“Electronic Start” Learning Package Item-No. 19 22 30

Version 07/09

O P E R A T I N G I N S T R U C T I O N S

These Operating Instructions accompany this product. They contain important information on setting upand using it. You should refer to these instructions, even if you are buying this product for someoneelse.

Please retain these Operating Instructions for future use!

A list of the contents can be found in the Table of contents, with the corresponding page number, onpage 2.

Table of contents1 Getting started ....................................................................................................................3

2 First tries with LEDs ............................................................................................................92.1 LED with series resistor ..................................................................................................9

2.2 Current direction ............................................................................................................11

2.3 Amperages ....................................................................................................................12

2.4 Signal lamp with pushbutton switch ..............................................................................13

3 LED circuit technology......................................................................................................153.1 Diode threshold voltage ................................................................................................15

3.2 Series connection ..........................................................................................................17

3.3 Little energy – a lot of light ............................................................................................19

3.4 Parallel connection ........................................................................................................20

3.5 Plays of colour ..............................................................................................................22

3.6 Flashlight ......................................................................................................................23

4 Test instruments with LEDs ............................................................................................244.1 Cable tester ..................................................................................................................24

4.2 Water detector ..............................................................................................................25

4.3 Alarm device ..................................................................................................................26

4.4 Polarity tester ................................................................................................................27

4.5 Battery tester ................................................................................................................28

4.6 LED as temperature sensor ..........................................................................................29

5 Transistor circuits ............................................................................................................315.1 Amplification ..................................................................................................................31

5.2 Follow-up control ..........................................................................................................32

5.3 Touch sensor ................................................................................................................33

5.4 LED as light sensor........................................................................................................34

5.5 Constant brightness ......................................................................................................35

5.6 Temperature sensor ......................................................................................................36

5.7 On and off ......................................................................................................................37

5.8 LED blinker ....................................................................................................................39

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1 Getting startedThis learning package is an easy introduction to electronics. The following is a presentation ofthe components.

Patch boardAll experiments are conducted on a laboratory experimenting board. The patch board with a total of 270 contacts in a 2.54-mm grid ensures safe connections of theintegrated circuits (ICs) and the individual components.

The patch board has 230 contacts in the middle section which are connected conductively by vertical lines in groups of five. In addition, there are 40 contacts for the power supply on theupper and lower edges consisting of two horizontal contact spring strips with 20 contacts each.The patch board thus has two independent supply rails. Figure 1.2 shows all internal connections.You can see the short contact rows in the middle section and the long supply rails on the edges.

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Figure 1.1: The experimenting board

Figure 1.2: The internal contact rows

Inserting components requires a good amount of force. The connecting wires might bend easily.Therefore, make sure to insert the connecting wires exactly from the top. Use a pair of tweezersor a small pair of pliers. Hold the connecting wires as closely as possible to the patch board andpress them down in a vertical movement. Proceed in the same way to insert sensitive connectingwires such as the tinned ends of battery clips.

For your experiments, you require different lengths of wire which must be cut off from the providedjumper wire. To strip the wire ends, it is a proven method to first cut into the insulation around thewire using a sharp knife.

BatteryThe following overview shows the components as they really look and the symbols used in cir-cuit diagrams. The battery can be replaced by e.g. a power supply.

You should not use alkali batteries or rechargeable batteries. Only use zinc-carbon batteries.Although alkali batteries have a longer lifetime, they might – just like rechargeable batteries –supply high currents above 5 A e.g. in case of a short circuit, which can cause the thin wires orthe battery itself to heat up considerably. The current supplied by a zinc-carbon battery duringa short circuit is usually below 1 A. This can destroy sensitive components but there is no danger of fire.

The provided battery clip has a connecting cable with a flexible wire. The cable ends are stripped andtinned. Therefore, they are rigid enough to be inserted into the contacts of the patch board. However,they can lose shape if plugged in frequently. For this reason, we recommend leaving the batterywires connected and just removing the clip from the battery.

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Figure 1.3: Battery and battery diagram symbol

A single zinc-carbon or alkali cell has a voltage of 1.5 V. Several cells are connected in seriesin one battery. Accordingly, the symbols show the number of cells in a battery. For higher vol-tages, it is common practice to indicate the middle cells by a dotted line.

