garage link

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All reasonable precautions are taken to ensure that the advice and data given to readers is reliable. We cannot, however, guarantee it and we cannot accept legal responsibility for it. A number of projects and circuits published in EPE employ voltages that can be lethal. You should not build, test, modify or renovate any item of mains‐powered equipment unless you fully understand the safety aspects involved and you use an RCD adaptor.

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Copyright © 2000 Wimborne Publishing Ltd andMaxfield & Montrose Interactive Inc

EPE Online, April 2000 - www.epemag.com - 251

Have you ever gone to getthe car out of the garage andfound that you left the door openall night? With luck, the car is stillthere and everything inside thegarage untouched. You breath asigh of relief and vow to be morecareful in future.

OPEN DOORBut what if the car had been

stolen? How would you squarethat with the insurance companywhen you declared that the car isleft overnight “in a securegarage”? What about theexpensive power tools, bicycles,and gardening equipment youkeep there?

These would be easilyremoved by any opportunistprowler. You could hardly showthe “forcible entry” needed tomake a claim on your householdpolicy when all he had to do waswalk in and take what he wanted!

WIRELESS LINKThis Garage Link circuit helps

to prevent the garage door (oreither door in the case of adouble garage having twin doors)being left open all night. It worksby establishing a radio linkbetween the garage transmitterand some point inside the house.The indoor receiver then providesan audible warning in the form ofa short bleep every 45 seconds.

The likely operating range isdifficult to predict. In the open air,

is essential to check that thereare suitable positions for the twounits. The garage Transmitterdoes not need to be particularlyclose to the door as long as apiece of twin wire can beconnected to it from a “remote”trigger switch there. It is better,in fact, if it is kept away fromthe door if this is made of metal.

Both units should be sitedclear of large metallic objects.There should be a mains socketnear the house-based Receiverbecause it is operated using aplug-in power supply unit.

The garage section isbattery-operated, using a packof four “AA” size cells inside thecase. This avoids the need for amains supply in the garage withpossible safety implications.The batteries should last for oneyear approximately.

Of course, applications forthis circuit are not confined tomonitoring garage doors andmany readers will have their

Have you left the garage door open all night again? Youneed this versatile, license exempt, coded radio link.

GARAGE LINK by TERRY de VAUX_BALBIRNIE

the prototype operated reliablyat a distance of over 20 meters(66ft). However, the range willbe much less when used inbuildings. The presence ofmetallic objects and evenordinary building materials willreduce the signal.

The prototype units were setup under “fair” conditions. Thegarage was built with singlebrick walls and the house withdouble walls made of brick andbreeze block. The easily-obtainable range wasapproximately 8 to 10 meters(26ft to 33ft). Obviously workingto as short a range aspracticable will give the mostreliable results.

ON SITEWith the likely operating

distance in mind and beforebeginning construction work, it

Self-contained Transmitter.

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own ideas about how to use it.Because the Transmitter is self-contained, it could be used tomonitor other doors, gates,windows, etc. In somesituations, it would be necessaryto use a waterproof enclosurebut this is left up to theconstructor.

LIGHT WORKSince people often wish to

leave the garage door openduring the day, operation is heldoff until the light falls to acertain preset level. Anotherpoint is that the door might havebeen left open in the evening onpurpose – perhaps because amember of the family isexpected home soon.

This is one reason why thewarning is given intermittently.It may then be ignored ifrequired. The other reason isthat it saves battery power.

Designing a circuit whichwould sound a warning if thegarage door was left openwould be easy if there was aclear path for a length of wire to

be laid between a switch at thedoor and a unit inside thehouse. Unfortunately, this is notusually the case.

Even where it would betheoretically possible to runsuch a wire, it is unlikely thatthere would be a neat andsimple way of doing it. It wouldalso involve drilling holesthrough walls or window frames.This is why it was decided touse a different approach andbase this system on a radio link.

FOLLOW THE BANDThe use of the radio

frequency (RF) spectrum iscarefully controlled with specificbands being allocated forvarious purposes. In the UK, thebody that oversees this is theDepartment of Trade andIndustry (DTI). Some frequencybands are reserved for radio

and TV broadcasting, some formilitary, some for radio amateurs,some for the public services andso on.

Some small bands offrequencies are left on a license-exempt basis and may be usedby anyone. However, strictregulations exist for their use. Inparticular, the power radiatedmust be extremely small so thatno appreciable signal may bedetected more than a shortdistance from the transmitter.

One such frequency is418MHz and this is used forcertain local pagers, car securitydevices, “wireless” house alarms

Constructional ProjectAERIAL AERIAL

TRANSMITTER RECEIVER

a

k

L.E.D.

