched towithin 5%. The preset R3 providesadjustment for minimum distortion andif matched components are used for DI,

... NS. C09011. ICIN5... Na s C04011. IC2DI... D2 s I tI4168

D2 and PI the total harmonic distor-tion should be less than 0.5%.The output of the Wien -bridge oscillatoris fed into N5, which is biased into itslinear region and operates as an ampli-fier. N5 and N6 together amplify andclip the oscillator output to give asquare waveform. The duty -cycle of thewaveform is somewhat dependent onthe threshold voltages of N5 arid N6,but it is close to 50%.The output of N6 is fed into an inte-grator constructed around N7 and N8,which integrates the square wave togive a triangular waveform. The ampli-tude of the triangular waveform is, ofcouIse, frequency dependent, and sincethe integrator is not perfect the linearity

also vanes with frequency. In practicethe amplitude variation is relativelyunimportant, since the generator willusually be used with a millivoltmeter oroscilloscope and the output can bemonitored. Adjustment of the triangleamplitude is provided by P3.As the CMOS gates cannot drive verylow load impedances an output bufferamplifier is provided, which greatlyincreases the usefulness of thegenerator. The amplifier is capable ofdriving loads of 4 S2 or greater, whichmakes it particularly useful for loud-speaker testing. If the generator islikely to be used to generate square -waves at low frequencies (less than100 Hz) it may be worth increasing thevalue of CI 5 to make the top of thesquare wave flatter. Quiescent current

adjustment is provided by P4 and thisshould be set to about 50 mA. PScontrols the output amplitude.As the impedance s around the CMOScircuits are fairly high the generatorshould be mounted in a screenedmetal boo to avoid interference pickup.If a mains power supply is used thisshould be screened from the rest of thecircuit to avoid hum pickup. Foroptimum results at high frequenciesCa (8p2) and Cb (33 p) can be added;note that these components are notshown on the layout for the p.c. board.

elector july/mspust 1976 - 713

VaP, 01 Re0 117 0 47'0 6-10

°-4M:'-1R. CHI -0

,Sjsni n en 5

1: j6r tn.0_14_0 .r7O

714 - Widow July/Impost 1976

6 rain synthesiser H. 0 tterwe I I

7 a steady hand

6This simple circuit has provedvery reliable and effective as a

background sound effect generator foruse by organists etc.Other simple devices of this type oftenuse reveral stages of amplification ormake use of special noise diodes whichare comparatively expensive. The ad-vantage of this design is that it employsan ordinary OA 91 or similar diode. The

9V

in emal noise produced by the diode isamplified by the single stage pre-amplifier, consisting of T 1 and itsassociated components, which is de-signed for high gain and low cost. TIcan be almost any silicon NPN transis-tor, a BC107 being used in the proto-type.The output at X may be taken straightto an amplifier if only a white noiseoutput is required. However, theaddition of the passive filter, composingC3 and PI enables a variety of effectsranging from light rain to a heavy stormto be obtained.

7This is a game of skill requiring`nerves of steel'. The object of

the game is to pass a metal rod throughone of the holes in a sheet of chickenwire and to touch a metal plate locatedsome 6-12 inches behind the wire -without touching the wire. When theplate is touched a buzzer will sound anda LED will light, but if the rod inadver-tently touches the wire before reachingthe plate the buzzer will sound and adifferent LED will light. The plate andthe chicken wire form the contact platesof touch controls, so no circuit connec-tion is required to the rod, the circuitoperates simply on hum picked up fromthe player's body.The heart of the circuit (figure IIconsists of two set -reset flip-flops con-structed around CMOS NAND gatesN 1/N2 and N3/N4. Initially the circuitis reset by pressing the reset button.This takes one input of N2 and N3 to'0' via diodes DI and D2, so the outputsof these two gates are high. The outputof N5 is thus low and the astablemultivibrator N6/N7 is disabled. The

outputs of NI and N4 are low, T1 andT2 are turned off and both LED's areextinguished.If the player succeeds in touching theplate iro without touching the mesh (A)the hum picked up from his body willset flip-flop N3/N4. The output of N4will go high, turning on T2 andlighting LED B. The output of N3 willgo low, so the output of NS will gohigh, allowing the astable to startoscillating. Via Nil this switches theDarlington pair T3 and T4 on and offat about I kHz, and produces a tonefrom the loudspeaker. The output ofN3 also holds one of the inputs of N2low, so that even if the mesh is sub-sequently touched with the rod flip-flop NI /92 cannot be set, but remainsin the reset state. Flip-flop N3/N4 mustbe reset by pressing the reset button,ready for the next player.Should the rod touch the mesh beforereaching the plate then flip-flop N 1/N2will be set. The output of N I will gohigh, turning on TI and lighting LED A,which indicates that the player hasfailed. The output of N I will be low,the output of N5 will be high and the'buzzer' will sound. The output of N2also holds one input of N3 low, so thatflip-flop N3/N4 cannot be set, even if

the plate is subsequently tou tied.A typical method of constru tion forthe game is shown in the photograph.The circuit can be mounted tea smallbox behind the metal bookplate. This,does not need to be solid metal of '

course, but can be a sheet of plywoodcovered with aluminium foil.

Variation on the GameA simplified (from the electronic pointof view) version of the game is given infigure 2, and many readers will recog-nise this. The object of this game is tomove a metal loop along a length ofwire that has various kinks and convol-utions - again without touching the

ForFor this game only one flip-flop is re-quired. As soon as the loop touchesthe wire flip-flop NI/N2 will be set,astable N3/N4 will start to oscillate andthe tone will sound, indicating that theplayer has failed. The circuit can bereset by pressing the button.

Constructional pointsIf the first version of the game is con-structed it is important that the spacers,which separate the frame holding thechicken wire from the backplate, should

TUN

alaktor juivratioust 1976 -115

8 kettlestat9 polarity indicator

1,5 made of insulating material. Thewhole frame should also have an insu-lated base, so that it will not begrounded if placed on a conductingsurface such as the earth.The same considerations also apply tothe second version of the game. Thelength of bent wire must be mountedon an insulating base ore an insulatedframe of some kind. The wire shouldalso obviously be fairly stiff, and 2 mmpiano wire is a suitable material. Theloop can also be made from a piece ofthe same wire.

8 Automatic kettles which willswitch themselves off after they

boil are very convenient, but also ex-pensive. Fora component cost of about£ 4,- a normal electric kettle can beconverted for automatic operation. Acircuit using an NTC for the tempera-ture scarlet element, will work well.

ralltiy7he NTC's resistance is high,which means that the input voltage to

the Schmitt trigger is low. Therefore, T1is off and T2 W on, so T3 is conductingand the relay is closed. As long as therelay remains closed, the triac will becontinuously triggered, allowing currentto flow to the kettle's heating element.When the water boils the steam heatsthe NTC, causing its resistance to godown, in turn the voltage to the Schmitttrigger goes up. When the voltagereaches the threshold voltage, TI turnson and T2 and T3 tum off, the relayopens and the triac stops conducting -switching off the current to the kettle.A LED indicates the water has boiled.A large degree of hysteresis W built intothe Schmitt trigger which ensures thatthe kettle will not switch on again untilthe NTC has cooled almost to roomtemperature. Them is little chance ofthe kettle boding dry unless it is leftunattended for several hours. On theother hand the water will be heated upto the boil every few minutes and willthus be kept hot. To bring the waterback to the boil manually a push buttonswitch (S) is provided. Pressing it resetsthe Schmitt trigger even if the NTC hascooled sufficiently for the input voltageto fall below the mm -off threshold.

ConstructionThere are two possible methods of con-struction. The circuit can be mounted ina small plastic box attached to thekettle plug with the NTC mounted on asmall arm. A hole about 4 mm diameteris drilled in the back of the kettle.(above the waterline) to allow steam topass over the thermistor. The secondmethod is to mount the components ina box with a 13 amp socket into whichthe kettle is plugged. The thermistormay then be mounted in a probe made

from an old ball point pen which isinserted in the kettle spout. In eithercase great care must be taken with -the insulation. All the mains connectionsshould be well insulated from the low -voltage circuits and the componentsshould be fixed in the case with nylonscrews.

th With this device it is possible to7 determine the polarity of test

points with respect to the circuitCOM111011.

The indicator is built around the 741opamp. It has an input impedance ofabout I MS2, so that there is little

.12V/100mA MOC100

circuit loading by this device.However, when checking circuits withhigh impedances, the loading effectsmust be taken into account.The opamp compares the voltage of

the test point relative to common. If thevoltage is positive so is the output ofthe opamp. Asa result LED DI willlight. If the voltage is negative, LED D2lights up. The pin numbers given on thedrawing are related to a TO -5 case.

716 - amino. juiviaugust 1926

10 aquastat H. Frakstein11 molestation alarm12 Hi -Z stereo amplifier for headphones

10 In situations where liquid levelsshould not transgress a given

maximum value it might be required tohave a pump automatically put intooperation in a case of emergency. Inthis respect we can think of well -headcontrol, ground water level control andother cases of emergency concernedwith waterworks in general. Perhapsthe circuit described here can mean thesolution.A P.V.C. tube is provided with threeelectrodes mounted horizontally in thespace c.q. place to be controlled(figure 2). Via a sturdy (weather -resistant) three -core cable these elec-trodes are connected to the controlcircuit. The bottom electrode is connec-ted to plus, electrode 'NI' to the baseof TI via a series resistor Rx, and 'N2'to the base of T4 via a resistor Rx. Thepositions of 'NI' and 'N2' on theP.V.C. tube correspond to the criticalliquid levels.The circuit functions as follows:- Suppose that for some reason the

water rises from an initial levelsituated between electrode andelectrode 'N 1'. As soon as the water

'N1', there will be anelectric contact between '+' and 'NI';T1 is driven open via Rx, the collec-tor of T2 is 'high', and T5 couldbecome conductive were it not thatT6 is not (yet) conducting. Thetransistor can begin to conduct onlywhen level 'N2' is reached; for thenT4 becomes conductive and T3 isblocked. When both T5 and T6 areconducting, the relay is energizedand the pump is switched on.The base and collector of T6 areshort circuited via a second makecontact on the relay; which meansthat the relay remains energized evenwhen the level is pumped downagain to below 'N2'. The latch isremoved from the relay only afterthe level has dropped below 'NI';then T5 is not driven, TS and TO

^l2

tes

)))-)block, the relay cuts out, and thepump stops.

The values of the resistors Rx must befound by experiment. Generally, thesevalues will lie around 150 kn. Thecurrent through the electrodes should bekept at a minimum in view of electroly-sis phenomena. Owing to the lowcurrent consumption the power supplycan be quite simple. Depending on theapplication, an aquarium or fountainpump can be used; if necessary, with theinclusion of an additional transformer.Warning - To ensure safe operation, theprimary and secondary of the mainstransformer (if used) should be wellinsulated! Furthermore, a series resistor(of, say 47 k) can be included in thelead to the electrode, provided thevalues of Rx are reduced toapproximately 47 k.

1 1 Take aasltatl:letimmueltiviCbrcrorewcitiht

a duty cycle of about 5%, use this signalto drive a miniature loudspeaker via aBD 136, and the result is such a soundthat any hostile intentions of a would-be attacker are converted (hopefully)into the well known urge to get away.At the same time, people in the areashould become curious and inclinedto track down the source of the noise.The circuit can be made very compactand consumes about SO mA when thepower supply voltage is 9 volts.

1 2 This amplifier is intended toteused in conjunction with the

popular Sennheiser stereo headphones(HD 414, HD 424). These headphoneshave a 2 k impedance. According to thefactory specifications, a voltage of1.41 Vnns is needed fora sound press-ure of 102 dB. (That is 4 Vp _ p, orI mW into 2 k).To meet the needs of the hard ofhearing, an output of at least 40 Vpcan be reached with this design, sothat a maximum sound pressure ofabout 122 dB is obtainable. This is oatrecommended, however....Each channel uses 2 of the 4 opamps inan LM324. The headphones (the load) isdriven by two emitter followers TI andT2 connected in a bridge circuit. IC laand T1 provide the necessary amplifi-cation, set by R3 and R4 at x 30. Atthe same time TI drives the otheramplifier in anti -phase via R6 and R7.The input impedance is quite high as aresult of bootstrapping via R 1 and C2,so that it is possible to obtain the audiosignal from a high -impedance point inthe pre -amp. Other sources am alsopossible, like the 'monitor' output of atape recorder.Except for RIO, R II and C4, each partoccurs twice on the p.c. board; com-ponents with an apostrophe belong tothe right channel. The space on thep.c.b. which is reserved for the heatsirdcsfor T1, T2 and T I', TV is somewhathmited, but these heatsinks may toucheach other.

le tor julyieugust 1976 -717

13 acoustic logic probe

The gain of the total amplifier is2 x (R3 R4)/114. Using the indicatedvalues for R3 and R4 the gain is ap-proximately 60. If the available inputvoltage is higher than necessary, it isrecommended to reduce the gain byselecting a higher value for R4.It should be noted that the amplifieroutputs are floating! This amplifier cannot be used with headphones which usea common return lead. In other words,the two earphones must be completelyisolated from each other.The frequency response is within 3 dBbetween 20 Hz and 25 kHz. The supplyokltage should be between 25 and30 volts. Current consumption will beless than 125 mA.

13 This logic probe is a littleunusual, as it indicates the logic

states by audio tones rather than thenorenal light display (LEDs, etc.).The audio tone generator used to makethe high and low notes, which corre-spond to the high and low logic levels,is formed by SI and R4. These twoparts form an astable which is tuned byCl or C2. The signal is buffered andamplified by S2 to drive the loud-speaker.The way the probe detects differentlogic levels is very simple. If the inputsignal voltage exceeds 2.4 V (this levelis set by P2) transistor T3 will be driveninto saturation. This in turn causes oneside of C2 to be connected to (probe)common, so that the oscillator producesa high frequency tone.When the probe is on a 0.8 volt level orless T1 will cause T2 to saturate, so that

the left side of CI is connected tocommon through the low resistance ofT2. This produces the low note. The0.8 V level is set by PI I.The capacitance of Cl should be abouttwice that of C2, this makes the tonefor the low logic level about one octave

below the high tone. The device isinoperative at test levels betweenapproximately 0.8 and 2.4 V, or if theprobe is not connected to the circuit.The acoustic probe can be powered bythe supply in the circuit that is beingtested.

718 - elektor ruly/august 1976

14 min -max temperature indicator15 test logic J. Kefer

16 solarstat1 4

Temperature reading does notalways require an analogue

indication. Often it would be nice onlyto receive warning when a certainmaximum or minimum temperature isreached. Such a temperature indicatorcan be used, for instance, to monitorthe temperature of water in anaquarium. Temperatures below, my,20°C and above 25°C are then indicatedby LEDs.The circuit is built mound the LM3900,an IC containing four Nortonamplifiers. In principle these Nortonamplifiers can be compared with acurrent -driven opamp. Two opamps areconnected to form a voltagecomparator. The other Nortonamplifiers serve as buffer stages.The entire circuit is fed from astabilised supply. A reference voltage isobtained by means of voltage divider(R3 -R4). The temperature sensor is anNTC-resistor. This NTC resistor (R.2),together with PI, forms another voltagedivider. The voltage at junction R2 -PI isthus temperature -dependent.The opamps Al and A3 compare thisvoltage with the reference voltage. When

the temperature exceeds the maximumlimit, the voltage at junction R2 -PI willbe higher than the reference voltage.Consequently, the output of opamp A3is positive. The inverting function ofbuffer stage A4 causes LED D3 to lightup. When the temperature drops, thevoltage at junction R2-Pl will drop.At a certain (adjustable) value thecurrent into the inverting input will beless than the current into the non -inverting input of opamp Al. As a resultthe output voltage of this opamp willrise. The inverting function of opampA2 now causes LED D2 to light up.

AdjustmentWith the NTC in water heated to themaximum permissible temperature,PI is adjusted so that LED D3 is justlighting up. Allowing the water to coolto the minimum temperature, adjust P2so that LED D3 just comes on.Note that the connections which areimmersed in water must be wellinsulated, of course.

A A4- LM3900

D2 = low lismTD3 = high limit

15 Trnhio.s,tuersitn: rtohbLisTsruiLt-akt for

levels: the usual levels '0' and 'I' andthe non -defined region between the two.When the circuit sees a'0' the TUP issaturated, turning on the '0' LED. If

the input voltage is between one andtwo volts, both transistors are blocked.This means the exclusive OR (EXOR)receives two unequal logic levels makingits output go high; the 'NC' LED lightsup. This LED will also be lit if the probeis not connected to a measuring pointor when connected to a floating IC in-put pin.Above 2 volts the TUN is driven intosaturation and the `I' LED lights up.Since a 7486 is comprised of fourEXOR gates, one IC and a small hatuulof parts could be turned into a4 channel logic analyser.

1 6 ,°.lizi`avrudeurehteoatthres:u7-rie:"inginterest in energy conservation. Water ispumped through solar collectorsmounted on the roof and, when hot, isused to heat up the domestic hot watersupply in a normal two circuit hot-water tank.However, it is pointless pumping waterthrough the solar collectors on a coldday when the temperature of the collec-tors is less than the domestic hot watertemperature (which will then be heatedby other means) as the solar collectOrsthen become very effective radiators.What is required is a differential ther-mostat that will start the pump onlywhen the temperature of the solar

aiektor tiny/august 1976 -719

17 frequency doubler using 4011

panels is higher than that of the water inthe tank.The circuit operates as follows: Tem-perature sensing is carried out by twonegative temperature coefficient ther-mistors, one on the solar roof and oneon the tank. These form two arms of abridge, the other two arms being formedby two fixed resistors and a preset pot.When at the same temperature' the twoNTC's have nominally equally resistanceand the bridge is balanced (PI allowsadjustment to compensate for differ-ences in the NTC'a). If the thermistoron the roof becomes hotter than that intit tank its resistance becomes lower,the voltage on the non -inverting inputof ICI exceeds the voltage on theinverting input, and the output goeshigh. The Schmitt trigger switches andthe relay closes, turning on the pump.If the temperature of the solar rooffalls below the temperature of the tankthe voltage on the -1 input of ICI fallsbelow the voltage on the - input, theoutput goes low and the relay dropsout. The Schmitt trigger ensures cleanswitching of the relay, which eliminatesrelay chatter at the switch over point.The unit is calibrated in the followingmanner. Firstly, both thermistorsshould be clamped to an aluminiumplate so they will remain at the sametemperature during calibration. Placethe thermistors in near boiling water,allow them to stabilize, then adjust PIso the output of the IC is low. Afterthis, remove the thermistors from thewater; monitor the IC output, makingsure the output stays low as thethermistors cool. This ensures that the

17

temperature of the roof thermistormust always be higher than that of thethermistor before the relay will switch

This frequency doubler uses oneCMOS quad, two -input NAND

gate package type 4011. The frequencydoubler proper consists of an inverterN2, two differentiating networks RI/CI, R2/C2 and NAND gate N3. NI andN4 function as input and outputbuffers.The incoming signal is buffered andinverted by NI, giving waveform (A) (itIs assumed the waveform has I I mark -space ratio). (A) is inverted by N2,giving waveforni B, which is the comp-lement of (A) (i.e. it is in antiphase).The negative,going edges of waveform(B) are differentiated by R2 and C2,giving waveform (C), while (A) isdifferentiated by RI and CI, givingwaveform (D). Waveforms (C) and (D)are fed into N3, and every time one ofthese waveforms is negative -going a

positive -going pulse appears on theoutput of N3, (waveform E). The out-put of N4 is . inverted version of (E).The switching threshold of CMOS logicis about 45% of supply voltage, so theswitching point of N3 on the risingexponential portions of waveforms (C)and (D) will occur at this point. Thetime taken for the waveform to rise tothis voltage is just less than the timeconstant RC, so the pulse duration ofwaveform (E) is approximately equal tothe time constants R1C1 and R2C2. Forreliable operation these time constantsshould be chosen to be much less thanthe shortest possible period of the inputwaveform. The mason for this is thatthe width of the positive -going pulses(E) is constant, but the length of thespaces between them diminishes as theinput frequency increases. If the pulsesare not of short enough duration theymay overlap at high input frequencies.