SWITCH

RADIO WAVES

Fig.1. Block schematic of a simple radio link.

+

B16V

(4 x AA)

22�R43M3

R2L.D.R.

R1470k

VR14M7

R566M

6IC17611

2

3

7

4

8

R33M3

R62M7

D11N4148

R710k

C1

OUTGND

THRES.

TRIG.

1

6

2

IC27555

+VRST

DIS.7

Rt10k

4 8

TEST LINKNORMALLYCONNECTSD TO C

(SEE TEXT)

R847k LK1

S6

3

6

14

S5 5

S3

S4

3

4

S2

S1 1

2

C347�

C2220p

R91M5

16

IC3HT12E

OUTPUTDATA

INPUTSADDRESS

VSS

9

17

12 13111087

OSC2

ADDRESSINPUTS

VDDOSC1

15

18

LOOPAERIAL

(ANTENNA)

VC12p TO 5p

R10680�

IC4AM-TX1-418

D21N4001

S7

+a k

*

*

*

*SEE TEXT

C

ED

+

ak

TE+

n.c.n.c. = NORMALLY CLOSED

DOOROPERATED

Fig.3. Circuit diagram for the Transmitter section of the Garage Link. Note that the normally closedcontacts of microswitch S7 are used and that closing the garage door opens them. The “TE”

designation at IC3 pin 14 means Transmit Enable (the bar over it means this signal’sactive state is

ON

OFF0 01 1 1 1

Fig.2. Transmitter code.

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and so on. However, due to so-called TETRA servicesoperating at around thisfrequency and more so in thefuture, the DTI have licensed433MHz for the same purpose.

This frequency is already inwidespread use in mainlandEurope. Note that these areactually narrow bands (that is,ranges) of frequencies but forthe sake of simplicity they arestated as spot values – 418MHzand 433MHz.

NO GUARANTEESAlthough 433MHz

equipment is probably lesslikely to suffer from interferenceproblems especially in thecoming years, there is alwayssome risk of this occurringwhichever frequency is used.Correct operation thereforecannot be guaranteed under allcircumstances.

The prototype unit operatesat 418MHz because thenecessary modules were readilyavailable at the time. However,there is no reason why similar433MHz modules could not beused.

Another choice is whetherto use AM (amplitudemodulation) or FM (frequencymodulation). Frequencymodulation is more immunefrom interference, wouldprovide a greater range and, forcritical applications, wouldprobably be better. However,here the less sophisticated AMsystem was used and itperformed perfectly well.

For those who areinterested, modulation is theway in which radio waves carry

data. With AM it is the signalstrength (amplitude of the waves)emitted by the transmitter whichis varied with the frequencyremaining constant. In thesimplest case, this is performedby switching it on and off. WithFM it is the frequency of thewaves which is shifted slightly

while keeping a constantamplitude.COMMERCIALMODULES

To allow the use of home-made transmitters would leadthe way to potentially botchedequipment causing interference

Constructional Project

COMPONENTS

All 0.25W 5% carbon film, except R2

See also theSHOP TALK Page!

$27Approx. CostGuidance Only

Transmitter(Excl. batts) $37Receiver

(Excl. mains adapter)

TRANSMITTER RECEIVERResistors

R1 470kR2 sub-miniature light dependent resistor (LDR) -- dark resistance approximately 5 megohm (see text)R3, R4 3M3 (2 off)R5 66M (2 x 33M connected in series -- see text)R6 2M7R7 10kR8 47kR9 1M5 (or 1M and 470k in series -- see text)R10 680 ohmsRt 10k (test -- see text)

CapacitorsC1 22u radial electrolytic, 10VC2 220p polystyreneC3 47u radial electrolytic, 10VVC1 miniature preset trimmer 2pF to 5pF

PotentiometerVR1 4M7 miniature preset, horizontal

SemiconductorsD1 1N4148 signal diodeD2 1N4001 1A 50V rectifier diodeIC1 ICL7611 micropower opampIC2 ICM7555IPA CMOS timerIC3 HT12E encoderIC4 AM-TX1-418 transmitter module (see text)

MiscellaneousS1 to S6 DIP switches (one strip of six)S7 lever-arm microswitchB1 6V battery pack (4 x AA)

PCB available from the EPE OnlineStore (code 7000261 -- transmitter) atwww.epemag.com; plastic case size118mm x 98mm x 45mm; 8-pin DILIC socket (2 off); 18-pin DIL IC socket;battery connector (PP3 type); bracketfor microswitch -- see text; connectingwire, solder, etc.

ResistorsR1 100kR2 10k

CapacitorsC1, C2 470n miniature metallized polyester -- 2.5mm pin spacing (2 off)C3, C4 220n miniature metallized polyester -- 2.5mm pin spacing (2 off)C5 100u radial electrolytic, 25V

SemiconductorsD1 1N4001 1A 50V rectifier diodeTR1 ZTX300 npn general-purpose transistorIC1 AM-HRR3-418 receiver moduleIC2 HT12F decoderIC3 78L05 5V 100mA voltage regulator

MiscellaneousS1 to S6 DIP switches (one strip of six)WD1 piezo buzzer -- DC operation 3V to 24V at 10mAFS1 250mA miniature PCB mounting fuse (see text)

Printed circuit board available fromthe EPE Online Store (code 7000262-- receiver) at www.epemag.com;plastic case size 102mm x 76mm x38mm; 9V 300mA (unregulated)mains adapter plus socket to suit;18-pin DIL IC socket; SIL socket forreceiver module, see text; connectingwire, solder, etc.

Both 0.25W 5% carbon film

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with vital services. The actualtransmitter (but not the circuitcontrolling it) must therefore becommercially-built to theprescribed specification. It isthen said to be “DTI MPT1340approved, W.T. licenseexempt”.

Appropriate commercialmodular transmitters areavailable quite cheaply. Thesimplest variety has only twowires, which are used for thepower supply and aerial(antenna), and this is the typeused in this project.

The receiver section isbased on a matching receivermodule. No traditional “radio”skills are therefore neededduring construction and setting-up.

BASIC LINKA simple radio link between

two positions using a transmitterand receiver tuned to the samefrequency is shown in Fig.1.Switching on (“keying”) thetransmitter would send out radiowaves from its aerial. Thesignal would be picked up by anaerial at the receiver and, aftersuitable processing, the LED(light-emitting diode) connectedto its output would operate. Byswitching the transmitter on andoff, the LED would flash insympathy.

However, this type ofsystem would be vulnerable tofalse triggering. Every time thereceiver picked up a signal fromany other source of radio wavesoperating at or about the samefrequency, the LED would comeon.

To avoid this, thetransmitter is keyed accordingto a certain pre-arranged digital


code. Only if this code ismatched at the receiver end willan output be given. Thereceiver may well pick upsignals which carry no code atall or carry the wrong code(from similar equipment) but, ineither case, it will have noeffect.

CODED LINKTo illustrate this, suppose

the code consists of the six-bitword: 1 0 1 1 0 1. In this case a“1” would be given by switchingthe transmitter on for a certaintime and a “0” by switching it offfor the same time. The signalgiven by the transmitter isshown graphically in Fig.2. Thereceiver would then be pre-setto “see” this code and no other.

In the Garage Link, thecode has twelve bits (althoughonly six of them may bechanged by the user). It is,therefore, very unlikely that anysignal, apart from the intendedone, would carry the correctcode. If someone within rangehappened to be operatingsimilar equipment and using thesame code then all that wouldbe necessary would be tochange it.

Unfortunately, any strongsignal at about the workingfrequency and not carrying the

code could swamp the receiverso that it would not “see” theweaker signal from thetransmitter. During that time, nooutput would be given.CIRCUIT DETAILS –TRANSMITTER

The complete circuitdiagram of the Transmittersection of the Garage Link isshown in Fig.3. While thegarage door is open, it allowsthe normally-closed (NC)contacts of microswitch S7 toclose and establish a supply tothe circuit from the 6V batterypack, B1. When the door isclosed, the switch contacts openand no current flows. Thismethod has the advantage thatfor much of the time, the batteryis not being drained.

Diode D2 prevents damageto the circuit if the supply wereto be connected in the wrongsense. If it was, the diode wouldnot conduct and nothing wouldhappen. For the moment, ignoreIC1 and IC2. IC3 is an encoder,

AM-TX1-418

0.35 IN.

+

MARK DENOTESPOSITIVE SIDE

Fig.5. Transmitter module(IC4) pin polarity

identification.

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which keys the transmitteraccording to the pre-arrangedcode. IC4 is the transmittermodule.

The twelve address inputsof IC3 are at pins 1 to 8 andpins 10 to 13. These may be setto logical 1 or 0 to provide thechosen code. Four of theaddresses could also be used tocarry separate data but this isnot done here.

To establish the code someof the address pins areconnected to the 0V line toprovide logical 0 status. Any pinleft unconnected automaticallyassumes logic 1.

CODESETTINGSetting up the code is

carried out using a set of DIP(dual in-line package) switches(S1 to S6) on the PCB (printedcircuit board). With a switch on,

a “0” is set and by switching itoff, a “1”. This gives a simplemeans of changing the code atany time if required.