2

720 - elektor july/august 1976

18 simple pulse generator J. Bonthond

18A pulse generator is an ex-tremely useful tool for investi-

gating the dynamic behaviour of logiccircuits. The circuit described here isbased on the versatile 555 timer, andprovides variable pulse length andrepetition frequency.

101,4

5001.

5001CI C C3 C- CI. C6 CL, C9 c, CI

IC1=555

I OOms

15066200 0o2 82",50, 820, 'us 8,12

Our

CND

zener diodes D 1 and D2 protect the in-put of N4 against excessive input volt-ages to the 'ext trig' and 'gate' inputs.With S4 in the 'single -shot' positionthe set -reset flip-flop comprising N1/N2can be set by pressing the single -shotbutton. The output of N3 will thus go

R1

C12

C,=oox

ext. trig. ga e

-0-

ezt single shot. MID4

47V oz

53

0

single shot.

ICI is connected as an astable multi -vibrator, whose repetition frequencydepends on R1, R2, P1 and theswitched capacitors Cl to C 1 1. Theswitched capacitors provide coarsecontrol of the pulse interval, whileP1 provides fine control. The outputof ICI is taken via a differentiatingnetwork R4/C12 to the mid -positionof S4.With S4 in the mid -position the pulsetrain from ICI is fed through N4 andN5 to the trigger input of IC2, whichis connected as a monostable multi -vibrator. This is repetitively triggeredby the input pulses and the outputpulse length is controlled by capaci-tors C14 to C24 and by P2. The outputof IC2 is buffered by the invertersN6 to N9 and a TTL compatible outputand its complement are provided atthe outputs of N7/N9 respectively.For testing other logic families such asHLL or CMOS a variable amplitudeoutput is provided by the emitterfollower T 1. The amplitude may beadjusted by P3. If this option is adopteda 15 V collector supply to T1 is re-quired.In addition to a continuous pulse trainthe circuit also has facilities for externaltrigger, external gating of the pulsetrain and single -shot operation. WithS4 in the mid -position the pulse trainfrom ICI may be interrupted byapplying a logic '0' to pin 5 of N4. WithS4 in the 'ext' position the output ofICI is disconnected and IC2 may betriggered from an external source. The

CO

NI ... N4 = IC2 = 7400N5... N10 = IC4 = 7406

4.7V

N5

02'"

low. triggering IC2 once.

ConstructionAll components with the exception ofthe switches, potentiometers and timingcapacitors Cl to CII and C14 to C24

i0On

fine

00k

pulse length.Ips

500

1000

500,

C.)

IC2=555 0.)t

050

00rn. 5

A

N6 N79 8 I 2

svlf

N8 Ng

so N10

C1I.C15 C16 CI1C121 CI9 C20 C21 C22 C231

R8

TIP31A2N5191

15V

0

15V JI

ITTTTTITTIT-C-0.1000 470,3 1 4n7 ton 4'm 100r 470, '0 407 100 91'I

91 9393 9 920 0 0 0 0 00 602

04 R1 140 0-44-4) 04R7

Oz1 2 1-0 01R 1 2 10 0-1R 9

0-IR3 1-0 0.1 R 5 1.0

C12 CH 1-0 I5C;13

04 R4 kl 0--S*12-11

foto Foto

tlk

IC2fff I

6 co au

T1 0-05v0-1R B 1-0 C3>

nvO o ocr

111/41.!1.1,!iiJL)

5V

19 crystal timebase for synchronous clocks20 rev. counter for diesels F.A. Heinrich

are mounted on the p.c. board. SwitchesSI and S2 can be made up from the'make -switch' type of assembly, usingan I I- or I2 -way wafer and a dummywafer for each switch. The timingcapacitors can then be mountedbetween the tags of the switch waferand the dummy wafer.

1 9 tIP.21!ef'.7fte.: digitalconven-

readout and under normal circurn-s.otchersotnhoeucaoonlovecnktpiornoavlidmesaignosoddriven

accuracy at low cost (typically E 5 -

£ 10) Unfortunately, in countries where' the mains supply frequency is subjec

to fluctuation, or in rural areas in thiscountry where power failures arefrequent, the mains driven clock is notsuch a good idea. The alternatives arebattery clocks that offer inferioraccuracy at about twice the price or, forthe very rich, crystal controlled batteryclocks starting at about f 35.A third alternative is to provide asynchronous clock with a crystal time -base, which divides down a frequency ofsay 1 MHz to give a 50 Hz signal whichis then used to drive the clock through astep-up transformer. The powerconsumption of a synchronous clock isabout I- 2 W, so if a 9 V batterysupply is used the current drain will beof the order of 250 mA, allowing fortransformer inefficiency and powerconsumption of the dividers, so a 2 Ah

NiCad battery would keep the clockrunning for 8 hours in the event ofmains failure.The circuit operates as follows:The reference frequency is provided bya I MHz crystal. Other frequencies may,of course, be used, but I MHz xtals arereadily obtainable. The output of thextal oscillator is divided down to 50 Hzby CMOS dividers and a complementarydarlington output stage provides thedrive to the primary of the step-uptransformer (which is simply a normalmains transformer reversed). The out-put capacitor and choke form a filterso that the waveform reaching thetransformer is reasonably sinusoidal.

4111% The pecularity of this rev.aiV counter is that it responds todifferences in luminous intensity.Consequently, if this circuit Es to beused as a rev. counter, the motor shaft

must be provided with a vane whichperiodically intercepts the light incidenton the light sensor. Little can be saidabout the choice of light sensitiveelement, because they come innumerous types. Instead of a photodiode,r photo transistors or photodarlingtons can be used. In practicallyall cases it will be necessary toexperiment with the value of RI. A firstsetting can be obtained by applyinghalf the supply voltage to point A bymeans of RI.For slow -running machines, DI cansometimes be replaced by an LDR. Assoon as more light is incident on DI, thecurrent through D I will increase so thatthe voltage on point A drops. Via CIand C2 this voltage drop is fed to themonostable multivibrator N2/N3.In the quiescent state both inputs ofN3 are earthed via R5, so the outputof N3 is 'high'. Consequently, the twoinputs of N2 are 'high' so that its outputis low'.As soon as a negative pulse arrives atone of the inputs of N2, the output ofN2 changes to 'high' and causes gate N3to change state, so that the second inputof N2 goes `low'. Even when the triggerpulse on the input of N2 cuts out, thecircuit remains in this condition. Onlyafter C3 (-,C4) is (are) charged to suchan extent that the voltage on the inputsof N3 are `low' again will the circuitreturn to the initial state. Thus themonostable multivibrator changes anyinput pulse on DI into a pulse ofconstant width. These pulses are fed tothe meter via buffer stage N4.The lamp in the supply line provides abetter stabilization than a resistor, atthe same time giving an on/offindication for the meter.The measuring range can be doubledwith SI. When S I is closed, the range isfrom 0 to 33 Hz (0 - 2000 r.p.m.);when SI is open the range is from 0 to66 Hz (0 - 4000 r.p.m.).


21 dark room aid H.F. Blom

22 battery indicator23 wide band frequency doubler

2 I This circuit is intended as anI aid in the dark room to ensurecorrect exposure time without effort.Before the paper is placed under theenlarger, the amount of light ismeasured by means of an LDR. Thisis the light-sensitive element.The LDR makes up one branch of abridge circuit, which is formed by theLDR, RI, R2 and part of P2. The otherbranch consists of P1, R3, R4 and a partof P2. With P2 in centre position, andP1 adjusted to a resistance value lowerthan that of the LDR, the voltage at the+ input of the op -amp is lower than thevoltage at the - input. With this con-dition, the output voltage of the op -ampis negative, and D1 lights up. On theother hand, when the resistance of P1 ishigher than that of the LDR, the outputis positive, and D2 lights. If the voltageson both inputs are equal, both LEDslight at half brilliance. This is due tothe fact that hum is picked -up by thehigh sensitivity op -amp. Thus the circuitindicates when the resistance values ofthe LDR and P1 are equal. When theLDR is placed under the enlarger, itsresistance value will correspond to thelight intensity.P1 is now adjusted until the bridge isbalanced, after which the exposure timecan be read from a calibrated scaleattached to Pl. By adjusting P2 slightchanges to the bridge balance are poss-ible. In this way it is possible to intro-duce corrections for different sensi-tivities of the photographic paper.This dark room aid has only one draw-back: the scale calibration of PI canonly be obtained by spending someevenings in the dark room. But, ofcourse, for the real enthusiast, this isno problem at all!The LDR must be mounted in a flatholder and be partly covered by a mask.This method allows spot measurement

and also helps adapt the LDR to thecircuit. To calibrate the unit, first of allensure that the bridge can be balancedwith PI over the entire range, fromextreme light to extreme dark. If notlarger or smaller apertures in the LDRmask can be tried. After this, by usingan ohm -meter, P1 can be provided witha scale.For example: PI = 500 S.2 gives onesecond, 1 kS-2 gives 2 seconds, and so onuntil P1 = 32 kS2 which corresponds to64 seconds. By means of test strips andan 'average grey' negative, the exposuretime can now be brought into accord-ance with the sensitivity of the photo-graphic paper by means of P2. To thisend, P2 is also provided with a scalewith the type numbers or gradationsfor different papers.The control range of P2 is equal to4 stops. If this scale is shifted too fartowards one of the extreme positions,the LDR mask must be changed.Since only a small area of the pictureis measured by the LDR, it is possibleto determine the contrast, and choosethe type of paper accordingly.

22 One drawback of primary cellsand batteries is that they often

go flat at the most inconvenient times.The circuit described here can donothing about dead batteries, but it cangive timely warning that the batteriesmust be replaced or charged soon.The battery indicator is suitable forvoltages between 3 and 15 V; thethreshold value can be adjusted with P 1.When the voltage drops below the setvalue, the LED (DI) lights up, giving anindication that the batteries needattention.

The circuit uses only a few components,requiring very little space so that itcan be built into miniature radio sets.As long as the battery is O.K., the cir-cuit draws about 1 to 3 mA; when th,!.LED lights up, this increases to 5 to15 mA. To save current, SI can beadded which can be either a toggleswitch or a push-button switch.

.14

23 Frequency doubling devicesusually employ a transistor stage

operating in class C. In its output circuitthere is a tuned circuit which resonateson the second harmonic. The amount offundamental frequency rejection isgoverned by the Q of this tuned circuit.However, when a frequency doublerbased on the SO 42-P is used, a 100 ohmpot is used to reject the fundamental.At 10 MHz, and as long as the inputsignal does not exceed 30 mV rms level,40 dB fundamental suppression can beobtained.The output capacitance of the circuit is6 p.

elektor july/august 1976 - 723

1

I

f

!f

24 liquid level indicator25 aerial booster26 variable slope filter

2 This circuit was originally4intended as a water level indi-

cator for use by blind persons, to givean audible indication when a cup, bowlor other container was full. It willfunction with any liquid that willconduct electricity, such as beer, tapwater, tea, milk. It will, of course notfunction with distilled or de -ionisedwater. The circuit has other applicationssuch as a rain sensor (when used with asuitable probe).The circuit is extremely simple. The in-put of NI is normally held low by a1M resistor. When the probes are im-rtersed in a conducting liquid the inputof NI goes high, so the output goes lowand the output of N2 goes high, en-abling the astable multivibrator N3/N4,which switches T1 and T2 on and offto produce a tone from the speaker. Anopen collector transistor output is alsoprovided to drive a relay or other cir-cuit. Probe construction for levelsensing and for rain sensing are shown infigure 2. The level sensor probes shouldpreferably be made of stainless steelwire for ease of cleaning, and the circuithousing should be watertight in case ofaccidents.

A very convenient way ofa improving5 the performance of anot -so -sensitive FM tuner is the additionof a VHF pre -amplifier stage in front ofthe existing equipment. Satisfactorygain improvement can be obtained if thebooster stage is designed using a

5...9VBC142BF Y52

BF200(BF 180) R

transistor having good VHF properties,such as the BI -7200 or BF 180. Thesetransistors also have good noise figures.

This booster design was tried in frontof a variety of receivers with goodresults. The average voltage gain figureproved to be about 12 dB.The preamplifier design is quite straight-forward, using a common base configur-ation conditioned for minimum noise.75 ohm antenna systems can be coupledstraight to Cl. For 300 ohm systemsa matching transformer LI can be made,by winding a 2 turn primary and a1 turn secondary on a ferrite bead.Use 0.3 mm enamelled wire (SWG33).The output matching transformer iswound on a 6 mm (1/4") diameter coilform. It should have a ferrite slug witha permeability pr = 12. Using 0.9 mmenamelled wire (SWG21) the primary(L2A) has 4 turns while the secondary(L2B) has 2 turns.The circuit is not particularly criticalto operate, provided that due care istaken in the construction of theamplifier and long connections areavoided. The transistor screening pincan be connected to the base, as shownin the drawing, or straight to thep.c. board common.The only circuit requiring adjustmentis the primary of the output trans-former, L2/C4. This adjustment iscarried out by tuning the FM set to astation at approximately 95 MHz. C4 isthen set to minimum capacitance, afterwhich L2 is set for maximum signalstrength by adjusting the slug. Tuning iscompleted by the adjustment of C4,also for maximum signal strength.

26 Most RC noise filters show afixed roll -off (slope) at

frequencies above the cross -over fre-quency. Such filters with a single RCnetwork usually have a 6 dB per octaveroll -off.Admittedly, with such filters, noise andhigh frequency distortion is made lessobtrusive. However, not only the noiseis effected, an important part of thehigh frequency content of the originalsignal may also have been wiped out.In such cases a filter with adjustableroll -off slope would be an asset.Figure 2 shows a noise filter circuit witha cross -over frequency of approximately7 kHz and a variable slope adjustablebetween 0 and approximately 25 dB/

1

fo -.7p=12n LC

Ro=

m= 0,85 0,9


26 variable slope filter (cont.)27 active oscilloscope probe28 TTL-insurance (Norbert Conrads)

oct. The network, analogous to thecross -over filter in the Quad 33 pre-amplifier, is based on the m -derivedlow-pass filter (figure 1). The inductoris a Toko 33 mH 5% coil.The frequency dependent characteristicsof this filter are shown in figure 3.Graph 1 represent a 0 dB/oct roll -offover the audio frequency range; this iswith P1 shorting out the inductor.Graph 2 shows a halfway position ofthe control, and graph 3 the effect of amaximum resistance across the inductor.The equation of figure 1 will be ofguidance to the constructor who wantsto design a filter after his own tasteusing other inductances and/or cross-over frequencies.

3 0

2

4

dB -6

810

12

14

16

18

20

-22

24

26

2 L1 33mH

2

3

20

27 The vast majority of oscillo-scopes possessed by home con-

structors have a fairly low bandwidth,between 3 and 10 MHz. On the mostsensitive voltage ranges the bandwidth isoften further reduced. The usual pieceof screened lead (coax) with prods hasa lot of capacitance. This is particularlynoticeable when testing logic or other

50 00 200 500 1K 2 5 OK 20

pulse circuits, as this load capacitancecan cause ringing.An active scope probe acts as an im-pedance converter. It presents a highinput impedance to the circuit undertest and has a low output impedance todrive the cable capacitance. Gain mayalso be incorporated into the probe,which can make up for lack of gain inthe scope.A simple probe is shown in the ac -

12V

(Hz)

50 00K 200 500 1MH29537 3

companying circuit. A FET connectedin source follower mode presents ahigh input impedance. This drives abipolar transistor voltage amplifier.The upper frequency limit is approxi-mately 30 MHz, provided sufficient careis taken with the construction. For highfrequencies the lead must be correctlyterminated at each end to minimisereflections.

20 It still happens too often that amains supply fails due to ther-

mal overload. The result is usually ashort circuit between collector andemitter of the output transistor. Whenthis happens in a TTL-supply, the out-put voltage may reach an extremelyhigh value, usually causing destructionof the TTL-ICs. The circuit describedbelow prevents such disasters becauseit switches off the supply if the outputvoltage goes higher than 6 V.

L.


29 simple headphone amplifier

HZ

re

it

IN

TI functions as the voltage detector;via R2 and D2 the emitter of thistransistor is held at a potential aboutequal to the TTL-supply voltage. If thebase voltage now becomes about 0.7 Vhigher than the emitter voltage, T1begins to conduct and triggers Th viaT2 and R4. When the thyristor istriggered, relay Re is energized andcuts off the TTL-supply voltage. Inaddition a LED is connected parallelto the relay to give the user visualwarning that the voltage is too high.Diggle D1 limits the negative base/emitter voltage of T I to a value ofabout 0.3 V. Furthermore, D4 and R6in combination with the thyristor,create a heavy load for the TTL-voltage.Since a 'broken-down' supply is nolonger stabilized, the TTL-voltageimmediately drops to a safe value underinfluence of the additional load whenthe thyristor is triggered.Consequently, the response time of therelay does not play a significant part inthis protective circuit.DVIe D4 prevents interaction of thetwo supply voltages. The supply for theprotective circuit (8 16 V) can beobtained from the non -stabilized out -

D4 5VBY103

0"" 8...16V

9389

put (electrolytic smoothing capacitor)of the 5 V TTL-supply. The thyristoris reset by switching off the supply.Diode D4, shown as a BY103, can bereplaced by any type that can handle atleast 2 A. Thyristor Th must be a3 Amp. type, and the operating voltageof relay Re should be about 10 V.

2n This headphone amplifier is7 intended for use with high

impedance headphones (greater than300 17) such as the Sennheiser HD414and HD424. It is extremely simple anduses only two transistors per channel.The circuit comprises a voltage amplifierT1, driving an emitter follower T2,which provides the necessary low out-put impedance to drive the 'phones'.The circuit was intended to be drivenby a control amplifier that can providea nominal 1 V r.m.s. output, such asthe TCA730/740 circuit published inElektor 8. It was found that a gain ofbetween 3 and 4 in the headphoneamplifier would provide an adequatevolume level in the headphones. Thegain is determined by the ratio R3:R4and is about 3.7. With such a small gainthe distortion is fairly low due to thelarge amount of negative feedback at

Resistors:

R1 = 330 kR2 = 56 kR3 = 6k8R4 = 1k8R5 = 470 n

Capacitors:Cl = 1 uC2 = 22014/16 VC3 = 22 g/16 V

Semiconductors:T1 = BC109CT2 = BC108132 of each componentfor stereo, except C3.

the emitter of T I. The unbypassedemitter resistor R4 ensures that theimpedance at the base of T1 is high(in fact [1 + hFE Ti times R4) so theactual input impedance of the circuit islargely determined by the biasingresistors R 1 and R2, and is greater than40 k. The amplifier can thus be drivenwithout difficulty by the control ampli-fier.The output impedance of 6k8 at thecollector of TI is stepped down by theemitter follower T2, so that the head-phones can be driven. The frequencyresponse of the circuit is determinedmainly by the input and outputcoupling capacitors Cl and C2 and issubstantially flat between 20 Hz and20 kHz. The nominal supply voltage ofthe circuit is 15 V but any supply volt-age between 12 and 18 V may be used.The printed circuit board layout is ar-ranged so that the input connectionsline up with the outputs on theTCA730/740 control board.