It seemed unnecessary toallow user selection of all theaddresses, so here only IC3 pin1 to pin 6 may be set using theDIP switches. The otheraddresses (pins 7, 8 and 10 to13) are tied to 0V together withpin 9 which is the 0V input,making them always logic “0”.

When the TE (transmitenable) pin 14 is made low(imagine this is so for themoment), the data present onthe address pins is givenserially at the data output, pin17. This is in the form of four-word groups and continues aslong as pin 14 (TE) is kept low.

If it is low for less than thetime taken for one word, it willstill transmit a four-word group.

When the low state of pin 14 isremoved, pin 17 finishes itscurrent cycle then stops.

The rate at which data istransferred is determined by thefrequency of an on-chiposcillator. This, in turn, is set bythe value of resistor R9connected between pin 15 andpin 16 (Osc1 and Osc2). Thespecified value sets a frequencyof 2kHz approximately.

The data from IC3 pin 17 isused to power the transmittermodule direct. When it is high,the transmitter (IC4) receivescurrent and sends out a signal.When low, it is off. A short loopaerial (antenna) is used toradiate the waves and trimmercapacitor VC1 is used to tune itfor maximum signal strength.

PULSETIMEIt is not necessary for the

R8

D1

A

B

R3

C

DELK1

IC3

S1S2S3S4S5S6

++

+

VR1

IC1IC2

R2

R4

R5C1

R6 Rt

R7

R9

C2

R10

IC4VC1 C3

D2R1

AERIAL(ANTENNA)

6V(VIA S7)

0V

Fig.4. Printed cir-cuit board compo-

nent layout and(approximately) fullsize underside cop-per foil master for

the transmitter.

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Transmitter to be providing datacontinuously – in fact, thiswould run down the batterieswithout good reason. IC3 pin 14(transmit enable) only needs tobe pulsed low for sufficient timeto provide the bleeps at theReceiver.

To provide these pulses,IC3 pin 14 is connected viaresistor R8 and test link LK1, tothe output (pin 3) of the astablebased on timer IC2. Acontinuous string of pulses isthen produced.

The frequency and mark/space ratio (that is, how longeach pulse is high comparedwith low) is determined by thevalues of resistors R6 and R7 inconjunction with capacitor C1.With the values specified, onecycle is produced every 42seconds with each “low” taking0��2s but this is subject to a fairlywide tolerance.

Test resistor Rt isconnected in parallel withresistor R6 to begin with. Thissets a much shorter time period(about half a second) so thebuzzer bleeps rapidly. This willbe useful for testing and setting-up purposes. At the end ofsetting up one of Rt end leads iscut to disconnect it from thecircuit.

SEEING THE LIGHTThe light-sensing aspect of

the circuit is based onoperational amplifier (opamp)IC1. This inhibits the action ofthe encoder when the light levelis high enough. The opamp is ofa type which requires very littlequiescent current (10uAapprox.). It therefore hasnegligible effect on the life ofthe batteries.

The non-inverting input (pin3) of IC1 receives a voltageequal to one-half that of thesupply (nominally 3V) due to thepotential divider action ofresistors R3 and R4. Theinverting input (pin 2) isconnected to a further potential


bc

e

R1100k

C147n

9 8 10 11 12 137

AERIAL(ANTENNA)

OUTDATA

IC1AM-HRR3-418

A.F. VCC

R.F.0VA.F.0V

DATA IN

R.F. VCC

VSS

C247n

14

15

17

16

DATAOUTPUT

IC2HT12F

ADDRESSINPUTS

OSC2

ADDRESSINPUTS

OSC1

18

VDD

S6 6

S5 5

S3

S4

3

4

S2

S1 1

2

C5100�

D11N4001

C3220n

WD13V TO 24V

10mA

TR1ZTX300R2

10k

C4220n

COM.

INOUT IC378L05

FS1250mA

3

1

10

12

15

2 711

14

+

ak

+

TB1/1

TB1/2

9V

0V

++

DATA IN(VT)

5V

Fig.6. Full circuit diagram for the Receiver section of the Garage Link. The designation “VT” atIC2 pin 17 means Valid Transmission.

PIN NO.

1

2

3

4,5,6

7

8,9

10

11

12

13

14

15

FUNCTION

R.F. VCCR.F. GND

ANTENNA

NOT CONNECTED

NOT CONNECTED

R.F. GND

A.F. VCC

A.F. VCC

A.F. GND

TEST POINT

DATA OUTPUT

A.F. VCC

13.7mm

38.1mm

1.27mm

P.C.B. HOLES ON 0.1 INCH PITCH

AM-HRR3-418

1 2 3 7 10 15

2mm

*

*

NOT USED

Fig.7. Receiver module pinlayout and function details.

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divider. Its top arm consists offixed resistor R1 connected inseries with preset potentiometerVR1. The lower arm is light-dependent resistor (LDR) R2.

When the LDR is brightlyilluminated, its resistance will belower than the R1/VR1combination and the voltage atpin 2 will be less than 3V – thatis, less than that at pin 3. Withthe opamp non-inverting inputvoltage exceeding the invertingone, the output at pin 6 will behigh.

This state is transferredthrough diode D1 to IC3 pin 14.Whatever the state of IC2output, IC3 “transmit enable” pinwill be made high so operationis inhibited.

FAILING LIGHTAs the light level falls, the

resistance of the LDR increasesand at some point will exceedthat of the R1/VR1 combination.The voltage at the invertinginput will then exceed 3V – thatis, greater than that at the non-inverting one. The opamp will

switch off and pin 6 will go low.This state is blocked by diodeD1 so it has no effect on theencoder (IC3) which is nowcontrolled by the astable (IC2)alone.

The exact light level atwhich the transition occurs isdetermined by the adjustment ofpreset VR1. Resistor R5, whichis connected between IC1 non-inverting input and the output,introduces a small amount ofpositive feedback and ensures asharp switching action at thecritical light level.

While actually transmittingdata the circuit requires some2mA, but between pulses theprototype used less than 95uA.Due to the short pulse length,the average current is verysmall. Remembering that whenthe garage door is closed thereis no current drain at all, theoverall current needed by theTransmitter is even less.

CONSTRUCTION –TRANSMITTER

Important Note: Thedesign of the aerial is specifiedby UK regulations. There aretwo configurations possible but,of these, a tuned loop is usedhere. The enclosed area mustnot exceed 700 square mm andit must be integral within the unit– it cannot be placed externallyand driven through a feeder.Radio amateurs please note:this transmitter is not typeapproved for use with a quarterwave or helical antenna .

All components for theTransmitter (apart from thebattery pack) are mounted on asingle-sided printed circuitboard (PCB). The topsidecomponent layout and(approximately) full sizeunderside copper foil trackmaster are shown in Fig. 4. Thisboard is available from the EPEOnline Store (code 7000261) atwww.epemag.com

Begin construction bydrilling the two fixing holes andsoldering the IC sockets, DIPswitches S1 to S6, and the twolink wires in position. One ofthese is soldered between


WD1

IC1IC2

IC3C2

C3C4

C5

D1 FS1

TB1

1

2

ak

e

b

c

TR1

C1

R1

S1S2

S3S4

S5

S6

+

+

+

INCOM

OUT

R21

7

15

10

9V

OV

AERIAL(ANTENNA) 262

Fig.8. Printed circuit board component layout and (approximately) full size copper foil track mas-ter for the Receiver.

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points A and B. The other is thetest link (LK1 – C-D-E). Thewire should be soldered asshown between C and D fornormal operation.

Next, the resistors,capacitors and diodes (takingcare with the polarity ofcapacitors C1, C3 and thediodes) can be mounted andsoldered in position. If a 1��5M�resistor is not available for R9,connect one 1M� and one470k� in series.

In the prototype, resistor R5(66M�) consisted of two 33M�units connected in series tomake up the value. You coulduse a single resistor having avalue of between 56M� and100M� if this is available.

Cut the LDR (R2) leads to alength of about 15mm andsolder it in place. Bend theleads through right angles sothat the “window” points to theside (see photograph). Solderthe positive (red) and negative(black) wires of the PP3-typebattery connector to the “+6V”(via switch S7) and “0V” pointsrespectively on the PCB.

LOOP AERIALThe prototype aerial was

made using a piece of light-dutysingle-core insulated wire cut toa length of 80mm. The end1mm or so was stripped and thewire bent into a loop. It was thensoldered into the “aerial”

position on the PCB.

TRANSMITTERMODULE

Before unpacking thetransmitter module, remove any

closely at it while rotating thetop screw). This gives theminimum capacitance of 2pF,which worked well in theprototype.

RECEIVER


The lever-arm microswitchmounted on a small metalbracket.

Garage door closed – mi-croswitch arm compressed,

power off!

Garage door open – mi-croswitch arm released,

power on!static charge that might exist onthe body by touching somethingwhich is “earthed” such as ametal water tap. This is becauseit is a static-sensitive deviceand such charge could damageit.

Cut its leads to a length of15mm and solder it in place onthe PCB, using minimum heatfrom the soldering iron. Takecare over the polarity – thepositive end is identified by ablack mark on the body.