-4.1001 ra4 ictRsiocoi PH5 to `0'

011

a


30 current source H. Frakstein

31 tapped code lock P. Cousin

32 wien bridge oscillator3n This circuit features a Darlington

V pair T2/T3 joined by TI tocomplete a negative feedback loop. Thiscan be explained as follows.Let it be assumed that, for one reasonor another, the source current I has atendency to increase. This would in-crease the potential across R3, causingthe T I base -to -emitter potential to riseand thereby the current through TI to

T1 T3 =TUN* see text

increase. As a consequence the potentialacross RI would also rise, thereby re-ducing the T2/T3 base -to -emitterpotential, which in turn would stronglycounteract the tendency of the sourcecurrent to increase.R2 has the effect of rendering thesource current relatively independent ofsmall power supply voltage fluctuations.This current is practically exclusivelydetermined by R3.

The R3 resistance can be found fromthe equation R3 = 600/I with R3expressed in ohms and'I in milliamps.

3 It is much easier to open a doorwith a code lock than always

having to carry a heavy bunch of keysabout. Besides, keys can get lost, andcode locks can't.The diagram in figure 2 shows a codelock that can be opened with a simplethree -digit code. However, this lockoperates in a slightly unusual way. Withthis lock, three figures must be touchedsimultaneously (i.e. with three fingers).One finger in the wrong place, and therelay cannot be energized.As figure 1 clearly shows, it is imposs-ible to deduce from the lock'sconstruction how it should be operated.This feature, together with theenormous number of possibilities,makes it impossible for an outsider toopen the lock. The code for the exampleis 9-11-4. When this code is tapped, theinputs of N2, N3 and N4 become

05V

( 4k17

rt -

i rI 40

11

I

1

I r,

,

14.

9

rr

40

)

C

)

r5V

Cr't

)1

141

I'D

I

195E1

) 91 1109E,

1

!

14'44440141.

logic '0'; so, the three inputs of N8 andthe output of N5 become logic 'I'.If none of the other taps are touched,all inputs of N7 will be logic '1', whichcauses the relay to be energized and theLED to light up.The relay can be used to switch anelectric door opener.

2 Th

12 2

6 4

5

10

> 2 1

5012 12

4

6

8

4

see text

7430

6

2

3N7

12

05V

DJS

2 BC141

Ni- N6= 7404N7-N8= 7410

9525.

This oscillator uses a 741 opanip(or 1/4 of a LM 324) as its

active component. If features aninteresting and effective method tostabilise the output amplitude. A pairof reverse -parallel strapped diodes isused instead of the more conventionalNTC resistor, incandescent lamp orFET. The circuit will produce 10 Vpeak -to -peak signal with no more than0.2% distortion.The amplitude stabilising networkconsists of the diode pair D1/D2together with P1, R3, R4 and R7. P1

sets the output amplitude. PI is connec-ted into the circuit in such a way thatwhen its resistance is turned too lowit will kill the oscillations. The ampli-tude range of P1 may be shifted to someextent by increasing the resistance ofR7.

The oscillation frequency can be found1by using the equation f =

2nRCin which R = R1 = R2 = 10 k, andC = C 1 = C2.

32


DI

33 porch lighter H.J. Wigbels

34 wind machine O. Corff

A frequency of approximately 1 kHz isobtained when C is 15 n.A balanced power supply of + and-15 V can be used in place of the single30 V supply, in which case the voltagedivider R5 and R6 plus C3 is notneeded.

82

CI 451" R3momoawn

R5

R6

®30V

CL

9538

IC = 741

D1.D2 .1N4148

33 This circuit will automaticallyturn on the porch light when the

door bell button is pushed, provided it'sdark outside. After about one minute itturns the light off again.In the door bell circuit, when the bell -push SI is closed, current flows throughthe two 1N4001 diodes (during alter-nate half cycles). The voltage acrossthese two diodes is about 1.2 V, suf-ficient to light the LED in the opto-isolator. LED current is adjusted by the

Al

40 -60vi

470 St pot. An alternative arrangementwould be to use a 6 V lamp in parallelwith the bell. The only disadvantage isthat it will burn out after a while.Initially, CI charges through DI, RIand.the porch lamp to the zener voltageof Z1 (24 V). The resistance of the LDRin daylight is low, therefore the connec-tion point between the LDR and P1 isheld high. Even when the door bellbutton is pushed and T1 conducts, thereis insufficient current to pull that pointlow. Since a negative pulse is needed bythe SCS (at point Ag) to activate thecircuit, nothing happens.However, when the LDR is in the darkit's resistance goes up, so that when T1conducts the voltage drop at the anodegate (Ag) will be sufficient to trigger theSCS. When the SCS has been triggered,point Cg goes high, causing T2 to con-duct which triggers the thyristor 'Th'.turning on the porch light.At the same time, CI is dischargingthrough the SCS and R5, R6 and R7.After about one minute, CI has dis-

RI-I 33k

CI

7/140v

82

charged to such a low value, avout thatof Z3, that the SCS stops conducting.The lamp is now switched off and thecircuit then goes back to the quiescentstate. CI charges up and the circuit isready for the next push of the button.The LDR must be mounted in a place sothat it can detect day and night. Careshould be taken so that outside lights(street lamps, passing cars) will notshine on it, otherwise the circuit maythink it is daytime.Warning: as mains voltages are presentat many points in this circuit, cautionmust be taken. Use good insulation,especially at the opto coupler.

34 This circuit gives a true imi-tation of the sound of wind.

Transistor T1 is connected as a zenerdiode and supplies T2 with a noisesignal. This signal, amplified by T2, isfed to a selective amplifier built around

15... 24V

2

T2

TUN ITUN

3

1014sv

R4

741

9586

4700

C81

11.17(:).

9582 C)

a 741 opamp. The negative feedback cir-cuit of the 741 contains a double -Tfilter. The centre frequency of thisfilter, and thus the 'wind timbre' isadjusted with the three -sectionpotentiometer P2a/P2b/P2c. Poten-tiometer P1 is for the wind forceadjustment and can be provided with aBeaufort scale, if required. Poten-tiometer P2 can be made of a 15 kquadro pontentiometer, with two of thesections connected in parallel (P2c). Ifrequired, the component values in thedouble -T network can be modified tosuit a different value for P2, accordingto the formula:

f - 1

, with R = P2a = P2b = 2P2c,27r RC

and C = 1/2C5 = C6 = C7.It is also possible to make P2 usingthree p.c.b. adjustment potentiometersmounted vertically and placed in line.Then some type of home made shaftcan be slid through all three pots.


35 regulated + and 15 V power supply36 triangular wave oscillator G. Bruggink

37 electronic lap counter M. Hund

35 This compact balanced powersupply makes use of the four

opamps in a single 324 IC. They areused to stabilise the output voltage aswell as limit the output current. Thecurrent limiter circuit is set at 60 mAand contains a minimum of components.It should be noted that under someconditions it may seem that the primary(input) power supply of only ±16 V ison the low side. The maximum outputvoltage will depend on the propertiesof the IC which is used. It is NOT safeto increase the primary voltage; anyincrease may damage the chip.A 5.6 V zener diode is used for thereference voltage. The zener voltage isnot very critical; if low, the outputvoltage will be slightly low.P1 is the voltage control for both the+ and - supplies. Through it, thereference voltage is divided and appliedto the + input terminal of the (top)opamp. This opamp regulates thepositive output voltage by controllingthe base current of the series regulatingtransistor (BC140).Output voltage stabilisation is effectedby a negative feedback loop with avoltage divider made up of the 22 k and10 k resistors.The negative voltage regulation is some-what more complicated. The + inputterminal of the lower opamp isconnected to the zero potential terminal'0', via a 6k8 resistor. The referencevoltage is applied via control P1 andsome other components to the - inputterminal. The negative output voltageis balanced against the positive referencepotential by the voltage divider `see -saw'formed by the 33 k and 10 k resistors

(which are bridged by a trimming net-work).Trimming control P2 neutralises theinfluence of minor tolerances in thenetwork components, and is adjusted tobalance the positive and negative outputvoltages. Current protection is ac-complished by the two remainingopamps in the IC. If the potentialacross either of the 10 ohm resistorsexceeds 0.6 V, the reference voltage willdrop to zero and, consequently, the out-put voltages with it. At the same timethe LED's will light up to indicate thatthe protective circuit is operating.

A triangular wave generator canbe designed and built with very

few components, using an operationalamplifier which has built in frequencycompensation. The circuit was designedusing the 741 opamp, however manyother opamps will work.When the power is switched on, thecapacitor begins to charge via thevariable resistor. As soon as the poten-tial across the capacitor rises above1/11 of the output potential, the out-put voltage begins to drop. The rate ofdrop (slew rate!) depends on the opamp.After a certain time the output voltagewill have fallen to a level where thecapacitor will begin to discharge. Assoon as the capacitor voltage drops toless than 1/11 of the output, the outputbegins to rise again. The 1/11 level isderived from the voltage divider madeup by the 100 k and 10 k resistors:

10k 10k 1

10 k+ 100 k 110k 11

36

16V

016V

5.6V

ion

N 4148

ir 1N 4148

lion

S

..)BC140

0

115P

aov

OGNI. 40V

0TN -15.-0V

BC160

9503

With the design parameters indicatedin the diagram the frequency can beset anywhere between 15 and 70 kHz.At 15 kHz the wave has a trapezoidshape with a peak -to -peak amplitudeof approximately 25 V. At 20 kHz theoutput signal shows an excellenttriangular shape.Since the slope of the triangle ramps arefixed, it follows that the amplitudewill decrease as the frequency increases.At 70 kHz the peak -to -peak amplitudeis approximately 5 V.The power supply voltage is not criticalbut will influence the amplitude of theoutput signal.The 741 opamp did not work wellbelow 15 kHz. Other opamps which useexternal frequency compensation can bemade to function at low frequencies,by using appropriate compensationnetworks. Symmetrical triangular waveshapes can be obtained in this way. Avery suitable opamp is the 748.

3T Mechanical lap counters for -/ model racetracks have a repu-

tation for unreliability. They eithersuffer from poor contacts, or themechanism easily jams. The electroniclap counter overcomes these difficulties.Triggering of the lap counter is carriedout optically. When the car passes the`checkpoint' it interrupts the lightfrom LI so that the resistance of theLDR increases. This takes the inputof N3 high, so the output goes low,setting flip-flop N1/N2. The outputof N2 goes low and the 7490 counter isclocked (7490 counts on negative -going edge). When the car has passed,the LDR is re -illuminated and itsresistance falls, the input of N2 goesto logic '0' and the flip-flop is reset.Two 7490 counters are cascaded so thata count of up to 99 laps may heindicated. This is displayed by twoseven -segment LED displays such asLitronix DL707, or MAN7 for example.The number of laps in a race can be


38 opto coupler with two LEDs

LI--r j-=

15 1 I 131121 111101 9

r 9 arrcc e

1C27447

8

6 2

08

800o

SI

11 8 9 12

9 .1192.

D C B = 801N

C47490 A"

Ro:' r02

6 7 2 13

4

LI751741 T3112 Fr

1 .9 a e

'Cl7447

C

11

2 1 7

9

/b /a

12

8DI N

C37490 AIN

991'892; ROI 0;2

014

6

2000

'4 b 1 1k

7

S2

resetgii

preset by SI to 20, 40 or 80. Three ofthe positions of this switch are connec-ted to the 2,4 and 8 outputs of the10 lap counter IC4. The input of N4is normally low so the output is highand T1 is turned on, energising relayRe. Power to the racetrack is suppliedvia the contacts of this relay.If the number of laps is set to say 40,then when the fortieth lap is com-pleted the 'C' output of IC4 will goNigh. The output of N4 will thus go low,turning off T I and opening the relaycontacts, which cuts off the power tothe track so that the car will stop. Onelap counter is required for each lane ofthe racetrack and the relay contactsshould be connected in series so thatas soon as the first car completes thecourse power to the circuit is cut.Power can be restored and the counterreset by pressing the reset button S2.The LDR should be mounted in acylindrical tube to screen it fromambient light, which might otherwisekeep the LDR resistance low and blockthe circuit. The sensitivity may beadjusted by Pl.

38 On the basis of the principle thatthe conversion of electrical

energy to light in a LED must he revers-ible, its merits for practical use were

N1... N4 =IC 5 = 7400

N3

Re DIDi 0

TI71 A 0

®TUN

- v1

LOA

investigated. The diagram shows positiveresults. A logic level of at least 2 V isapplied to the input of Tl. The photocurrent generated in LED 2 is fed to athree -stage amplifier where it is broughtto a level suitable to drive T5 and T6.At very low frequencies the duty cyclevaries, but this forms no objectionfor most applications. The maximumfrequency could not be determinedowing to the capacitive cross talkbetween the two LEDs, which beginsto play a role above about 40 kHz. Theresults with all LEDs are not unani-mously favourable. The best resultswere obtained with HP types. Owingto the LED's selectivity as a light

:5.20V

see text

Ti . T6.TUN

5V

CI9381 1

sensitive diode, the system is hardlyaffected by ambient light. Only in afew exceptional cases will it benecessary to screen the LEDs againstambient light, particularly from fluor-escent tubes.Rx should be chosen according to theformula:

VbRx -

ILEDi max.

Ry represents the load resistor; forTTL applications, the supply shouldbe 5 V and Ry is 270 a

5-15V

4Empsiekat9517

7.7


39 seat belt reminder

39 In view of the proposed govern-ment legislation to make the

wearing of car seat belts compulsory,and the prospect of a £50 fine for notcomplying with the law, some sort ofdevice that reminds driver and passengerto wear their seat belts would be ex-tremely useful. The circuit given here isnot intended to be a foolproof devicefor compelling people to wear seat belts(as is the case with some commercialsystems) but is intended simply to jogthe memory of the well-intentioned(but absent-minded) driver andpassenger.The circuit senses the fact that someonehas entered either the driver or thepassenger door by making use of theinterior courtesy light door switches.These are normally connected inparallel, but for this purpose they mustbe isolated by diodes. The interior lightwill then still function normally, but itis possible to sense the opening of adoor when the other is already open.

When the driver's door is opened theflip-flop comprising N1/N2 is set. Theoutput of Ni takes the input of N4high, so that when the ignition isswitched on the astable comprisingN3/N4 starts to oscillate, switchingTI and T2 on and off and flashing thewarning light. When the door has beenclosed flip-flop N1/N2 may be resetby pressing the reset button S3, thusdisabling the astable.For the passenger door the same func-tion is performed by N5/N6 and N7/N8,

Batt.

06..12V

Dt 02

with one difference. If the passengerleaves the car but the driver remainsthe passenger's warning light will startto flash. This is also the case if thedriver enters the car alone but thepassenger door has been opened atsome time in the past e.g. to loadparcels into the car before the start ofthe journey. For this reason a secondreset button S5 is provided for thepassenger warning light, which ismounted on the driver's side of thedashboard.The reset switches S3 and S4 may bemanually operated buttons mountedon the dashboard, or with a littleingenuity they may he linked to theseat belts. An example is shown forbelts having the buckle rigidly mountedon the transmission tunnel. A reedswitch is mounted on the buckle andthis is activated by a small magnetglued to the hasp of the belt wheneverthe hasp is inserted into the buckle.If this type of system is used then flip-flop NI /N2 may be dispensed with byomitting the link between the output ofNI and one input of N2 and connectingboth inputs of N2 to S3 and RI. Thishas the advantage that if the seat belt isremoved after the driver's door has beenclosed S3 will open, taking the input toN2 high. The output of N1 will thusalso go high, enabling the astablemultivibrator and flashing the driver'swarning light.It is not possible to do this on thepassenger side however, because of themanual reset button S5. If N5/N6 were

S4

R3-L

T8

SS

Q+ 2

R7

70n 14

470nR10100k

6112V

L2

11 6112V

9678.1 °

not connected as a flip-flop then S5would have to be a latching type sothat the input of N6 could be held loweven with S4 open (i.e. no passenger).The possibility then exists that S5 mightaccidentally be left closed on leaving thecar, in which case the passenger warninglight would flash only while the doorwas open. S5 must therefore be amomentary action switch, and N5/N6must be connected as a flip-flop.

S1 = passenger's doorS2 = driver's doorS3 = driver resetS4 = passenger resetS5 = passenger override

N1 ... N4 = IC1 = 4011N5 ... N8 = IC2 = 4011D1 . 02 =1N4001TI . T4 =TUN

Batt.6...12V Ign


40 SSB adapter41 microammeter

4A Several of the better portableV radios have a number of short

wave bands, with adequate stability forSSB (Single SideBand) reception. How-ever, they are not provided with thenecessary SSB detector, and very oftenthe selectivity is also insufficient. Asuitable extension circuit is required ifone is interested in short-wave SSBreception.In the circuit shown, a FET input stage(T1) is used so that the input impedanceis sufficiently high to allow for connec-tion of the adapter to practically anyexisting IF strip.lite limiting amplifier in IC1 is used asan oscillator, the high gain of this ampli-fier means that the tuned circuit (LI,C2 . C4) need hardly be loaded, sothat high stability of the oscillator canbe achieved. Furthermore, the internallimiting stage in this amplifier is alreadydesigned to reduce the influence ofsupply voltage variations to a minimum.The TBA120 (or SO41P) also contains amultiplier stage, and this is used in this

BFOtuning

Co

R2

10p

E300

A

Ca

60P

CO

Cl

100n

220P

spa

L1

470pH

6

ICITBA12r+

S041 P

5-12V

11 R3

C631 41

To. Toon To.

C9

BC547b

RO

18n

circuit as a product detector. Toincrease the selectivity, the output ofthis decoder is fed through a low-passfilter with a cut-off frequency ofapproximately 3.4 kHz (R1, R4,C9 ... C11). The output stage (T2) issimply an emitter -follower; it can drivepractically any headphone direct.The alignment procedure is relativelysimple:- set C2 in its mid position;- using C3, set the oscillator frequency

to match the original IF frequency(455 kHz). This can be done with afrequency counter, if one is available;failing this, tune in to a normal AM

C10

77

T2

CI I

770p

R5

C13R6

470 $2 I ---2µ2

C12 10VI=10p

TI6V

9641

I

Osignal and adjust C3 for zero beat. C2should now give a tuning range of±3 kHz around the centre frequency.

- tune in to a strong SSB signal, andadjust PI so that the output is notaudibly distorted.

The p.c. board should be mounted in ametal screening box. A BNC connectorcan be used for the input; check that itmakes good contact with chassis,certainly if an aluminium box is used.The only control on the box is the BFOtuning capacitor C3. This can be con-nected to the p.c. board using screenedcable. The actual value of C3 is not socritical, provided it gives a total tuning

range (capacitance variation) ofapproximately 10 p. If the only tuningcapacitor obtainable is too large, it isalways possible to either strip out someof the plates to bring it into therequired range, or else add a small seriescapacitor.The connection to the receiver shouldbe coax, and not more than 3' (1 m)long. It should be connected to the finalIF stage of the receiver via a 10 pcapacitor that is mounted as close aspossible to the IF stage in question. Thiswill, of course, detune the final IF stageslightly, so that it will have to beretrimmed.