Taking the same anti-staticprecautions, unpack IC2 andIC3. Insert them in their socketstaking care over the orientation.By leaving IC1 position emptyfor the moment, the light-sensing aspect of the circuit willbe disabled and this will simplifytesting.

Adjust trimmer capacitorVC1 so that the plates are notmeshed or only slightly so (look

The complete circuitdiagram of the Receiver sectionof the Garage Link is shown inFig.6. The receiver module IC1requires a 4��5V to 5��5V supply.

The total currentrequirement of the circuit is5mA approximately, whichcould not be maintained by abattery over a long period ofoperation. This is why a mainspower adapter (sometimesreferred to as a batteryeliminator) is called for.

The power adapter suppliesa nominal 9V to the input ofvoltage regulator IC3, via fuseFS1 and diode D1. The outputof IC3 provides the 5V neededby the receiver module, and thisis also used by the rest of thecircuit. Fuse FS1 preventspossible damage in the event ofa short-circuit.

Diode D1 prevents damageif the supply were to be

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connected the wrong way round.This is a possibility where plug-in power supply adapters areused, because the outputpolarity is sometimes uncertain.If the supply was reversed, D1would not conduct and nothingwould happen.

The receiver module is inthe form of a single-in-linepackage – that is, it has onlyone row of pins. Not all the pinsare present and gaps are leftwhere they would have been.The numbering takes intoaccount those which are presentas well as those which are notso, although there are 15numbered “pins”, only 10 ofthem actually exist. The pinlayout and designations areshown in Fig.7.

There are separate pins forthe positive supply feed to theRF (radio frequency) and theAF (audio frequency) sections.These are pin 1 and pins 10, 12and 15 respectively. There arealso separate ground (0V)connections for these (pins 2and 7 for RF and pin 11 for AF).

The same power supply isused for both sections, but theyare decoupled separately usingcapacitors C1 and C2. Theaerial is connected to IC1 pin 3(Data In). The amplified dataappears at output pin 14.

DECODINGThe decoder IC2 is, in many

respects, similar to the encoder(IC3) in the Transmitter unit.The receiving code is set up inthe same way using a set of DIPswitches S1 to S6. The non-settable address pins 7, 8 and10 to 13 are fixed with a logicstate of 0, by tying them to the0Vline. Pin 9 is connected alongwith these because it is the 0Vinput. Data is applied to pin 14(Data In) from the receiver

module output, pin 14.Resistor R1 connected

between pin 15 and pin 16(Osc1 and Osc2) sets thedecoder oscillator frequency.This needs to be approximatelyfifty times higher than that usedin the transmitter section andthe specified resistor sets it at100kHz approximately.

When correct data arrivesat IC2 pin 14, pin 17 (ValidTransmission) goes high.Current then flows, via theresistor R2, into the base (b) oftransistor TR1 and the buzzerWD1 in the collector (c) circuitoperates. Since data is receivedin short bursts as determined bythe Transmitter output, thebuzzer will sound with regularbleeps.

CONSTRUCTION –RECEIVER

All components for theReceiver (apart from the supplyinput socket) are also mountedon a single-sided printed circuitboard (PCB). The topsidecomponent layout and full sizeunderside copper foil trackmaster are shown in Fig.8. Thisboard is available from the EPEOnline Store (code 7000262) atwww.epemag.com

Begin construction bydrilling the two fixing holes thensolder the terminal block TB1,link wire, IC sockets, and DIPswitches S1 to S6 in position.Use pieces of single in-line(SIL) socket for receiver moduleIC1 – do not solder this ICdirectly onto the board. Youcould make these by cutting upa dual-in-line socket.

Solder all resistors andcapacitors in position takingcare over the orientation ofelectrolytic capacitor C5. Addfuse FS1. In the prototype this

was the PCB-mounting type;this is convenient because it willprobably never blow.

Follow with diode D1,transistor TR1, regulator IC3and buzzer WD1, again, takingcare over their orientation. Notethat the flat face of the regulatoris downwards and that of thetransistor to the right. Someregulators have a different pinarrangement so check this pointif necessary.PRELIMINARYSET-UP

Attach a PP3-type batteryconnector to terminal blockTB1, taking care over thepolarity. A 9V battery will beused for testing but it will bereplaced with the plug-in, mainsadapter, power supply at theend.

Solder a piece of light-dutystranded wire 18cm long to the“aerial” point. This correspondsto one-quarter of a wavelengthapproximately. Note that, unlikethe Transmitter aerial, this couldbe placed outside the case. Youcould even use a shorttelescopic aerial, if you wish.

Observing the anti-staticprecautions again, insert IC2and the receiver module, IC1,into their sockets. IC1 will onlyfit one way – that is, with thecomponents side facing IC2.Take great care when insertingit. If too much force is used, thepins will bend and possiblydamage it. Note also that thepins are fairly long and will notpush fully “home”.

PRELIMINARYTESTS

Decide on a code for thetwo units. It does not matterwhat it is, but the DIP switches(S1 to S6) in each unit must beset in exactly the same way.


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Connect a PP3 battery tothe Receiver and pull out theaerial into a straight line. Placethe two units approximately 2m(6ft approx.) apart. Insert thecells into the Transmitter batteryholder and connect it up. Notethat the maximum voltage to beused with the Transmitter is 6V– more than that will damage it.

With luck, the buzzer willbegin sounding with rapidbleeps! Remember, resistor Rtis in the circuit and the timeperiod has been reduced fortesting.

If it fails to work, change thealignment of the transmitteraerial. Try moving the unitscloser together to see if thatimproves matters. Experimentwith the adjustment of capacitorVC1. If it still doesn’t work,check that the code switches ineach unit are definitely set inthe same way. A faulty solderedjoint at a DIP switch in eitherunit could set the wrong codeand prevent the system fromworking.

AT FAULTIf there is still a fault, it is

more likely to be in theTransmitter, because this hastwo distinct sections. These arethe encoder and transmitter onone hand and the light sensor(but this part has beentemporarily disabled) andastable on the other. If there isa persistent fault, you could tryisolating it to one of thesesections.

First, remove the ICsobserving the anti-staticprecautions mentioned earlier.Now, change the connection ofthe “test link” LK1 on the PCBso that C connects to E. Thistakes IC3 pin 14 to 0V andallows the Transmitter to senddata continuously. If it now

works, check the earlier stages.If nothing happens, it is morelikely that the fault lies in theReceiver. Assuming the twounits are operating over a shortrange, try increasing it. Movethem to the point where thebuzzer operates intermittently orin a “chirping” way due toperiods which lack proper data.Adjust VC1 using a plastictrimming tool (a metalscrewdriver blade will affectoperation) to tune theTransmitter aerial for the bestsignal. Increase the range to 10meters and make furtheradjustments as necessary.Experiment with the orientationof the aerials.LIGHT WORK

To check the light-sensingstage (IC1), first disconnect theTransmitter battery. Observingthe anti-static precautions,insert IC1 taking care over theorientation. Adjust preset VR1fully anti-clockwise (this meansit does not have to be very darkto operate and simplifiestesting).

Re-connect the battery andtest the system. With sufficientlight reaching the LDR (R2)sensitive surface, the buzzershould stop sounding. When theLDR is covered, it should beginagain. If this does not work, trycovering the LDR morecarefully – perhaps sufficientlight is still reaching it. Coverthe LDR with black opaque PVCtape so that the transmitterworks continuously again.

ON TRIALWith the aid of an assistant,

hold the two units in various trialpositions to find the best ones.As with any very low-powerradio equipment, there will begood and bad spots. Check withthe car in the garage. The

orientation of the Transmitterloop is important. Set this andthe Receiver aerial for besteffect.

Do not use metal boxes tohouse the units – only plasticones. Metal boxes would screenthe circuits and prevent radiowaves passing in or out!

FINAL ASSEMBLY –TRANSMITTER

Place the Transmitter PCBand battery holder on thebottom of the box in theircorrect positions. Whendeciding on the orientation ofthe PCB take account of thedirection from which the LDRwill receive light. Ideally, itshould end up pointing towardsthe garage door so that whenthis is open, it will receive“outside” light. Alternatively, tryto direct it towards a window.

Mark through the fixingholes, remove everything againand drill them through. Holdingthe PCB in place, a smalldistance above the base of thebox, mark the LDR position.Measure the position of presetVR1 and mark the lid directlyabove it. Remove the PCB anddrill these holes.

The one for the LDR shouldbe about the same diameter asits window. The hole for VR1should be large enough to allowit to be adjusted from theoutside using a smallscrewdriver or trimming tool.Drill a hole near the right-handside of the PCB for the wiresleading through from the garagedoor switch.

Before attaching the PCB,drill two holes in the back of thebox clear of all internalcomponents. These will be usedfor attaching the unit to the walllater. Secure the PCB using


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plastic washers on the boltshanks. The LDR leads shouldbe bent so that the window liesa few millimeters behind thehole drilled for it. Secure thebattery holder using adhesivefixing pads or a small bracket.