41This circuit can be used forcurrent measurements in five

ranges, from 1µA to 10 mA f.s.d.(D.C.).The opamp is used in a virtual earthcircuit. The current Ix to be measuredflows direct to supply common, and thecircuit supplies an equal current fromthe opamp output through the corre-sponding feedback resistor to theinverting input terminal; the polarity ofthis current is indicated in the diagram.Inspection of the circuit reveals that lxis supplied by the opamp output circuit.Since the inverting input terminalremains at earth potential (this is what`virtual earth' is all about!), the opampoutput terminal will be positive withrespect to zero; the voltage at f.s.d. willbe 1 V for the feedback resistancevalues indicated in the diagram.The resistance of the meter plus theseries resistor R must give full scaledeflection at 1 V. With, for instance, ameter of 1 mA f.s.d. the sum of theresistances must be 1 1d2; for a 100µAmeter the sum must be 10 k2, and soon. A preset potentiometer can be usedfor R.

sof =IV


42 perpetual solar clock43 MOS monostable

4, Using a modern CMOS watchA chip driving a Liquid Crystal

display, it is a perfectly feasible toconstruct a perpetual clock that willobtain its power from the sun. The onlycomponents that may eventually failare the LC display and the NiCadbatteries used to power the clock duringdarkness. The solar power supply isextremely simple. 5 silicon solar cellsprovide about 2.75 volts in bright sun-light, slightly less in average artificiallighting, which powers the clock andcharges the batteries. The two diodesdrop the 2.44 volts of the NiCadbattery down to about 1.6 V which isthe voltage from which the clock ICoperates. (Suitable solar cells cost about70 p each.)

T

A

COM

MC 14440

VSSVDDMin Hrs SecOsc,

LST

Display -test

VSS

HF

HF

-( 100k

041405 1.4

2xNiCad

2,44V

43 The basic circuit of a mono -stable using CMOS NOR -gates

is given in figure 1. The disadvantge ofthis simple circuit is that the outputpulse width depends not only on thetime constant RTCT, but also on theswitching threshold of the outputinverter N2. As this is subject to a ±33%manufacturing tolerance it is not poss-ible to accurately determine the pulsewidth, and if a defined pulse is re-quired then RT must be made adjust-able, which is costly and inconvenient.This difficulty is overcome by the cir-cuit of figure 2, which automaticallycompensates for the tolerance inthreshold voltage. The circuit istriggered by the negative -going edge ofthe input waveform (1). When thisnegative -going edge occurs the input ofN1 is pulled low by C 1, which is un-charged. The output of NI thus goeshigh, while CI charges through R I . (2)When the voltage on Cl reaches thethreshold of Ni, the output of N1goes low. The duration of the outputpulse from N1 (3) is denoted by t1.

4.-4 100k

I 0-1 100k

100k

51 52

.1 1DI ... D4 = 1N4148

D5 = 0A47

S3

x-tal32768 Hz

6...350

22M 1-411

While the output of N I is high C2 ischarged up via D 1, so the output ofN2 is low. When the output of NIgoes low C2 begins to discharge (4)through R2 until the voltage on itreaches the threshold of N2, whenthe output of N2 goes high.The output pulse duration (T) is thesum of t1 and t2, i.e. the time takenfor CI to charge to the threshold volt-age of N1 and the time taken for C2to discharge to the threshold voltageof N2, respectively.The compensation of the output pulselength depends on the fact that twogates fabricated on the same chipusually have equal threshold voltages,so it will work only if N I and N2 arein the same package.The compensation operates as follows:if the threshold voltage is the nominalvalue (approx 45% supply voltage)then, assuming that the time constantsRI Cl and R2 C2 are equal, it willtake approximately the same time forCl to charge to the threshold voltageof NI as it takes C2 to discharge to thethreshold voltage of N2. If the thresholdvoltages are higher than nominal it will

75k± 20k

MIM7 P

1

DI

D2

D3

9532

N1,N2=1/2 x4001

47n

O

9199-1

44 spike monoflop

eleK(Ur ti ny I dtlyU31. - I

take Cl longer to charge to thethreshold voltage, but it will take C2 ashorter time to discharge to thethreshold voltage. If the threshold volt-ages are lower than nominal it willtake Cl a shorter time to charge, butC2 a longer time to discharge.The net result is that the overall pulselength stays more or less constant.

2O

With the manufacturing tolerance onVt, k can vary between 0.33 and0.66, but this will cause only an 8.5%variation in the monostable outputpulse width. With the nominal valueof threshold voltage the pulse lengthis approximately 1.4 RC.Literature:RCA Application Notes

--NA'--------

This can be shown mathematically asfollows:

T = t1 + t2

tt = R C In Vb - Vt

Vbt2 =RC InVt

where Vb = supply voltageVt = threshold voltage

C=C1 =C2R= R1 = R2

rewriting these equations

Vb

t1= R C ln 11 k

and t2 = R C In -1

where k =V--t

Vb

therefore

1 1T = R C In + R C In -k

which simplifies to

N1.N2----h x4011

44 Only four NAND -gates areneeded to build a spike mono -

flop. The delay time of three series -connected gates is used as the time -governing element (output pulse width).These three gates also function as aninverter for the incoming signal. Duringstatic conditions when the input signalis either '0' or '1' the gate N4 receivesinput signals which are not the same.This means the NAND output will behigh (1).Understanding the static condition ofthis circuit is simple, however fordynamic conditions it is best to think

1 Ac)0

of the input signal as a 'wave front'rather than just levels. This wave frontpasses through two different circuitpaths. One path is direct to N4 and theother is through the delaying inverterNI ... N3 then to N4. Since one wavefront arrives slightly before the otherit is only during this short time thatthe output of gate N4 will change state.Furthermore, this only occurs duringpositive going input transitions: if theinput signal was low, the inputs to N4would be '0' and '1' (top to bottom);when the input changes to '1' the topinput to N4 changes instantaneously.This makes the inputs to N4 '1' and'1' which gives a '0' at the output.The output will remain at '0' as long asthe wave front is moving through thedelay gates, the typical propagationdelay for each gate is 11 ns. Thereforethe output pulse is only about 30 ns.A negative going input transition willnot produce an output pulse: at themoment the input goes '0' the inputsto N4 are '0' and '0' which still gives a'1' at the output.If a positive going spike is need at theoutput, an inverting transistor stage can

2

20/DIVbe added. The use of TTL inverters isnot a good idea, because there is agood chance they will not respond tosuch short spikes (30 ns). To increasethe pulse width, five gates could beconnected in series to form the delayinginverter, which would make a pulsewidth of about 50 ns. This pulse shouldbe TTL compatible.The circuit was tried using the TTLIC 7400, but it should also be possibleto use COSMOS gates. The photographshows the input signal (A) and theoutput signal (B). The negative spikeoccurs on the positive edge of the inputsignal.

N1- N4= 7400

OB

1T = R C In (1 - k)k

734 -eleimar julyteummt 1376

45 ignition key reminder46 car clock using watch IC47 VCO with 74123

45 This circuit is intended toprevent a car driver inadvertently

leaving the ignition key in the lock. Itwill provide a warning if the driverattempts to leave the car while theignition key is still in the lock, whetherthe ignition is switched on or off.The fact that the key is in the lock withthe ignition switched off is detected bya light source and sensor arrangement.When the car door is opened the interiorcourtesy light switch will close and LI,which is wired into the same circuit, willlight. T3 will be fumed off and oneinput of NI will go high. If the key isnot in the lock then the phototransiator T2 will receive light from LI and theother input of NI will be held low.If the key is in the lock with the ignitionoff the light from LI will be blocked bythe key and the input of NI will be heldhigh by R2. The output of NI will below so the output of N2 will be highand the multivibrator comprising N3/N4will start. Similarly, if the ignition isfumed on when the door is opened TIwill be turned on, holding the input ofNI high, and the alarm will sound.

2

n r7 17 17I/ LIDM 72024

z14"

';1?';1;

46 Using the Interal ICM7202A(new version of the 7202) an

extremely compact car lock can beconstructed. The circui is basicallysimilar to the digital watch (Elektor 10),but with three differences.I. The display runs continuously. Thisis quite permissible and will not exceedthe power or current ratings of the IC.2. The p.c. board can be larger, thusmaking for easier construction.3. A stabilised supply is incorporatedto allow the circuit to be run from 12 V.The DL34M display used in the watchcircuit should be quite visible evenwhen mounted on a car dashboard.(Anyone who can't mad it shouldn't bedriving!)The master control S3 is normallyclosed so the display is on continuously.

47 The TTL-1C 74123 comprisestwo monoflops (MEI and MF7.)

which can be triggered by a positive edgeon the respective B -input. In this circuitQ2 is connected to BI, and QI to 132, sothe two monoflops MFI and MF2 to-gether form an astable multivibrator. Thecycle time is determined by the sum ofthe pulse times of the monoflops.

Normally, the pulse times of the mono-flops are determined by an externalcapacitor (between the points 14/15and 6/7, respectively) in combinationwith an external resistor (between thepoints 15 and positive supply, and 7 andpositive supply).This standard configuration has been

elaktor julytamust 1975 - 235

48 three channel mixer49 tap doorbell B. Segor

changed in so far that the diodes Dl andD2 have been added. This is importantwhen electrolytic capacitors are used forCI and C2. Furthermore, the externalresistors are replaced by the currentsource transistor T I . The charge timesfor CI and C2, respectively, are nowdetermined by the collector current ofTI which in turn is determined by thecontrol voltage (VC) supplied via R6.From the graph it appears that thefrequency decreases linearly with thecontrol voltage. Since the capacitors inthis case are not electrolytics, the graphcorresponds to the situation where DI

, alad D2 have been replaced by wirelinks.At equal values for Cl and C2 the dutycycle is 50%. Any value between 1 nand 100 g will do for CI and C2. Thecontrol voltage should not be higherthan 6 V.

27ya

CI =C2=100n

°

48 The LM3900 and the CA340Iare both quad Norton -type

clamps; the inputs can best be con-

sidered as current -driven instead ofvoltage -driven. For this reason resistorsmust be included in series with eachinput. This type of opamp is ideal forvirtual earth mixer circuits (figure I).An obvious application is to use three ofthe four opamps in one IC as inputpreamps (AI, A2 and A3 in figure 2),and to use the fourth as a summingoutput buffer stage, i.e. the mixerproper.The gain of the basic virtual earthcircuit shown in figure I is determinedby the ratio of the two resistors RI andR2. Furthermore, to obtain a correctDC balance, the value of R3 should behalf the value of R2. If variable gain isrequired, RI should be made variablebut R2 and R3 remain fixed.In figure 2, the gain of the input stagescan be preset with P4, P5 and P6respectively; P7 sets the gain of themixer stage.A further interesting feature of aNorton -type opamp is that the amplifieris blocked if the non -inverting input isgrounded; the output voltage is thenpractically equal to the positive supplyvoltage. This means that switches SI, S2and S3 can be used to completely blockany unwanted input, reducing noise andcrosstalk.

4 n A reliable tap doorbell can bebuilt using a few cheap com-

ponents, which usually can be foundin the junk -box.In the circuit described here the touchcontact (tap) is connected to the baseof T1. When a finger is placed on theTAP, mains hum on the skin will driveTI. This changes the bias on the baseof T2, which in -turn drives thedarlington T3/1-4, ,witching on thebuzzer and the lamp. A small pieceof printed circuit board, aluminiumfoil or something similar can be used forthe TAP contact. TI must be mountedvery close to the TAP or the connectingwire will pick up too much hum.In the quiescent state the circuit drawsabout 4 mA. It will work on any supplyvoltage from 6 to 12 volt. Of course thebuzzer and lamp must be suitable forthe supply voltage which will be used.


50 AMV without R and C51 aircraft communication receiver

50Thisastable multivibrator isbuilt from an odd number of

inverters. These inverters can be eitherTTL or COSMOS. The frequency ofowillation depends on the total propa-gation delay time of the inverters. Theoscillation is the result of circulatinginverted pulses. The cycle time of thesquare wave voltage equals twice thetotal propagation delay time.File frequency can be calculated byusing:

2 n Tpwhere f = oscillating frequency

n = number of inverters (odd)Tp =propagation delay time

(per gate)If the circuit was built using 5 TTLinverters such as a 7404, the propa-gation delay time would be 10 ns pernote. The resultant frequency would be:

2 x5 Mlle

-1>>->A>I>°*

51The simplest VHF receiver N thesuperregenerative.

Unfortunately, it is also one of the mostdifficult to get working properly ...Most of the problems stem from thefact that the oscillator is self -quenching.The receiver described here is lesscritical than the conventional super-regen,but it also uses mom components:the various superregen functions areeach performed M separate stages.The first stage (TI ) is an RF preampli-fier. The input (aerial) is not tuned,both in the interest of a simplealignment procedure and to reduce thedanger of feedback from the oscillatorto the RF input.The LC tuned circuit at the collector ofTI doubles as the tuned circuit for theoscillator. The oscillator stage (T2) isoperated in a grounded base configur-ation. A multivibrator (T3 and T4)supplies a quench signal which turns theascillator on and off. For proper oper-tion of the circuit, either the turn -on

or tum-off must be exponential; in thisdesign, the turn-off was madeexponential.The quench voltage at the oscillatoroutput is passed through an RC filternetwork (C7, C9 -C12, R9 -51I4, TO); theremaining (audio) signal is amplified by

T7 and T8 to a sufficient level to drive acrystal earphone or audio amplifier.The measurement results on the proto-type were as follows:

sensitivity: 2 pV for 12 dB S/N;- bandwidth: I ... 10% of the centm

frequency, depending on the inputsignal level;

- audio output level: 50 mV ... 2 V(PI);

- quench frequency: 3 30 kHz(depending on supply voltage);

- type of detection: logarithmic law,i.e, the audio output level N almostindependent of the RF input level;

- residual quench voltage at theoutput: to 50 my (p -p);

- tuning range: 90 ... 180 MHz.

Construction detailsThe p.c. board (and, if required, a 4.5 or9 V dry battery) should be mounted ina metal box. The tuning capacitorshould be mounted in such a way thatthe wires connecting it to the p.c. boardare not longer than half an inch ( I cm).Note that the connection to the rotatingvanes must also be connected to thep.c. board - the capacitor will not workvery well if only one side isconnected ...A good choice for the aerial input socketis a coax -type UHF connector. This hasthe advantage that a 'banana plug' fitsinto it perfectly. The aerial can now bemade of a large metal knitting needle(about 15 inches long), soldered intothe banana plug.If an aluminium box N used, make surethat all panels make good contact toeach other and to supply common -check this with an ohmmeter.When the receiver is switched on, a loudhissing should be audible. Set PI formaximum hiss level. It should now be *possible to tune in to several trans-mitters, certainly in the VHF FM band.The aircraft communications are con-centrated around 123 MHz and137 MHz.If no stations can be received, some-thing is wrong with the receiver. Thefollowing trouble -shooting procedureshould be carried out,

I. Turn the slider of PI up to the R5end (shorting out PI).

2. Tune a portable VHF -FM receiver toapproximately 100 MHz, hold itswhip aerial about an inch away fromthe oscillator coil LI , and turn C3slowly. If the portable FM receiversuddenly starts to hiss loudly, theoscillator and quench generator ofthe aircraft receiver am workingproperly. If an unniodulated carrier

is found instead of hiss (the portablereceiver goes quiet instead of hissingloudly), this means that the quenchmultivibrator T3/T4 is not workingproperly. An AC voltmeter can beconnected via a capacitor to point 'X'to check this.

3. If the above measurements did notlocate the fault, the next step is tocheck the audio stages. The voltagesat points 13 and D should be

- 1 volt and .1Vb respectively:

4. The RF amplifier stage can bechecked by measuring the emitter -base voltage of TI (0.2 V); thevoltage at point A should be1Vb 0.2 Y.

5. If the oscillator and the quenchmultivibrator are both working - seepoint 2, the portable receiverindicates an unmodulated carrier, butthe AC measurement at point Xshows that the multivib is working -diode D5 must be of the wrong type.

Note: be warned that under no circum-stances should this receiver be used onboard an aircraft. This is strictly "-forbidden by international aweement,and the penalties am very high!

Resistors:RI - Ok762,193,194,R6,R7 = 10 kR5 .470 51R8 = 27 kR9 - 220 k610 470 k5111 5112,5113,5114 -33 kR15 2k2R16,K19,R20,R27 = 1 k

RI7,R18,R22 = 47 k621= 22kR23 100 k1924,1926 3k3R25 - 33 SI

Semiconductors.PI = PF239T2 = BF199T3,T4,T6,T7 - 50547, BC1471-5,T8 = BC557,13C157DI ...DS = 1164148 -

CapCI - 47 pC2,C4 = 820 p03 = 2...20 p, tuning capacitorC5= 22 0/3 V66 -C7 I2nCS= 680 pC9,C10,C11,C12,C22 1515

C13,C14 = 470 pC15,C18,C21 - 100 nC16 = 47 nC17 = 10nC19 -471/10C20 = 470 p/10 V

&alms july/august 1976 - 737

LI a 75 turns 1 mm Cu 0 = rn


52 'wireless' bell extender

541 The front -door bell or telephone& cannot always be heard all over

the house. Extending the bell usuallymeans running wires all over the house,which is not a particularly elegantsolution to the problem. However, themains wiring already runs all over thehouse, so if this could be used therewould be no need for additional cables.The circuit shown here makes use ofthis possibility.The transmitter (figure 2) consists of anoscillator (T5/T6), running at 150 kHz,which is triggered by a monostablemultivibrator (T4). This monostable canbe triggered either by the front -doorbellpush (input 3) or by the telephoneamplifier (input B). If it is notconvenient to run the front -door belland the telephone over the same trans-mitter, more than one transmitter canbe used.Since it is not officially permitted toderive the telephone signal from electri-cal pulses inside the receiver, aninductive pick-up or microphone must

3

100r,

8 10V,

Omem

1000P16V

p56V

B=4X 1N4001

680n

be used. An amplifier boosts this signalto a useful level (figure I ). T3 is alsoused for rectifying the AC signal pickedup from the telephone.The receiver (figure 3) consists of a two-stage amplifier (T7/T8), a rectifier cir-cuit with a sufficiently long time -

constant to reject brief interferencepulses, and a multivibrator (T9/T10).To avoid problems with the neighbour'sbell extender, it is advisable to set thesensitivity of the receiver to theminimum value consistent withdependable reception. This sensitivitycan he adjusted with the 1 k preset

4k7

L._

BC547b

77

1N4148

1

(

1 N4148

25V

BC557b

I 4k7 1

BC557b

CO

Tp56V

SIM 475

IFloor, 1N4148

1N4148 1°

T5

BC547b

potentiometer. The components shouldnot be a problem: an equivalent for theBC547b is the BC107b (see `TUP-TUN-DUG-DUS', elsewhere in this issue);similarly, the BC557b is equivalent to aBC177b. The supply transformers usedin the circuit can be normal bell trans-formers; transformers TI and T2 are(/) 20 mm ferrite pot cores with an air -

gap, such as the Al 250. In each case,

(47012F®1N 4148

BC160

BC547b

9588-2

11

N 4148

erg

1k

3W

BC140

100n1000V

100n1000V

r\i

9588 -3

yielder july/august 1976 - 739

53 single transistor sawtooth generator54 class A amplifier re -considered55 0-30 V/1 A, stabilised G Ebner

winding 'a' consists of 40 turns of0.3 mm copper wire (31 SWG) and

. winding' is 20 turns of the same wire.

53 This simple sawtooth generatormakes use of the 'reverse'

characteristic of an NPN transistor: theemitter is positive with respect to thecollector and the base is not connected.Under these conditions, certain types oftransistor show a Vce/Ic characteristicwith a negative resistance kink (over acertain limited operation range), similar

I to the tunnel diode or unijunction tran-slator characteristic.

Medium power transistors such as the2N2218 and 2N2219 show this phenom-enon to a pronounced degree. It is nouse trying ordinary TUN's. Finding themost useful specimen is a question oftrial and error, either by measuring theVeefIc characteristics in the circuit offigure 1, or by hooking -up the circuit offigure 2 and plugging in various transis-tors until one works.Figure 2 is the simple sawtooth oscil-lator circuit. The R and C values and theinegistori breakdown potential deter-mine the sawtooth frequency. Thecapacitance value (C) can be between10 n and 1000 p. P is used to vary thefrequency over a certain range. Forfrequencies of over 45 kHz the outputwave shape will become mom likea sine -wave. The sawtooth peak -to -peakamplitude will be about 2 V.

Popular Electronics

54 id7ftfhri:nacmepIlikfioe,r5-with a2-0

output stage operates in Class A. Due toa sliding bias arrangement, the quiescentcurrent increases as the drive swingincreases.The class distinction between A and Bmodes of operation becomes apparentwhen considerations of high fidelity are

weighed against efficiency considerations. Class A stages are inherently freefrom cross -over distortion.The drawback of class A systems is theirlow efficiency compared to class Bstages.With this design, and using a 44 Vpower supply the quiescent current willbe approximately 960 mA. An outputpower of about 15 W will be deliveredinto an 8R load, or 20W into a 452load.Harmonic distortion will remain below0.1%. The input sensitivity will be about360 mV for 15 W into 8 R and about300 my for 20 W into 4 52 loads. Theinput impedance is approximately150 k.For preamplifiers with a 1 k sourceimpedance, capacitor C2 will be 6n8, for2 k source impedance it will be 3n3, and

The amplifier is short circuit proof;if there is a short it will draw approxi-mately 1.6 A.Control PI 0 used to offset the no -signaloutput voltage at the R18/1119 junction(approximately 21 V).Each output transistor (T6 and T7)requires a generous heat sink, thethermal resistance should not be less than3.3°C/W; drivers T4 and T5 require aclip -on heat sink.

Mulford Technical Communications.

55 aPu,eir.spuelieos,alwatys useful,

integrated voltage regulators are nowbecoming quite readily available, acircuit using only standard componentsmay be of interest.To avoid wasting too much power, andto limit the dissipation in the seriesregulator, the total 0-30 V control rangeis subdivided into three smaller ranges.Each of the three ranges corresponds toan appropriate secondary supply voltage(selected by Sla) and an appropriatereference voltage (selected by SIb).In order to obtain continuous controldown to 0 V, a negative auxiliary supplymust be added. In the circuit, this isderived (via 135 and C2) from a separate12 V winding on the mains transformer.An altemative would be to use anauxiliary mains transformer; a cheapersolution was described in an earlier issueof Elektor (-110/- from one transformerwinding', Elektor 5, p. 724).The results measured on the prototypeare quite good: ±35 V mains voltageswing caused only ±25 mV swing of theoutput voltage, at full load (I A) theA.C. component of the output (hum)was less than 15 mV.The circuit operates as follows. Areference voltage, derived from the

740 -dektor My/august 1976

55 0-30V/1A, stabilised (cont.)56 TV modulator

01 .. DO = 4 x 50V/2 A... B50 C2200D5,D13=1N4001010,011,0191114 =164141100.02 = 10 V400 rn4 5,0908 = 51,1 400 mW 5%

zener diode(s) D6 -D9 and set with PI, isapplied to the base of T2 via DIO andTI. T2 and T3 operate as a differentialamplifier; the output voltage n appliedto the base of T3 via DI2. The outputof this differential amplifier is applied,via D11, to the base of the 'compound'series regulator consisting of T4, T5 andT6. Even if it looks a bit complicated,this is a standard regulator circuit; itkeeps the output voltage practicallyconstant over a wide range of outputcurrents.T7 and TO with associated componentsare a current limiter stage. When thevoltage across RIO reaches a specificvalue (set by P2) T7 starts to conduct.This, in turn, causes T8 to conduct; thebase drive to T4 is reduced, pullingdown the output voltage so that theoutput current remains within thepreset limit.Position 1 of SI corresponds to anoutput range of 0-10 V, position 2 gives10-20 V and position 3 gives 20-30 V.P I gives a fine adjustment within therange set by SI. The maximum outputcurrent is set with P2. This control caneither be preset to give a maximumoutput current of 1 A or used as anadjustable output current control. Note,however, that the current limiter is notoperative when P2 is turned fully

T1,T9T3,T4,TB =BC102= BC177

T5= B0132TO= 203055

towards the emitter of T7; either handlethis control with cam, or else add aseries resistor between P2 and theemitter of T7 of approximately 2k2.

5().°;.':i'lfpleheviTiZr.doldtvta.Tirthat the stability of the oscillator isusually insufficient. This leads to driftand unwanted frequency modulation.A far better modulator can be built

using a cheap 27 MHz crystal. The firsttransistor in the circuit is the crystal -4-controlled oscillator. This is followedby a two -stage amplifier/pulse shaper,which converts the basic oscillator signalinto short pulses with a rise time ofapproximately 1 nanosecond. Due tothis extremely short rise time, allharmonics of the 27 MHz fundamentalup to 1000 MHz are present in theoutput signal.The series -type mixer stage has a videobandwidth of approximately 7 MHz. Itshould be noted that this simple circuitinverts the modulation signal.The prototype proved capable of givinghigh quality TV throughout the wholeVHF and UHF band.

sfilitor july/suaust 19Th - 241

57 poker58 schmitt trigger

57 Sometimes one wonders whysome people seem to want to do

everything electronically. A case inpoint is the set of poker dice describedhem. Admittedly, the circuit has theadvantage that it makes it almostimpossible to cheat - even whenplaying bluff poker ('liar dice') ...The 'dice' in this circuit are 6 -bit shiftregisters driving six LEDs each. EachLED represents one face of the 'die': 9,10, jack, queen, king or ace.A poker set consists of five dice, so thecircuit shown in figure I must berepeated five times. 'Rolling the dice' ismhieved by starting the oscillator (N13)N 14). Since there are five oscillatorswhich can each be set to a differentfrequency, them is even less chance of a'controlled throw' than in normalpoker.The circuit shown in figure 2 controlsthe number of dice to be rolled as wellas the length of the roll.A game now proceeds as follows:The first player rolls all five dice. To dothis he first sets all five set -reset Rip -flops which control the five oscillators,by briefly touching each of the fivetouch switches ('I' to '5'). He thentouches the 'start' contacts, starting allfive oscillators.When he feels that the dice have rolled

al long enough, the player touches thebtop' contacts. This stops the oscillatorsand resets all five flip-flops. The resultof the throw can be read off on theLEDs.The next player must now try to get ahigher score by rolling one or more ofthe dice again. He selects the dice thathe wants to roll by touching Vie corre-sponding contacts. Then he, too,touches the `start' button and ends the

throw by touching the 'stop' contacts.The game continues in this way untilone of the players either fails to beat hispredecessor's score, or else gets fiveaces.In some variations of the game, six diceare used. It is a simple matter to extendthe game for this: one extra unitaccording to figure I and one extra set-mset flip-flop are added.

58As

triggerlongas

rctchieinurill

is lesstvl

ttatane tothisthelower threshold level, TI is cut off; T2will be saturated through R2.Consequently, the T2 collector voltagewill be low, determined by the T2saturation voltage (approximately100 mV) and the voltage across thepositive feedback diode D2. The lattervoltage can vary over a limited range asa function of the power supply voltageand the R3 resistance. Both theseparameters may be selected betweenwide limits without upsetting theoperation of the circuit.If the level of the incoming signalexceeds the upper threshold level,which is determined by the voltageaccuse D2 and the TI base -to -emitterthreshold, TI will conduct. When TIreaches the saturation point, T2 will beblocked, so that the voltage on itscollector will equal the power supplyvoltage.RI limits the TI base current should theinput signal be high and positive. DImay be fitted to protect the TI base -to -emitter diode should the input signal gotoo far negative.The Schmitt trigger hysteresis isdetermined by the relation between R2and R3. For R3 R2 it is practicallyzero; as the value of R3 is reduced withrespect to R2, the hysteresis willincrease. The maximum hysteresisobtainable in this way is approximately100 mV.


59 on -off -TAP J. Tiernan60 speech shifter61 piano tuner E.H. Leefsina

59This circuit can be used forcontrolling CMOS analog

..witches of the types 4016 or 4066.The special thing about it is that theswitch can be repeatedly turned onand off via only one touch contact.Ike gates N3 and N4 form an SRflipflop which is set and reset by nega-ice pulses on point 13 and point 9,

respectively. By touching the contact,a logic 1 is produced on points I and 6.This results in a reset pulse on point 9if the flipflop is set (Q = 1), or a setpulse on point 13 if the (11010p isreset (Q = 0). Each time the contactis touched, the flipflop changes state.The Q. and /or Q -output of the flipflopcan be used to drive the CMOS-switch(es).

Note:

The contact should be touched verybriefly (shorter than about 1 s) asotherwise the circuit will functionas an astable multivibrator with theoutpu,t of the flipflop changing stateevery second.

60 oMfaionny breacvoerstizernatihhuor peculiarsiats

wishes. INS device is usually one ofthem. It is used to alter the pitch of the

human voice, so that it can be made toand like anything from a chattering

Donald Duck to a croaking frog.The circuit uses one TBA 120. Themultiplier section is used as a ringmodulator, and the limiter section as atuned oscillator. The outnut load shouldnot be less than 5000. P2 is used to setthe effect, while PI sets the input level.Using the TBA 120 the gain is about 20;if the SO 41 Pis used instead the gainwill be about 10.

61 Ptut:g1radjObecitorrs 1 lerren2p erts

with a good musical ear and sufficientpatience and perseverance. However,since such experts are not always easyto find, it is very tempting for do-it-yourself enthousimts to have a try.Regrettably. the result of several hours

BC 5a7P

Top octave frequencies fora crystal fre-quency of 6,384,068 Ha

Note Divisor Frequency

A 3616 879.39 HzG sharp 3831

4059830.6078335

F sharpF

43004558

74301698.43

E 4827 659.220 sharpD

51145418

622.22587.31

C sharpC

57406061

554.36523.27

8 6443 493.87A sharp 8826 466.16

work is usually neither a well -temperedpiano nor a well -tempered tunerThe digital tuning aid described in thisarticle was designed to aid the amateur.It uses a stable crystal -controlledoscillator (N4 -N6) followed by aprogrammable divider to provide allrequired reference frequencies. The

division ratio can be pre -scaled inpowers of 2 (octaves) using S2, and theexact division ratio required for aparticular note is selected with SI. Theresulting accuracy is better than0.006%.The programmable divider is a faitlystraightforward circuit: IC4 is theoctave prescaler. Its output is fed toa three -stage counter (ICTIC3) with amaximum count of 4096. However,when the count selected by SI isreached, the resulting input pulseto NI causes the first half of ICS to becleared. This flip-flop in turn clocks thesecond flip-flop in ICS and simul-taneously resets the counter chain(ICI -IC4).

L

r(wimt e

onz_._. aerie

=Do

+0

4! 46%;I:OOtio;OOOOOOO

0-11-0 0-0GP

01 -40

:54&g(-'4,1)is?--eb121 4\t 01 ,440 ..k) 4,3 53. o---0(t; gkre) ULM/I.:WO I

111.111141111111

I .. 8

e 0 e

DM IIMILI6NIMISMOIL1.4111011111111.11 =MELEIRBIEUIMISIBININNEIMMEI IIIIMILMIIMEEINEIMMILIMMEIEELMILIMEIMINKIIIIIMEN MEM.OEM LIM 1131.1111EWE 1110101 MOMMIEWSLIMIERII ELIMOILMIBINEINIMILVALUMILIBILLIIMINNIENI BIIMMIENIN

144 - Nat« July/mall. Me

62 MMV for ACG63 peak indicator

62 The automatic call generator ormorsm-mat (El 0, February

1976) has only one drawback: IC4 isnot easy to obtain. This IC, the 4098 or4528, comprises two monostable multi -vibrators which are used to determinethe durations of the dots and dashes.Without this IC the entire circuit is use-less. Therefore an alternative has beensought that will not affect the perform-ance of this circuit.The problem here is that the originalMMVs are retriggerable. This excludesthe possibility of using simple MMVsbuilt from two NAND -gates. However,closer study of the original circuit showsthat only the dash-MMV needs to beretriggerable. The dot-MMV can neverreceive a new trigger pulse during thetime that it is producing an output pulse.This means that the dot -MM V can be asimple circuit with two NAND5 (N 1 andN2 in the diagram).The fact that the dash-MMV must beretriggerable means that it needs a lotmore components. The solution chosenis to use a flipflop dividing the inputsignal by two. Each new trigger pulsecauses this flipflop to change state, andthis is used to switch between two RCcircuits.Directly following the switch -over, thediode across the resistor of the inactiveRC circuit ensures that the capacitor israpidly discharged, so that this delaycircuit can be used again, if necessary,without the resulting pulse durationbeing affected.The only restriction of this circuitrelative to the original circuit is that thetransmission speed cannot be variedbecause the pulse duration is determinedby fixed resistors.The terminals shown in this diagramcorrespond to the terminals of the 4098

or 4528 originally used. If the circuit isbuilt on a small p.c. board, these con-nections can be mounted in theappropriate positions so that the panelcan then be used m a plug-in unit in theoriginal IC base.

63Awhpoenak. Isznelalinetieceseldosr ninc

certainmaximum value. It can be quite useful,for instance, with tape recorders.One of the most important require-ments of a peak level indicator is that itshould respond to very short signal.The indication will then have to remainon long enough to be observed. The

3

O

circuit is built using a CMOS invertingSchmitt trigger, type MM74CI4. Thepin connections for this IC are shownin figure 2. The input impedance of thepeak indicator is fairly high; the valuedepends to some extent on the positionof Pt.The unit functions as follows. The inputis half -wave rectified by diode DI. Whenthe voltage across RI exceeds the uppertrigger threshold, a logic I level is pro-duced at the output of the secondSchmitt trigger. The trigger thresholddepends on the supply voltage; with a5 V supply the trigger level is 3.6 V. Thepositive pulse at the output of thesecond trigger is fed back to the input -

capacitor C2. If the input signal wasvery short, the logic 1 level will be

9039-350 ms/Div.

1/211,110 raven

OLO2 1 0108

maintained during the charging time ofC2. This time is determined by thevalues of C2 and resistor RI.

- During this time the output of the thirdtrigger is logic 0, so that LED D4 lightsup as an indication that the level of theinput signal is too high. The currentthrough the LED is internally limitedby the trigger; hence no current limitingresistor is needed.Figure 3 shows a sine wave input signalonnan oscilloscope screen. The lowercurve is the output voltage of thesecond trigger. Pot PI is adjusted to themaximum desired level that the peakindicator should begin to function at.As soon as the pre-set maximum valueit transgressed, the LED lights up forat least the charge tune of C2 (about225 ms). If after this period the signalis still too large, the LED remains onuntil the input signal has droppedbelow the maximum value.Diode D2 prevents attenuation of thepositive edges of the input signal whichWOuld cause the indicator to respondless quickly. Diode D3 ensures that C2can discharge rapidly when the outputof the second trigger becomes logic 0.As a result, the indicator has immedi-ately returned to the initial state whenthe discharge time of C2 has elapsed.Diode Dl has two functions; it preventsthe input of the first trigger from goingnegative (which would destroy the IC)and it ensures that the charge time ofC2 is independent of the position of Pl.The MM74C14 comprises six invertingSchmitt triggers, so that only one IC isneeded to build a stereo peak indicator.

64This squelch circuit blocks theaudio signal path to the ampli-

fier when the amount of noise exceedsa certain level. The unit works in thefollowing way: The noise around80 kHz is filtered, amplified and rec-

64 squelch65 tremolo R Dower66 one shot

elektor july/auport 1976 -745 1

ITdi9 1 RR

g1Jr TUP

linit:died; this signal is used to control T3

and T4. The result is that when notation is tuned in (so that the noiseevel is high) T3 and T4 will be con-

ducting and the audio signal will behotted to ground.

The frequency selective amplifier isormed by a 741 opamp. An LC net-

work, tuned to about 80 kHz, is con-nected in the negative -feedback loop.P2 controls the selectivity of theamplifier (set it so that the circuit doesnot oscillate), while PI controls thesquelch level.Since the 741 cannot handle largesignals at 80 kHz, an amplifier stageTI has been added. This is followedby rectification (DI, D2). Theswitching transistors are BF494's, sincethey provide a better suppression thanlow frequency transistors in this circuit.The signal suppression (squelch on) of aBF 494 was measured at 53 dB, asopposed to 45 dB using a BC 547 B.Connection to the receiver is relativelysimple: the audio output from thedemodulator before the de -emphasiscircuit is taken to the input of theselective amplifier; the audio outputsfrom the stereo decoder am fed throughthe switching stages.

65 tThehc

associatedn ct i o transistorcomponentswithfo

rma pulse generator, which can be setbetween I and 15 Hz by means ofcontrol PI. The generator drives an NPNtransistor with two light emitting diodesin its emitter circuit. One LED gives avisual indication of the tremolo rate, theother modulates the LDR (tight depen-dent resistor) in the optocoupler. Thecircuit is switched on by opening S I.The LDR resistance varies in sympathywith the generator. Since it has beeninserted in series with the signal path,it modulates the signal to create the

k --LA

tremolo effect. This tremolo can beapplied to the sound from electronicallyaided musical instruments, or used withrecording equipment.The LED current is about 10 mA in theconductive state of the transistor. SIoverrules the action of the tremolocircuit by causing the LED's to remainpermanently activated.

66

706 - °hams july/august 1976

67 cascode current source68 simple front-end for VHF FM

67The performance of the normalcurrent source using a common -

emitter connected transistor can beimproved by the addition of just onemore transistor. This is shown in thecircuit diagrams. The influence of theT1 collector voltage on the sourcecurrent is greatly reduced through theaction of T2.The output characteristics of thewscode circuit approach those of theeal constant current source much

more closely than those of the singletransistor circuit.In figure 2, diode D3 sets the Vice oftransistor TI just above the 'knee'.

68 '.,°;rtrrTnnsomriet:rp.tizho:',1c.adlooradvial, a very simple front-end isequate.