– RECEIVERDisconnect the battery and

remove the connector from theterminal block TB1. Place theReceiver PCB on the bottom ofits box and mark through thefixing holes. Take it out and drillthese through. Measure theposition of the buzzer and drill ahole in the lid larger than that inthe buzzer itself for the sound topass through.

Check the type of connectorfitted to the mains adapterpower supply unit. Drill a hole inthe side for a socket of thesame type and attach it. Drilltwo holes in the back of the box(clear of the PCB) to attach it tothe wall later. Secure the PCBusing plastic washers on thebolt shanks.

Solder two pieces ofconnecting wire to the powersocket. Take care that thecorrect tags are used. Checkthe polarity of the power supplyunit output and connect thewires to terminal block TB1observing the correct polarity.

If you are unsure about this,do not worry. If the receiverdoes not work at the end it willbe simply a matter of reversing

these wires. If you are using apower supply unit with anadjustable output, you may findthat the “6V” setting actuallyprovides over 9V when usedunder the low-load conditions ofthis circuit.

Attach the Transmitter andReceiver units in their finalpositions.

SWITCHED ONDecide on the switching

arrangement for switch (S7) atthe garage door. In theprototype, a lever-armmicroswitch was used. This wasattached to a small aluminumbracket (see photograph) whichwas, in turn, secured to thedoorframe. The microswitch hada large paddle-style lever, whichallowed for some tolerance infitting, although any type couldprobably be used.

The switch should beoperated by some part of thedoor mechanism, which movesrelatively slowly when the dooris operated. This will avoidheavy jarring as the doorcloses.

Hold the switch assembly inposition and check that thelever will be pressed to the pointwhere the switch clicks as thedoor reaches its closed position.Check carefully that this doesnot interfere with normaloperation of the door.

Attach the switch and makeany adjustments as necessary.

Make sure the switch lever stillhas some movement left whenthe door is closed so that it isnot placed under any unduestrain.

CONNECTING UPIdentify the switch contacts

that “break” (open) when thedoor is closed (that is, thenormally-closed contacts).There is usually a diagram ofthis on the side of themicroswitch. Using spadereceptacle connectors, attach ashort piece of light-duty twinstranded wire to the appropriatetags. This should be sufficient toreach a small junction box (theburglar alarm type is ideal)attached near the doorframe.

Referring to Fig.9, completethe external wiring. Any light-duty twin stranded wire will besuitable. You will need to placea 2-way piece of screw terminalblock TB2 inside the transmittercase.

Cut the red batteryconnector wire and connect itsfree ends to the terminal block.Connect the switch wires to theblock, via the junction box, asshown. If two switches are usedfor two doors, connect them inparallel.

Connect the power supplyunit to the Receiver and test thewhole system. If it fails to work,reverse the polarity of the powersupply wires.

The Receiver aerial wirecould be either routed aroundthe inside of the case (makesure the end is insulated so thatit cannot make metallic contactwith any internal components.Alternatively, it can be allowedto hang outside through a smallhole.

LIGHTING-UP TIME


TRANSMITTERP.C.B.

6V0V

6VBATTERY

PACK

TB2

TERMINALBLOCK JUNCTION

BOX

LIGHT-DUTYTWIN WIRE

DOOR-OPERATEDMICROSWITCH

++

Fig.9. Interwiring between the Transmitter and remote door-operated microswitch.

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It is now time to remove thetape from the LDR in theTransmitter so that the light-sensing part operates. Wait untilit is dark enough and, with thelid in place and the garage dooropen, adjust preset VR1 so thatthe system just responds at thispoint.

You will find that the lightlevel at which the unit starts tooperate (going dark) is not quitethe same as that at which itstops operating (going light).This is due to the effect offeedback resistor R5 in theTransmitter. If the effect is toopronounced, increase its valueor remove it.

You may find that the LDR“sees” the garage light whenthis is switched on. Of course,this would hold the buzzer off.This would probably be anadvantage because if someone

was working in the garage atnight with the door open, thebuzzer would not sound.

If you want it to operateunder these circumstances,shield the LDR so that thegarage light does not reach it.Bending its leads so that it liesfurther behind the hole anddirecting the unit more carefullyat the source of “outside” lightwill also help.

Remove the Transmitter lidand cut through one of the leadsof test resistor, Rt. Move the cutends apart to prevent them fromtouching. The buzzer shouldnow give a short bleep every 45seconds approximately.ON APPROVAL

Before putting the systeminto permanent service, it isimportant to display a mark onthe transmitter stating that it

conforms to DTI SpecificationMPT1340. This must state thewording “MPT1340 W.T.License Exempt”. The size mustnot be less than 10mm x 15mmand the figure height must notbe less than 2mm.


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