The input stage (TI) is operated ingrounded base; band-pass filters areadded at in and output to reduce thelevel of transmitters outside theFM band. It would be possible, ofcourse, to use a much sharper tunedcircuit at the collector of T1. Thiswould improve the selectivity and thesensitivity, but fora simple receivermob as this it is just not worth the extraexpense.The second stage (T2) W a self -oscillating mixer. The output is matchedfora 330 52 load. At a supply voltage of15 V the tuning range will be up to atleast 104 MHz, so the same supply canbe used for the front-end and varicaptuning voltage. It is advisable to use agood IC voltage regulator, such as the723, in the interest of minimum humand noise on the supply to the varicap.For the same reason it is advisable touse a good quality tuning poten-tiometer ('crackle -free'); the value canbe anything between 10 k and 100 k. Ifthe potentiometer proves to 'crackle'more than is acceptable, a 1 p capacitorcan be connected between its slider(Point 'VT' in the circuit) to supplycommon - although this will result in'sluggish' tuning.When used in combination with a simpleIF strip, the sensitivity will be about10 pV for 26 dB S/N. The imagerejection will be about 15 dB, and theoscillator voltage fed back to the aerialinput is about 1 mV.These 'specifications are not at allspectacular, but they are adequate forreceiving local transmitters in mono ona simple whip aerial. The receiver shouldnever be connected to a high gainoutdoor aerial, since the radiation fromthe local oscillator would be sufficientto cause interference on any otherreceivers m the vicinity. The same holdsfor most portables, for that matter.

elektor julyrauoust 1976 - 747

69 sample/hold synthesiser70 single sideband adapter

69 Thistcsynthesisercin portable

lt

communications receivers which useup' conversion. The 100 k ten turn potis used to set the output frequency.However, when this frequency becomesclosely related to a harmonic of the1 MHz frequency standard, the VCOlocks on and gives a stable output signal.This lock occurs every I MHz within thetuning range of the coils.The inductor values as shown in thediagram work over a frequency range ofapproximately 16 to 32 MHz. Bychanging these coils, different frequencyranges can be covered; the maximumusable frequency will be about 70 MHz.If contraction is carried out with duecre and shielding is used around thedifferent circuit blocks, spurious out-puts will be more than 60 dB down.The output frequency accuracy isdependent upon the accuracy of theI MHz crystal in the pulse -box. Thepulse -box generates the standard fre-quency; the output contains strongharmonics extending up to about70 MHz.The sample/hold circuit uses a pairof t300 FET5.

Il

The circuit efficiency can be improvedby using a balanced configuration inthis stage. An E420 dual FET willwork well. This should also greatlyreduce the influence of power supplyfluctuations upon the circuit perform-ance. The job of this circuit is tocompare the harmonics from thepulse -box with the frequency of theTVCO (tuned voltage controlledoscillator) and produce a controlvoltage which is used to tune theTVCO. This feedback mechanismresults in the desired frequency lock.The TVCO is buffered to keep theI MHz pulses from finding their wayinto the output, as this would produceundesirable side bands. If frequencylock is lost an alternating current isproduced at the output of the sample/hold circuit. After amplification, thisis used to light a LED.

70 It is very easy to make a goodSOB adapter using the SO 42-P

IC. Its output enables a symmetricaloscillator to be built with great ease.In order to minimise the influence ofdifferent source impedances, the inputis terminated with a low resistance. It isrecommended to limit the workingfrequencies to below I MHz.The coil in the diagram is a 455 kHzIF transformer.

Equivalents:BF494 - F194, BF.BC547 - BC.), etc.BC.) e BC157, etc.E300 = BF245, B0258,

2E1381910180 e BF185, BF200

748 - sisktof juiv/asgvst 1976

71 digital speed readout for turntables72 driving LEDs from TTL w. y. ..,e73 variable regulated supply7 The basis of the system isa/1 magnetic sensor that picks uppulses from a magnetic tape around theperiphery of the turntable. The tape isa premagnetised plasti-ferrite strip asused on wall planner charts, and soldunder the name Sasco Magna Tape. Thisis available in 5 mm and 10 mm widths,either of which are suitable for thisapplication. The tape is magnetisedalternately N and S along its lengthwith a pole -pitch of approximately6.4 mm.The tape may be stuck on the inside oroutside of the outer rim of the turntable,depending on the type of turnable used.It should be as near the bottom of therim as possible to avoid any pickup offield by the cartridge, although withsuch a small pole -pitch the field isnegligible more than a few mm awayfrom the tape.Taking a numerical example, if the tapeis stuck around the outside of the rimof a 12 inch (300 mm) platter, themwill be approximately 148 pole pitchesaround the periphery. At a turntablespeed of 33.3 r.p.m. this will give riseto a 82 Hz signal. To give a 3 digitreadout the counter chain must count

333 pulses, so the counter gate periodmust be 4.06 sea. This seems slightlylong, even though the speed beingmeasured is nominally constant, butfortunately the necessary gate periodcan be halved by full -wave rectifyingthe input signal, thus doubling the fre-quency.

The circuitPulses from the pickup coil are ampli-fied by ICI and inverted by IC2. Thepositive half -cycles of the ICI and IC2outputs are selected by D3 and D4 andare clamped to CMOS compatible levelby R5, R6 and D7. The pulses amcounted by a 3 -decade counter, the gatesignal being provided bye 555 timer,which can be adjusted to suit the pulserates produced by different lengths ofmagnetic strip on different diameterplatters.The pickup coil can be made by winding500 tums of 0.2 mm woe (36 SWG) ona large nail.On turntables having a built-in strobearound the rim of the platter a retro-reflective optical source/sensor arrange-ment may be used. In this case, on two -

speed 33/45 turntables the 33 strobeshould be used as this has the mostmarkings. The 33 r.p.m. strobe has 182dots around the periphery of theturnable so the existing counter gatingcircuit can be used with no modifi-cation of component values.

72 The accompanying figure showsthe output circuit of a normal

TTL gate.The usual way to drive an LED fromsuch an output is shown dotted: theLED and a series resistor are connected:between the positive supply and theTTL output. The LED is on when theoutput is low'; T3 is'n saturation, sothe series resistor is needed to limit thecurrent.

However, if the LED is connectedbetween the TTL output and supplycommn, as shown, the series resistorcan beoomitted. When the TTL outputtries to go 'high' (TI and T3 areblocked) the internal resistor R3 willlimit the output current to a safe value,Note that this circuit can only be usedwith 'normal' TTL gates. It should notbe tried at flipflop outputs, opencollector gates, etc. Furthermore, notmore than two outputs of one chipshould be loaded in this way.It should also be noted that the outputin question cannot be used to driveother TTL circuits: it will not give atrue 'high' level output.

VlS The riA780 can be used toa construct a very simple power

supply which will deliver I A at anyvoltage from 5 - 30 V. The IC has fourpins: the usual 'I', 'out' and 'common'pins plus a control input.The input voltage must be at least 5 Vhigher than the required output voltage;the maximum input voltage is 40 V and

1elektorjuly/august197.3 -749

74 PIP meter

790

the maximum dissipation is IS W. Itis not easy to blow up this le it hasbuilt-in thermal and current overloadprotection circuits.Using the component values shownin the diagram, the maximum outputvoltage will be approximately 28 V,but if a 25 k potentiometer is used forPI the voltage can be set to over30 V. An alternative solution is toreiruce the value of RI slightly byadding a 68 k resistor in parallel.Capacitors C2 and C3 are included toimprove the stability; they should bemounted as close to the pins of the ICas possible.The required transformer secondaryvoltage Can be calculated to asufficient degree of accuracy by simplymultiplying the desired raw supplyvoltage (i.e. the maximum outputvoltage plus 5 volts) by 0.7.For a negative stabilised supply adifferent IC can be used in the samecircuit: the pA79G. In this case thiscomponent values shown in bracketsshould be used.Either circuit can be mounted on thep.c. board shown; for the negativesupply version the polarity of thebridge rectifier and of Cl should bereversed. Furthermore it should benoted that the pinning of the two ICs isnot identical, so some extra wire linksare needed when mounting the 79G.The board is designed to accomodateeither a fixed resistor (R2) or a presetpotentiometer for PI; it is also possibleto run leads to a potentiometer on thefront panel.If the fixed resistor option is chosen,the value can be calculated as follows.

4700R, = S x (Vont - 5) for the 78G; or

22002(Vout - 2.2) for the 79G.

Fairchild application note.

oeoO z 0 o0

0 0

122.'a0-o

olo

74 Itapisarelatively gtLenxecend thec

counter so that it is possible to makepulse, interval and period measurements.An extension circuit which contains

very few parts, and is easy to build isshown in figure I. The unit can be usedwith no internal connections to thecounter, but if it is connected into theinternal counter circuitry it will bemuch easier to work with.The basic idea of period measurementis the inverse of normal frequencymeasurements: For frequency measure-ments a time base is used to open andclose a gate, for a specific length oftime, allowing the input signal to flowinto the counters during this specificperiod.For period measurements, the inputsignal controls the gate, allowing astandard frequency from the time baseto flow into the counters during theperiod to be measured. This standardfrequency time base, not shown in thecircuit, is connected to input 3.1 kiln is used, however any standardfrequency such as 10 kHz or 100 kHzwill do equally well. The higher thestandard frequency, the higher thereadout resolution will be.The PIP meter makes 3 different typesof period measurements:1. Pulse: the period between thepositive -going edge and the negative -going edge (+ to - of the input waveform).2. Interval: the period between thenegative -going edge and the positive -going edge (- to +).3. Period: the total period of the inputsignal (A- to +).TTL-compatible input signals can beconnected direct to input 2. If the signallevels or rise -times are not TTLcompatible, input I can be used: Rs andZ I limit the signal levels and NI and N2sharpen the edges. Rs should be chosenaccording to signal level; usually 470 5-2will be a good choice. The input signal,selected by SI, passes through gate 615to control the clock input of FF1.The flipflop responds to negative goingtransitions, so that positive -goingtransitions at the input of inverter N5will clock FF1.When S2 is in position I, and assumingthat initially both flipflops have beenreset, the circuit works as follows:When the first positive edge of the inputwaveform occurs, FF I `flips'. Its Qoutput goes high causing one of theinputs to N6 to be high. However, N6remains blocked as long as the inputsignal remains high, since the output ofinverter N3 is then low. As soon as theinput signal goes low, (this does notcause FFI to change state) the outputof N3 becomes high. With two of itsthree inputs high, N6 can pass the1 kHz standard frequency connectedto its third input. Gate N6 remainsopen until the next positive going

150 - Weldor july/august 1918

74 PIP meter (cont.)75 electronic voting system76 handy dark room timer H.F. Blom

*40 :

.. APP1119

NIN3N5

See text FFI

transition of the input waveform, which'flops' FF1 and closes the gate.The readout in the counter will nowcorrespond to the number of pulsesfrom the I kHz time base that werepassed during one interval of the inputwaveform. If the counter reads, say,3000 this would mean the gate wasopen for 3 seconds. So when a standardfrequency of I kHz is used, the readouthas a resolution of 1 msec.When FF1 'flops' at the end of the firstinput period, it 'flips' FF2. The Qoutput of FF2 goes 'low', blocking N5so that no further clock pulses caneach FF1. The system now remains

disabled until both flipflops are reset.In switch position 2, the only differenceto the circuit is the deletion of inverterN3. This means that when, he positive -going signal flips FF1, all inputs to N6are high and the 1 kHz is passed to thecounter. At the end of the `high' part ofthe input waveform ('pulse') N6 isblocked. The readout now correspondsto pulse length.When SI is in position 3, only theQ -output of FF I is used to control thegate, so that it remains open during thewhole period from the first to thesecond positive -going edge of the inputwaveform.The output of gate N6 can be connecteddirect to the counter input. Thecounter's gate must be modified so thatit will stay open during the entire periodon the input waveform. To do this, theoutput of N7 is fed to the control inputof the original flipflop in the countergate circuit. The flipflop output can beused to clear FF1 and FF2 after onecount; a pulse input to N4 will nowinitiate a new measurement.

75 ,"?igusicri:mcuointtporfob,:etyomae:ttcs,the

conferences, in which each participantshould be able to give hN vote (prefer.ably without cross talk).A handful of components is sufficientto enable analog -electronic voting. Eachvoter gets a push button with which the

Ni -ants.NI13N> =7410FF2 = 703

corresponding TUP/TUN thyristor canbe reset (if required). If the thyristor isset, a current flows through it, whichdepends on the values of R, R0, supplyvoltage V and the saturation voltage Vdof the thyristor, which is usually about600 to 700 mV. When a certain criticalnumber of push buttons are depressed,the voltage drop across Rd exceeds thethreshold voltage Vdr, causing the 741comparator to switch and the LED tolight up. The resistance is selectedaccording to the formula:

_ Vdr Rm(V - Vdr - Vd)

where m is the number of votes requiredin favour of a given proposal. Thisnumber depends on the number ofvoters and the type of majority requiredChaff I' or two-thirds majority).The voting procedure is as follows: if avoter is in favour of the proposal hedepresses his button; if he is against theproposal he'd better do nothing!If after the voting (and with the correctvalue for Rd) the LED lights up theproposal has been accepted.A LED can be included in series witheach resistor R, provided the value ofthe latter is suitably adapted. Thisprovides a visual indication of theindividual voting.To make the apparatus more flexible, itis recommended to replace R5 by a

number of switchable potentiometers, orto use a potentiometer with a cali-brated scale, which should be markedwith different numbers of requiredvotes (m).For high values of m, a certain inaccu,racy arises due to the offset voltage ofthe 741. This can be remedied bymeans of the appropriate compensationcircuits. However, it might be consideredthat even democracy is not quite 100%perfect.

When taking a photograph, theamount of light incident on the

negative is influenced by both thediaphragm and the shutter time. Bothof these controls operate in such a waythat one full step of either halves (ordoubles) the amount of light. Thislogarithmic system was chosen becauseit enables easy work without (toomuch) thinking and knob turning.A linear scale is rather unpractical; if,say, the sequence 5-10-15 ... up toand including 90-95-100 was used, thefirst step from 5 to 10 doubles theamount of light, whereas the final step(95-100) is no more than 5% and wouldmake no visible difference in the picture.

76

Furthermore, the total control rangefrom 5 to 100 is only 1:20, whereas19 logarithmic steps provide a controlrange of 1:500,000. Therefore it isobvious that a good design for a darkroom timer will also use a logarithmicscale. To enable a somewhat finerexposure control, it should preferablyalso be possible to use half -stopadjustments. This is achieved by usinga factor 7/5 instead of exactly s/ inthe circuit.This timer does away with all calcu-lation, one diaphragm stop on the

enlarger lens now slop y correspondsto two positions on th timer.Operation is as follows The countingpulses am generated by means of aOJT (T1). T2 acts as a buffer stagebetween this oscillator and the firstdivider (IC I). This divides by 5 or 7to provide the factor of INT= 0.7.This is achieved by sensing the 5 or 7outputs via an AND -gate (D2 ... D4),and feeding the signal to the reset inputof the divider.The network comprised of C4 and R9form a wave shaping circuit which

elaktor juiv/ausust 1978 -151 1

changes the square -wave output to ashort pulse.When the 'stare button is depressed,fliPflop (NI -N2) changes state and theclock pulses are passed on via N3. Thefirst pulse causes flipflop (N6 -N7) tochange state. This energizes the relayvia T4, and the enlarger lamp is turnedon. The clock pulses which are beingpassed by gate N3 are counted by thedividers IC2 and IC3. The rate at whichthe successive outputs of the dividersgo 'high' has the desired logarithmicprogression: each output goes 'high'after twice the time needed for thepreceding output to go 'high'.Therefore no decoding is required; it issufficient to connect the outputsdirect to switch S lb. As soon as theselected output changes from `high'back to `low' flipflop N1 -N2 is reset.This, in tum, resets flipflop N6 -N7(turning off the enlarger lamp), resetsthe counters IC2 and IC3 and blocksthe count gate N3.The time is calibrated by adjusting PI .The circuit draws about 100 mA at5 V, so that a simple IC stabilizer canbe used. It is recommended todecouple the supply near the ICs bymeans of a capacitor of 10 to 100 n.


77 touch activated dimmer78 transistor tester A. Schiplage

79 audioscope J. Schmidt

77 feature ofpossibility

dimmer (IC2, U112B) alternately onand off by means of one touch activatedcontact. An additional feature is thepossibility of connecting one or more

mote control units (ICI, U113).In its basic design the device uses asingle U112B type IC made by AEG)Telefunken. The brightness of the lightis controlled (or preset, as the case maybe) by Pl.Since the voltage across the triac powersthe IC, the triac will be triggered at aphase angle of 22 degrees or mom,leaving sufficient voltage to power theIC. Admittedly, the lights will notreceive full power, but the differencewill be barely noticeable. The R6 + C8combination limits the current to theIC. Note that C8 and CIO must safelystand the full mains voltage; this meansthat they should be rated for 450 V DCwkg. The tap contact is protected bytwo 4M7 resistors.These resistors are an absolute necessityto prevent electrocution!A short switching delay has beenintroduced in the touch switch inputcircuit to reduce the influence of inter-ference. This delay is set by C5, and byC7 and C2 for the remote controlcircuits. The values which are given canbe altered if necessary.Any number of remote control ICs canbe added between pin 14 .d pin 3 ofthe U112B.The device will not work satisfactorilyunless the live and neutral mainsconnections are made as indicated in thediagram.All resistors can be 1/4 watt. Unlessotherwise indicated, capacitors are100 volt types. Inductor L is an inter-ference suppressor coil for triac control.Practically any type of friar will besu liable.With some skill and patience, the basic

circuit around IC2 can be built in to atraditional light switch.

AEGITelefunken Application notes

78 Teotisucnircctlitnw,lehn ausf%dgiunm,

counter will give a direct readout of thegain of the transistor under test (TI).A current determined by R2 and PIis sent through the base -emitter junctionof T I . The gain of this transistor deter-

mines the charge current of Cl.The Schmitt trigger NI is driven by T2,so that when the voltage across Clreaches a certain value NI will switch.When all inputs of NI are at logicits output is at logic '0', and so the out-put of N2 is 'I'. At the same time,capacitor CI is discharged via DI. As aresult, the voltage at the emitter of T2drops rapidly until it reaches the levelwhere NI switches back. The shortcircuit is removed from Cl and the out-put of N2 becomes low.From the above, it appears that with afixed base current, the gain of T1

determines the charge time of Cl andhence the frequency of the outputsignal. The base current can be adjustedwith PI so that a current gain of 100corresponds to an output frequency of1000 Hz. The frequency of the outputwill be proportional to the gain over awide range, therefore g can be maddirectly from the counter.If required, the output frequency for

= 100 can be adjusted to 100 Hz or10 kHz; this is achieved.by changingthe values of R2, Pt, and Cl.The collector.hurrent of T I is usuallyabout 1.8 mA.

7ra Audioscope is a circuit which7 enables low frequency signals tobe seen on the screen of a televisionreceiver. The electronic circuit producesvertical bars on the screen, moving inthe rhythm of the LF signal. The circuithas the advantage that no modificationsneed to be made to the TV set.The heart of the circuit is the astablemultivibrator (T2 and T3), which oscil-lates at a multiple of the line frequency.The following transistor T4 amplifiesthis signal and at the same time im-proves the edges of the square wave.This signal will now contain harmonicsthroughout the VHF band, and eveninto the UHF band,. it can be feddirect to the aerial input of a televisionre.iver. After synchronisation with theline frequency, the screen shows one ormore black bars, according to adjust-ment. Synchronisation and the requirednumber of bars are set with P2. Thispotentiometer is adjusted until thepicture is stationary with PI set atzero (no LF signal).After P2 has been adjusted, PI can beturned up slowly. The bars will movehorizontally in the rhythm of the inputsignal.Further interesting effects can beachieved, such as by mixing a normal

80 dissipation dumper

elektor july/sugust 1976 - 763

Ps. list

01',04,05,06 =10 kR2= 22kR3 = 100k07,08= 39kR9 4k7R10 =15 kR11 8k2P1 =100 k logP2 = 47 k

capacitors:01= 33nC2 = 22nC3.04= 330PC5 = 1 n06,C7 - 1 n/400 V

semiconductors:T1 - TUNT2,T3,T4 =13F494,5F194

picture with the accompanyingis sound which is also made visible. The

output of the circuit is simply connec-it ted to the (VHF) aerial input of the

telrision receiver. The describedelect shows up best on channels 2, 3and 4. The current source of this cir-cuit is provided by a 9 V battery.

80 NsuepaprIclulltaabseilirsieed,

or.transistor. The dissipation in thistransistor increases with increasing,load current and with increasing volt -

e age across it. This means that it isoften necessary to use several tran-sistors in parallel for the series regulator,

d and to incorporate a large heatsink -certainly if the power supply is to havea variable output voltage at high outputcurrents (lab power supply).It is cheaper to dissipate as much aspossible of the excess heat in a resistor,provided some way can be found to do

t this without interfering with the voltagestabilisation.The circuit shows a replacement for theoriginal series transistor, consisting of

two power transistors, a power resistor(12.2) and a few extra components. Thecomplete circuit is equivalent to a singleNPN power transistor. The 'collector',`base' and 'emitter' are marked witharrows.The interesting point about this circuitis that the maximum dissipation in thetwo power transistors is only about one

quarter of the maximum total dissi-pation; the rest (three quarters) isdissipated in the resistor R2.The circuit operates as follows. T I, DI,D2, RI and II3 form a current source,which biases the zener diode 05. Atlow output currents, the voltage at thecollector of T3 will be higher than thevoltage at the collector of T I. This

754 - e1ektor 'illy/august 1976

80 dissipation dumper (cont.)81 sound effects generator J Klasek

means that T2 must be cut off. Thevoltage at the collector of T3 is equalto the input voltage minus thevoltage drop across R2. In this situation,the circuit behaves like a normal singletransistor series regulator (T3) with aseries resistor (R2). The maximumdissipation in T3 occurs when halfthe voltage difference between 41'and 4-2. is dropped across it; the loadcurrent is then:

L2R2

As the current increases further, thevoltage drop across the resistor increases,and the drop across T3 decreases. At acertain output current (twice the valuefound above) the voltage drop across13 would be zero, i.e. the transistorwould be M saturation - and thatwould be the limit of the stabilisationrange. However, the zener voltage ofD5 is chosen in such a way that T2 willstart to conduct just before T3 goes intosaturation. At higher load currents T2forms a variable shunt across R2,maintaining a voltage drop across T3that is just sufficient to keep this tran-sistor out of saturation so that it willstill work as a series regulator.

In this 'second leg' of the seriesregulator operation, the dissipation inT2 will increase progressively. However,the dissipation in T3 is now very low,and the dissipation in T2 will reach thesame maximum found earlier for T3.For this reason, both transistors can bemounted on a common heatsink(provided insulating washers are used);the heatsink must be designed for onequarter of the maximum total dissi-pation.The formula for calculating the value ofR2 is:

11

where is the DC input voltage andImpx is the maximum output current,limited by a fuse or a current limitercircuit in the power supply. The maxi-mum dissipation in R2 equals:

PR' ,max- R2

This type of power resistor is used inthe electrical system of some makesof car.The value of RI is chosen so that afew mdliamps pass through DI and D2.R3 determines the collector current of

TI ; this current must be sufficient tobias D5, even when T2 requires basedrive in the second leg of the regulator.Be-ir in mind that this current throughDI flows into the output, so that atvery low loads it may have an effecton the regulation. In this case, an extraload resistor may be needed across theoutput.

81 vTl.i:isetcytint ogzerfaftee:t:tube

used as a background to, or be mixed..with, other sound effects; it cansimulate, for instance, a helicopter'sswish as well as grasshopper's chirp.The circuit operates as follows.The signal from an astable multi -vibrator (T I, T2, Cl, P4, R4) is fedthrough the lowness network R6, C4,to the modulating input of a tonegenerator (T3, T4). The basic fre-quency control of this generator is P2.Emitter follower T5 buffers the outputsignal of TI. As a result, a frequencymodulated oscillation is available atcontrol P3.Another astable driven tone generatOr

82 7 watt IC audio amplifierelektor july/august 1976 - 755 1

(T6 ... T10) functions in the same way.Transistor T II functions as a noisesource. The signal across R30 is ampli-fied by T12, T13 and can be superim-posed, via R35 and SI, upon the signalgenerated by the first astable.The signals from these three sources arepassed, via controls P3, P6 and P7, tothree tone filters. These filters includethe tone controls P8, P9 and PIO.Finally, the output signals from thetone filters are mixed (R45 ... R48) toproduce the desired noise effect at theoutput of T14.

82 The TBA 810 has been inproduction for several years, and

by now the price has dropped to a veryreasonable level. It has built in thermaland short-circuit protection circuits, soit should have a reasonable lifeexpectancy.

2

Without any additional cooling, the ICcan deliver 1 Watt into a 412 load witha 6 V supply. With a sufficiently largecooling fin and a 16 V supply it candeliver up to 7 W into 4 Cl, the inputsensitivity in this case is 240 mV. If 8 nloudspeakers are used, the maximumoutput power is about half.

IOV

aa dimensions in inin.

The input impedance is practicallydetermined by PI (I M), so it is possibleto connect a crystal cartridge direct tothe input. If this high input impedanceis not required, the value of PI can bereduced.There are two versions of the TBA810:the IS' and the 'AS', with differentlyshaped cooling fins. The additional finshown in the drawing is suitable for theIS' version, but it will need some slightmodifications to fit the `AS' type.The frequency response is ±3 dB from40 Hz to 18 kHz. The voltages shown inthe circuit were measured when the unitwas powered with a 16 V power supply.Note that the pin numbers in the circuitdo not take account of the cooling fin;the IC has a total of 12 pins.

756 - Neater july/august 1976

83 DSSC generator84 battery charger

85 read-out brightness regulator

83It is possible to maintain acarrier suppression better than

40 dB up to 10 MHz using this circuit.The load inpedance can differ from theindicated value provided that the DCresistancedoes not exceed 2k2.Optimum operation will be obtainedif the input circuit is driven by a signalof approximately 100 my peak -to -peak.If it is required to 'clip' the outputsignal the audio signal (input) level mustbe increased. Carrier suppression will bereduced if the crystal oscillator signallevel is too high.

8A It's summer now, but based onw. past CX111Crieilce it is to be ex-

pected that winter is just around thecomer. This circuit will (hopefully)keep motorists happy on cold mornings.It automatically charges the car'saccumulator overnight so in the bleakmorning light the engine should wakeup readily.When designing an overnight chargingunit, care must be taken to avoid thepossibility of overcharging the battery.To avoid this, the output voltage shouldbe limited to a safe value. For 12 voltbatteries the maximum safe voltage isabout 14.1 V and for 6 V batteriesabout 7 V.These voltage levels are adjusted by P2and P3. Overcharging is now prevented

A

6axtn A diode

DI . .054 Dll

Xi the following way. During thehanging period the battery voltage goes

up; when the battery reaches a 80 or90% charge the voltage will havereached the maximum safe voltage asset by P2 or P3.The current will then be switched offvia TI and T2. PI sets the chargingcurrent between 2 and 6 Amps. Theoutput current can be monitored byadding a simple ammeter. Almost anymeter can be used, Rs being selectedto calibrate the meter. Cp prevents themeter needle from vibrating at 100 Hz:the only other smoothing device in thecharger is the car battery!

I

Sometimes it seems impossiblelIghai to set the correct read-outbrightness for LED displays. If thebrightness is set for good daytime con-trast, at night the mad -out becomesblinding. The purpose of this device isto automatically adjust the brightness ofseven -segment LED displays in such away that the visible contrast comparedwith ambient light is maintained over awide range of ambient light levels.This is done by using a transistor whichchops the positive supply voltage to thereadout device. The chopping frequencyis about 1 kHz, the amount of ambientlight determines the duty cycle (on/off).Due to the persistance of the humaneye, the readout will not be seen toflicker. To give an example, if the on/off ratio is assumed to be I:I, theaverage readout current will be abouthalf the non -chopped value. Asa result,the LED brightness will also be abouthalf.The device uses two LDRs (light depen-dent resistors), one to sense the sur-rounding light and the other to sensethe display intensity. Since it is not con-venient to mount an LDR in front ofthe display itself, a 'slave' LED (D1) isunneeded in parallel with the displaltandrt used to sense the readout brightness.A sawtooth signal is generated by ICI aand ICI b at a frequency of about 1 kHz,

242

set by PI . This signal is applied to theinverting input (11) of ICI. A controlvoltage derived from the two lightsensors is applied to input (12). ICI.

elektor july/eugust 1978 - 757

of

Y

D.

86 hafler circuit for quasi-quadrophony87 bird -bell H. G-,

now operates as an analogue-to-pulsewidth converter. The design par-ameters are such that under averagelighting conditions the output on/offratio is about I:1. This duty -cycle variesas a function of the control voltage: anincrease in control voltage causes theon/off average to increase, with theresult that the display brightness (andthe brightness of the 'slave' LED)

Thcontrol voltage is derived from

theof the ambient light and

the brightness of Dl in the followingway. Assume that the system is initiallyimequilibrium, i.e. the display contrastis set correctly. If the ambient lightincreases the resistance of LDR I willdrop, so that the input voltage to the

; error amplifier (IC Id) in es. This increastam increases the control voltage sothat the display will brighten, asdescribed above.The resistance of LDR 2 will also drop(as Dl becomes brighter) counteractingthe drop in resistance value of LDR Ito a large extent. The resulting actualvoltage change at the LDR 1/LDR 2junction will be just sufficient, after

'51

malification by 'Cid, to re -adjust theduty -cycle at the output so thatequilibrium between the values of

a

LDR I and LDR 2 is maintained. InEr, effect, the circuit is a negative -feedback

controlled brightness -follower!The desired degree of contrast can beset with P2; if the range is notsufficient, some experimenting withmasks between DI and LDR 2 may berequired.Power to the display can be supplied viaseries transistor TI , which will safelypass up to I A when mounted on asa...ill heat sink. While on the subject ofthis final stage, it is worth noting thatsome types of BCD to seven -segment.decoder ICs are provided with a strobeinput terminal. This can be used for'dousing' the light emitted by thedisplay for the duration of the `spaces',in which case the series transistor is nolonger needed.

86 Although there are, on themarket,many expensive 'quadro-

phonic synthesiser' boxes that make useof the Haller effect, there have, as yet,been few if any designs for the homecontractor. This is a pity, as Hafler-effect boxes can provide an economicintroduction to experiments inquadrophony.The circuit is extremely simple. Two

I two -pole six -way switches and twobanks of resistors provide 6 steps ofIrtenuation to control the volume of

the rear speakers.A third switch introduces six steps ofresistance between the midpoint of theloudspeakers and ground, thus intro-ducing a degree of front -rear blend.Stepped attenuators are used in pref-erence to continuously variable resistorsas high wattage resistors and switchesare more readily obtainable than high -wattage low resistance pots.

87THi:ereel,,,o: newthe

oldtwist: doorbell?7that rugs like a bird. In the

circuit the doorbell transformer isequivalent to bird feed. in other words,the existing doorbell transformer can beused to power the bird.The song frequency is controlled bythe loudspeaker impedance and C2.The length of time the bird sings iscontrolled by C5. If C5 is made toolarge, the bird will sing too long; on theother/sand, if C5 is made too small,the bird will HUM at 50 Hz. Which ismost undesirable.Note that it is also possible to have thebird sing with a Japanese accent, byusing old parts from a broken transistorradio.

88 linear scale ohmmeter89 symmetrical power amp.

The ohms resraingnaedseogfuTtemost mu]-ti

ineasuring high and low resistancevalues. Resistance ranges below 10011cr above I M often do not exist onheap meters, and anyway the scale is

non-linear. A linear scale resistancemeter that will measure between100 mft and 10 M full-scale can beconstructed using two op -amps, and canhe used in conjunction with a mul-tuneter on any voltage range betweenI and 8 volts, depending on the scalesavailable on the meter.The circuit operates by passing a

constant current through the resistorunder test, so the voltage developedacross it is proportional to the resist-ance. This voltage is amplified by anop -amp and is measured by the mut-timeter. The voltage reading on themeter corresponds directly to theresistance value, i.e. on the kilohmsrange 1 volt corresponds to 1 k, on theohms range 1 volt corresponds to1 O etc.The constant current through theresistor is produced by two differentmethods. On the higher ranges aconstant voltage is applied to a knownresistor connected to the invertinginput of an op -amp. The unknownresistor is connected between outputand inverting input, so the same currentRows through it. The inverting inputis a virtual earth point, so the voltageacross the resistor is equal to the voltageat the op -amp output. Put another way,the op -amp amplifies the known inputvoltage with a gain proportional to thevalue of the unknown resistor.On the lowest resistance range about10 mA is required to flow through theresistor to give even I mV across it,which is the minimum that can easily bemeasured (even so the op -amp offsetmust be nulled). At this sort of current

the output resistance of the op -amp ICIhas an effect, so the aforementionedmethod cannot be used. Instead thecurrent is supplied from the supply railvia a constant current source.FET input op -amps am employed fortheir low input currents. This is necess-ary as on the I mn range the current isonly 100 nA.

8n This class B amplifier has a few7 characteristics that makes it

stand out from the crowd.For one thing, not only is the final stagesymmetrical and complementary, but allof the other stages are as well. Thedifferential input stage requires an extraoutlay of about 40 p, when comparedwith an asymmetrical preamplifier stage.The balanced design abolishes the needof expensive (and unreliable) couplingand decoupling capacitors. The loud-speaker connects straight to the finalstage.Switching phenomena are inaudible.The required frequency compensationis

transient

in such a way thattrient intermodulation distortionis eliminated. This was more fullydescribed on page 452 of the April1976 issue of Elektor (nr. 12).To make sure that the amplifier isstable, the open loop gain must rolloff at 6 dB/oct above the break fre-quency fo. In conventional circuitsthis would mean the driving signalwould have to increase at frequenciesabove fo, also at 6 dB/oct. Under thoseconditions the driving signal at higherfrequencies could rise to such a level

that clipping would occur. This design-,features a network composed of the

output impedance of the precedingamplifier in series with R7 plus C2and R18/1219. The signal across C2is the effective signal driving the ampli-fier; its level decreases as its fre-quency rises above fo.The amplifier is of universal design,both in theory and in practice. Thisleaves plenty of scope for the do-it-yourself constructor. A fewexamples follow.I. Omission of R20 and R21 causesthe final stage to function as a balancedcurrent controlled circuit.2. Open loop gain and DC adjustmentof all stages are practically independentof the balanced power voltages, pro-vided zeners DI and D2 always drawa current of approximately 10 mA. Theprinted circuit board will accommodatethe half watt resistors RI and R2.3. The T9/T 10 transistors can be eithercomplementary darlington pairs ordiscrete complementary power transis-tors. Components R22, R27,R14... R17 and T5 ...TIO must bechosen in accordance with therequirements of the other activeelements in the circuit. Unfortunately,limited space prohibits full discussionof all possibilities.The complete amplifier when poweredby a balanced plus and minus 30 Vsource, will have an output of at least40 W into an 8 ohm load, with aharmonic distortion of approximately0.05% at I kHz. The gain is about 22.Some construction tips:I. The PNP darlington (T9) can bechosen from the following selection:TIP 145 (60 V), TIP 146 (80 V),TIP 147 (100 V) and similar types;the NPN (T10) from TIP 140 (60 V),TIP 141 (80 V), TIP 142 (100 V) andthe like. T9 and T 10 can have a commonsink with a 2°C/W thermal resistance.2. Control P2 adjusts the quiescentcurrent of the final stage to 25...50 mA.The exact adjustment procedure isdescribed on page 530 of the May '76issue of Elektor (nr. 13).3. Zero offset control, PI, balances theamplifier output terminal (the R25/R24junction) in the absence of an inputsignal. This adjustment requires auniversal meter with a low DC millivoltrange. The balancing effectiveness canbe verified by reversing the meterpolarity. It is recommended to verifythe offset balance again when P2 hasbeen re -adjusted.4. Capacitor C3 must be bipolar, not anelectrolytic.5. The 2 z 24 V centre tapped powertransformer and bridge rectifier must becapable of supplying 2 . .. 3 A.

760 - elsktor julyhmpuss 1976

90 200 MHz sample/hold adapter91 hand -effect organ92 voltage regulator for motorbikes B. Paul

an Using this device it is possibleto observe VHF signals up to

200 MHz on oscilloscopes which havebandwidths as low as 100 kHz.The sample & hold switch proper is T6.T7 operates as a heterodyning mixer, itwill produce a zero beat when the inputsignal is at any harmonic multiple of thefrequency being generated by TI.Assume that a 30 MHz signal withsecond and third harmonics must beinspected. When CI is adjusted toproduce a 10 kHz signal on the oscillo-scope, the second harmonic wouldproduce a 20 kHz signal and the thirdharmonic would produce a 30 kHzsignal.12 ... T5 are required to sharpen theedges of the oscillator wave -shape. Thecircuit has a gain figure of about 20which permits it to be used on low levelinput signals. The maximum input levelis approximately 30 mV.This adapter will allow the inspection ofsignals to 200 MHz. However, it doeshave its inconveniences, for one, it isimpossible to determine the frequency

of the input signal. Also, when un tablesignals are inspected, tuning maybecome a problem, since the signal driftis multiplied by the heterodyningprocess. An equivalent for the BC547Bis the BCI47B; an equivalent for theBF494 is the 8E194.

91 v'J controlled oscil-latorsNP this

'organ', one to produce the tone andone to introduce vibrato. The oscillatorfrequency and the vibrato speed arecontrolled by moving the hands to andfro over a pair of LDRs.The vibrato depth is preset with the I Mpreset potentiometer. The 100 k and10 k presets in series with the LDRs areused to set the frequency and vibratorange; the setting will depend on theamount of ambient light.

OwBC54713

93 ro

To avoid hum if the unit is used underfluorescent lamps, the output from theLDRs is first passed through RC low-pass filters.The VCOs are well-known emitter -coupled oscillators; the oscillationfrequency depends on the currentflowing through each lower pair oftransistors.Practically any general purpose smallsignal silicon NPN transistor can be usedinstead of the specified BC547.

92 Only a few components areneeded for this electronic volt-

age regulator for motorbikes. Comparedto conventional regulators it has the'advantages of high reliability, long lifeand accurate voltage control.

12V

elektor july/augun 1976 - 761

93 quiz selector A. Schnieusser

Values brackets for 12 V version

soon as SI (ignition switch) is closed,battery supplies current via DI.

rant flowing through the baseor R2 drives the power-darlington

uit (T2/TI) into saturation, so thate field winding of the dynamo is -

retied.en the motor is mnning, the dynamo

pplies the necessaryMeal installation via D2, blocking

I and taking over from the battery. Asas the output voltage becomes

er than the sum of the voltageross the zener diode D6 and the base-itter voltage of T3 (0.8 V), T3 will

start to conduct. This pulls down thevoltage on the base of T2, reducing therurrent into the field winding of thedynamo. The remit is that the outputvoltage of the dynamo will drop. Whenthe voltage drops below the sealer volt-age, T3 is blocked and the dynamo is'opened up' again. This control systemkeefis the output voltage almost con-stant at 0.8 V more than the zenervoltage.Diodes D4 and D5 are included as aprotection against negative spikes atthese points. The battery is chargedthrough diode D3 and limitingresistor RI.Diodes DI and D2 are power diodes,capable of carrying 25 amps. Thereverse breakdown voltage should notbe a problem: it can never be muchmore than the battery voltage. Asuitable type would be the SiemensE 1110 used in the prototype, if it isavailable ....After the circuit has been tested, it is agood idea to pour polyester resin overa: when this has hardened it is quite agood shock absorber. TI will need amall heatsink; this should not becovered with polyester, but it must bewell insulated from the frame of the

IFprototype was designed and built

fora 6 V electrical system, but if thevalues shown in brackets are used thecircuit can be used in a 12 V system.

as In a 'professional' quiz, someform of electronic detector is

invariably used to determine which can-didate is first to indicate that he thinkshe knows the answer. The circuit shownhem can be used to do the same job.Push-button S5 is under control of thequiz master. As long as he keeps thisbutton depressed, none of the indi-cation circuits will work. As soon as hereleases it, the other circuits becomeoperative. The first candidate to pushhis button (SI -S4) triggers the associatedmonostable (ICI -IC4). The monostabletams on an indicator LED and simul-taneously blocks the other three mono -stables via Ni. The monostable time isset at approximately 8 seconds; afterthis time the indicator lamp goes outand the other players can have ago.The quiz master can reset all mono -stables at any time with his 'override'button 05.

162 - Makin, july/august 1976

94 infra-redphone

94 ZddVnycri?tgiuftgr,tconnected to the main hifi equipmentvia a length of cable. This is unsightly,and people can trip over it.In recent years, a new solution to thisold problem has been proposed: theinfra -red transmission link. Althoughprice, distortion and signal-to-noiseratio are all 10- 20 dB worse than theoriginal cable (yes, the price too ...)there seems to be quite a demand forsuch a system. It is particularly popularas an 'extra' in TV sets - the pricedifference isn't so obvious there.The transmitter (figure 1) consists of

2

an audio preamplifier with pre -emphasis.The output from this is fed to the VCO,an XR 2207. The VCO characteristic ofthis IC is highly linear, and it is verystable. The main reason for choosingthis IC was the high reliability: almostany voltage controlled rnultivibratorcould be used without a noticable effecton the overall distortion.The output from the VCO drives aclass -C amplifier; this, in turn, drivesthe infra -red LEDs.The receiver (figure 4) is a simple coinci-dence detector using a TBA 120 (or aSO 41 P), followed by a single audioamplifier stage. The source follower at

the input is needed as a buffer stagebehind the high impedance input. Thephotodiode doubles as varicap to tunethe LC input circuit.The measuring results were as follows:- Operating frequency: approx.

100 kHz;- Distortion: 3% at maximum output

ON my) into 1 k. The input voltageto the modulator (pin 6 of theXR 2207) is 40 my under theseconditions;

- Frequency response: -6 dB at 15 Hzand 15 kHz;

- Signal-to-noise ratio: depends onambient lighting conditions.

5

sms71

Lt A - 260 room. 02 mm Cu 136 SWG/LIB = CG sums, 0,2 mm Cu 06 MG)

600 turns, 0.2 mmCu IBS SWG)

slaktse luly/xuaust 1979 -7e3

95 symmetrical regulated supply96 stereo indicator

nce the infra -red LEDs are extremelypensive, an alternative solution shouldof interest: replace the LEDs by

rrite rods ... The circuit shown inre 2 can be used instead of the

LEDs in figure I. It is simply connec-d between the collector of the BC 160d supply common. The cod is wound

te fon an 811(20 cm) ferrite rod with adiameter of about 01'(1 cm).In figure 4, the receiver LED andassociated components are replaced bythe LRC circuit shown in figure 5. Thecoil is wound on a similar ferrite rod.The tuning capacitors am adjusted untilthigreatest range is achieved (approxi-mately 10: or 3 m). Be careful not totune in to a harmonic of the transmitter- this will reduce the range drastically.4 suitable power supply for the trans--litter is shown in figure 3.

95 This symmetrical, variable,regulated power supply uses two

,Irage regulator ICs and one additionalTamp, The maximum output current is

A one half of the supply runs into

18

current limiting it automatically reducesthe output voltage of the other half aswell, so that the output voltage remainssymmetrical.Basically, the circuit consists of a posi-tive regulator (jaA78G) and a negativeregulator (µA79G), each with its ownoutput voltage adjustment (P1 and P2,respectively). This means that the circuitcan also be set to give a double asym-metrical output. However, it is easier tounderstand the circuit if it is assumedthat the output voltages am equal.In this case, the voltage at the R2/R3junction is zero as long as the twooutput voltages remain equal andopposite. The output of the 741 willalso be 0 V under these conditions, andall is well.However, if the output from, say, thenegative regulator drops for somemason, the voltage at the R2/R3junction will swing positive. This causesthe 741 output to swing negative, off-setting the common reference point toboth regulators to such an extent thatthe output remains symmetrical.The maximum input voltage isdetermined by the maximum supplyvoltage permissible for the 741, this is36 V ( 2 x 18 V). The minimum outputvoltage is determined by the µ,478G:5 V.

15..15V1A

Note that single supplies using these twoICs are described elsewhere in this issue.

Fairchild application note.

96 aThis .lbeocui

aboutcircuit, p,g:evs.ing

ence of stereo information in a musicsignal, is connected to the loudspeakeroutputs of a stereo amplifier. The inputlevel is adjusted with stereo poten-tiometer PI.Both the R- and the L -signal are half.wave rectified via the base -emitterjunctions of TI and T2, respectively,producing 'pulsating' collector currents.If the L. and R. signals am equal (monoinformation), the collector voltages ofTI and T2 are equal, too, so neither ofthe LEDs will light up. If the signals areunequal (stereo) one or both of theLEDs will light.The input level must be sufficientlyhigh: the peak signal level at the base ofTI or T2, respectively, must be at least0.6 V.

12. 15V


97 moisture indicator98 super -bootstrap RC oscillator

97 fOanLogfmtlyli.mnaneoyfpLiotbolenn:tshi

s

summer is, when must I water mygarden?Perhaps this little gadget will help. Itmakes use of the contact potential atthe junction of two different con-ducting materials. When two rods ofdifferent materials are pushed into thesoil close to each other, they will oper-ate like a battery. The internal resist-ance of this 'battery' depends on theamount of moisture in the soil. If asufficiently sensitive microammeter isconnected between the rods, thedeflection of the pointer will give agood indication of the amount ofmoisture in the soil.

The choice of electrodes is mainlydetermined by the materials available.Some preliminary experiments haveshown that if one electrode is copper,the other electrode can be eitheraluminium or steel. In both cases thecoppers the positive electrode and thealuminium or steel is negative; a metersensitivity of 50µA f.s.d. will always besufficient - if the electrode materials

are well-chosen, even 100... 250µAf.s.d. will do.A possible construction is shown in thefigure. If copper gas pipe is used, thesecond electrode (a piece of aluminiumantenna rod or the blade of a steelscrewdriver) can be mounted inside thepipe. Needless to say, the centreelectrode should be insulated from theouter pipe, furthermore, it is a goodidea to seal the gap between the innerand outer electrode at the top andbottom of the pipe with polyester resinor some other material that will give astrong and watertight seal.The only way to calibrate the indicatoris by adding resistors in parallel with themeter. This is only necessary if thepointer goes off the end of the scale inwet soil. A useful range should beobtained if the pointer just hits the endof the scale when the electrodes areimmersed in a glass of tap water. Areading of less than one quarter of fullscale will then mean that the soil isfairly dry, whereas a reading of morethan three-quarters of full scaleindicates an underground river.After using the meter, always rememberto wipe it clean and dry; never leave itpushed into the ground for long periods,as this will cause severe corrosion of thecopper electrode.

RC98 nCeetrwCoinn,stypheoswofapreacoivnea:

effect, that is to s y, their outputvoltage is slightly arger than the inputvoltage at a specific frequency. Two

2 Wvi

le

lb

Sts

id

0 ..

slektorjuly/august 1976 -765

99 quad symmetrical supply100 preset aerial amplifier

typical networks that demonstrate thiseffect are the lowpass filter of figure laand its highpaz counterpart offigure I b. Figure 2 indicates that bothfilters have a 6 dB per octave roll -offwith a slight hump around the breakfrequency (curves a and b respectively).The 'gain' near the break point isroughly 0.8 dB. Since the slope changesfrom positive to negative this wouldindicate that somewhere near the breakthe phase angle is zero. Further RC net-works can be added (figures Ic and I d)which results in a more pronouncedmagnification. This is about I dB(figure 2c and 2d).With these facts in mind, it should bepossible to design an RC oscillator usingan emitter follower whom gain iajustbelow unity. Its emitter is connected tothe input end of the filter network, andthe base to the output end.This arrangement would seem to satisfythe fundamental condition for sustain-ing oscillation, namely, an in -phase loopgain not less than unity at the criticalfrequency.These theoretical considerations lead tothe bootstrap -like circuit of figure 3,empipying the selective network offigure Id. With the RC parametersindicated the oscillation frequency isapproximately 2 kHz. The variableresistor and the pair of reverse -parallelstrapped diodes stabilise the amplitude.The variable resistor is set to a pointslightly above the threshold of oscil-lation, in which case the oscillation ispractically sinusoidal (distortionipproximately 0.2%l and the amplitudeit its minimum (approximately0.5 V rms).The distortion figure can be consideredremtrkably low, especially when it isremembered that the circuit employsjust one transistor and a relatively crudeamplitude stabilisation. The circuitIllustrates how a simple theoreticalconsideration can be turned into apractical circuit.

99 13uepsipclieysvioltet.seix?(f.slyimnmtheetrific;1

are), it is often handy to have availablea second set of symmetrical supplyvoltages (32 in the figure). These volt-ages are higher than the ±1 voltages, andan supply only a relatively low current.With this circuit it is possible to obtainthese auxiliary voltages from the sametransformer windings as used for themain voltages.The circuit operates as a symmetricalvoltage doubler. Suppose the secondaryof the transformer gives 2 n V volts rms

d that the diode threshold voltages arc

neglected. Then the voltages ±1 areequal to tV x VT. Capacitor C3 ischarged from CI via D2 during one halfcycle; during the next half cycle C4 ischarged from C2 via D3. Consequentlythe points 32 carry a voltage of02 V x relative to supply common.For the practical circuit 1N4000 seriesdiodes can be used for DI, D2 and D3.The values of CI , C2 and C3 am 100 to470µF with a maximum voltage ratingof at least V x s/7 volts.

BFIBOBF200

1 00 This amplifier is intended foruse in the 88-108 MHz VHF -

FM band. It uses fixed input and outputcircuits.Figure 1 shows how the amplifier isconnected for use with a separate powersupply cable; figure 2 shows the connec-tions for supplying power via the coaxcable. For optimum results, a trimmingpotentiometer of 2 k can be connectedin series with the emitter resistor, and atrimmer of 10-40 p between the junc-tion of the 22 p capacitor/0.15 µHchoke coil and common. These controlsare set for maximum S/N ratio. Thegain is 12 to 15 dB at a noise figure of2 dB.


101 SSB exciter with HF compressor

within the duration of one syllable) butit does not give rise to audible distor-tion. Even if it may seem rather peculiarto connect a shorting control systemto the output of a ceramic filter, severalmeasurements failed to show anynoticeable distortion of the band-passfilter characteristic. The high qualityof the SSB signal is only obtained atthe expense of a reduced average out-put power. If maximum power isrequired, the 100 k preset poten-tiometer at the base of the controltransistor can be set so that it is shortedout - this puts the gradual compressorout of action. The signal will now beclipped in the second 703, and it iswell-known that clipped SSB has thehighest efficiency (and the worstquality ... ). Of course, any compro-mise setting between high quality andhigh efficiency can be chosen byadjusting the 100 k trimmer to taste.Since the second 703 can always bedriven into clipping, there is noguarantee that its output signal will besufficiently clean. For this reason asecond filter cascade has been added.The spectrum analyser photo shows theresulting selectivity, and the oscillatorfrequency relative to this. As can beseen from the photo, the result is USB(upper sideband). To convert this to

101 Them arebasic types of

I. Filter -type circuits: these are ex-pensive because of the filler and crystalrequired but have the advantage ofbeing easy to adjust!2. Phase -type and Weaver circuits: theseme cheap but not so easy to alignproperly.An intermediate solution is describedhere, using cheap ceramic filters in ahigh quality SSB exciter that does notcall for awkward alignment procedures.The microphone signal (crystal mikeor high impedance dynamic) is ampli-fied and fed to the S042P balancedmixer. The 100 12 preset poten-tiometer between pins 10 and 12 isset for maximum carrier suppression.The local oscillator uses a BF494 (orBF 194) and a ceramic filter.The output signal from the S042P isDSSC (double sideband suppressedcarrier). This signal is passed through aseries filter chain and amplified in thetwo 703 opamps. The output from thesecond 703 is rectified and drives asecond BF494; as soon as the basedrive to this transistor exceeds about0.5 V the transistor will start to limitthe input signal to the first 703. Thisgives the desired gradual compressorfunction. It is fast (complete control

1

, -boas 0001 lt1 193z.,.../..7-.,

LSB, a mixer can to used: the exciteroutput and the rat a harmonic of thecarrier are fed to this, and the mixeroutput is tuned to (n - 1) times thecarrier frequency. MI components canbe left in circuit even if one wishes toswitch back to USB: the only differ-ence in that case is that the (n - 2)n-harmonic of the carrier my5t be fed tothe mixer instead of the rt1". Thismethod for switching from USB toLSB and back has the advantage that itdoes not introduce zero -beat shift.An experienced amateur should haveno problems using the S042P, the filtersand the 703s for reception as well ad fortransmitting. The IF can be injected atpoint 13 of the S042P (marked 'X'in the diagram).

ION

elekror Italy/august 1976 -161

102 trawler band converter

p

1 02 Theresst ienegq transmitters in the

frequency band from 1.8 to 5.4 MHz:ships, aircraft, amateurs and broadcasttransmitters in the tropics. With the aidof a simple converter this band can bereceived on a normal MW receiver.The converter described here gives acertain amount of gain, so that it canbe used with practically any normalreceiver - even one with relativelypoor sensitivity. PET TI is includedlea source follower circuit to reducethe loading of the LC tuned circuit atthe input. The limiting amplifier in IC Iis Msed for the oscillator circuit, andthe product detector is used as amixer.The output circuit (L4, CIO, Cl I) istuned to approximately 1.7 MHz1175 m). This is the frequency that theMW receiver is tuned to; it should be atone end of the MW scale. If necessary,CIO and C11 can be increased slightly. accomodate receivers with a shorterMW band.The balanced mixer has the advantagethat the local oscillator frequency doesnot appear at the output.The vutput is designed torn 50 17Mad 'Impedance (standard aerial input);if the receiver hasn't got . aerial input,about 4 turns ma right around thereceiver should give adequate coupling. the (ferrite) aerial. The output of themnverter can be connected direct tohoe 'coil'.It is impossible to say in advance whatThe results will be when the converter

connected to a portable in this way,for the following reasons:I . The oscillators in portable radiosmually radiate quite strongly, and thisradmtion is picked up by the converter.This results in `dead' areas and beatfrequencies, which sound, respectively,Eike areas where there is no receptionand areas where there is nothing butnterfering whistles.2. The ferrite aerial of the portablevceiver is still connected to this receiver.This gives the same effect as IF Mier-terence: any broadcast transmitter on ornear the output frequency of theconverter is mixed with the output fromthe converter.Alignment of the converter is fairlyAmple, since the oscillator coil is fixed.C2B is turned until a transmitter at5.4 MHz is received. CIB is nowadjusted for maximum signal strength.After this adjustment, tune in to atransmission at 2 MHz and adjust LI Aand LIB for maximum signal strength;slide one or both of these coils to andfro along the ferrite rod). Repeat thef'gnment procedure until no further

provement can be obtained.

CIA,C1B,C2A,C2B = 2 A 25NITI (2 x 335 plLIA 25 turns OA rnm CuLIB x 3 turns 0,2 mm Cu

If an outdoor aerial is to be used, ashort ferrite rod is sufficient for Lt.However, if it is not certain that thereceiver will be used with an outdooraerial, it is advisable to use a longferrite rod, say 8" long and Yi" diam-eter. The printed circuit board is meantfor a Toko tuning capacitor type2A25MT1. This contains the necessarytrimmers, which can be connected tothe correct points. Only one set oftrimmers are needed ....Prototype measurement results:- conversion gain (50 S2 in and out):

> 20 dB:- frequency range: 1.7 ... 5.4

(or 6) MHz, according to the settingof C2B;

- image frequency rMection: >30 dB,These results cannot be guaranteed ifa standard portable MW receiver isconnected via the suggested 4 turns ofwire round the case!

look out!Have you already thrown away themailing wrapper that this Msar came in?If so, you'd better start digging in thewaste paper..As compulsive odd -comer -fillers, wecouldn't resist the temptation to usethe inside of the wrapper for one morelittle circuit. We intend to do somethinglike this every month from now on.If you haven't got a subscription, you'llbe miming one interning item eachmonth. Sorry, but we haven't got momfor it in the magazine.

766 -Nekton july/august 1976 advertisement

evOtss 00,\to* stastet,

volt D.C. MOS ohm MIPSGP MA CararniulDianros0 .

Soeakers. Car Stereo...us, Ows. In ulamas.

ALL ABOVE ITEMSFlumescent 12vot Camping LIMIo.

o type OC,annel flashing lists units. Bullo.:In 1;14. No too... noades1

:tars AC 0-1000V. DC 0.1030V...100 rnA.C,160 kohm

zoo-'

0.C. current 0/10.1AA% 0/E00s1A. mops.Pasigmnes 0.II30 duns DODO moso.rsi. filoPs.Decibel. -SO to 62,03

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12

POSTAGE: Skalow

Canola ktedELECTRONIC COMPONENT DISTRIBUTORS

P.O. Box 25 Canterbury KentTelephone: Canterbury (0227) 52139

Elektorprinted circuit boards and binders.

Prices as in this issue

Component kits and individual componentsavailable by return of post from stocks at

Canterbury. All prices include VAT and P.+P.741-8/14DI L £0.22 E.1109 £6.087400 £0.15 7038A £3.687401 £0.15 TBA231 .. £1.027405 £0.19 CA1310AE £2.277447 £1.20 CA3080 .. £0.797473 £0.30 CD4011AE 10.237495 £0.69 CD4017AE £1.7674121 £0.32 CD4049AE f0.67MM5314N E4.77 TBA120 . £1.24SFE 6 mA (6 MHz Filter) £0.90I/C EXTRACTOR £0.55 NE555V . £0.65I/C INSERTER £1.40 2N3055 £0.54

Crystals for any requirementPH 100 5 -digit 30 MHz freq. counter £99.50

Above types are partial examples.Write or phone for details.

7 p.m.. 9µm. phone Dover 103001 812332Stock List free on request.

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summei14 circuits - americanradiohistory.com 08, 1976 · circuits. contents elektor july/augues...

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