LEDThe learning package includes two red LEDs, one green LED and one yellow LED. The polarityof all LEDs must always be observed. The negative connection is called cathode. It is at the shor-ter connecting wire. The positive connection is called anode. The cup-shaped holder that holdsthe LED crystal at the cathode is visible inside the LED. The anode connection is connected withan extremely thin wire to a contact at the top of the crystal. Caution! Unlike light bulbs, LEDsmust never be directly connected to a battery. A series resistor is always required.

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Figure 1.4: Diagram symbols for different batteries

Figure 1.5: LED

- Cathode

+ Anode

ResistorsThe resistors included in the learning package are carbon film resistors with tolerances of ±5 %.The resistor material is applied on a ceramic rod and covered with a protective layer. Rings ofdifferent colours indicate the resistor type. The resistance value and the accuracy class areindicated.

Resistors with a tolerance of ±5 % are in the E24 list. Every decade includes 24 values withabout the same distance to the neighbouring values.

Table 1.1: Resistance values according to the E24 standard list

1,0 1,2 1,3 1,4 1,5 1,6

1,8 2,0 2,2 2,4 2,7 3,0

3,3 3,6 3,9 4,3 4,7 5,1

5,6 6,2 6,8 7,5 8,2 9,1

Begin reading the colour code from the ring closest to the edge of the resistor. The first tworings represent digits whereas the third ring is a multiplier for the resistance value in ohms. Thefourth ring represents the tolerance.

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Figure 1.6: Resistor

Table 1.2: Resistance colour codes

Colour Ring 1 Ring 2 Ring 3 Ring 41. digit 2. digit Multiplier Tolerance

Black 0 1

Brown 1 1 10 1 %

Red 2 2 100 2 %

Orange 3 3 1.000

Yellow 4 4 10.000

Green 5 5 100.000

Blue 6 6 1.000.000

Purple 7 7 10.000.000

Grey 8 8

White 9 9

Gold 0,1 5 %

Silver 0,01 10 %

A resistor with the ring sequence yellow, purple, brown and gold has 470 ohms and a toleranceof 5 %. The learning package includes two resistors of each of the following values:

100 Ω Brown, black, brown 220 Ω Red, red, brown 330 Ω Orange, orange, brown 470 Ω Yellow, purple, brown 1 kΩ Brown, black, red 10 kΩ Brown, black, orange 100 kΩ Brown, black, yellow

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TransistorsTransistors are components used for the amplification of small currents. The used BC547 resis-tors are silicon NPN transistors.

The connections on the transistor are called emitter (E), basis (B) and collector (C). The basisconnection for both transistors is in the middle. Looking at the label with the connection pointingdownwards, the emitter is on the right.

CapacitorThe capacitor is another important electronic component. A capacitor consists of two metal sur-faces and an insulating layer. When electric voltage is applied, an electric field in which energy isstored builds up between the two capacitor plates. A capacitor with a big plate surface and asmall distance between the plates has a high capacity, i.e. it stores a lot of energy when voltageis applied. The capacity of a capacitor is measured in Farad (F).

Electrolytic capacitors reach high capacities. The insulation consists of a very thin layer of alu-minium oxide. The electrolytic capacitor contains a fluid electrolyte and aluminium foil with a bigsurface. Voltage must only be applied in one direction. In the wrong direction, leakage currentwill gradually reduce the insulating layer and finally destroy the component. The negative terminal is indicated by a white stripe and the connecting wire is shorter.

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Figure 1.7: Transistors

Figure 1.8: Electrolyte capacitor

2 First tries with LEDsYou can take a battery and a small light bulb and try different things until the bulb lights up. Youshould not try the same with an LED as it will be destroyed quickly if connected directly to a battery.You have to proceed a bit more systematically: Observe the correct voltage, the right polarity, anduse a suitable series resistor. It is not really difficult. Try out the circuits described below to becomefamiliar with working with LEDs.

2.1 LED with series resistor

Set up your first circuit with a battery, an LED and a series resistor. Use a red LED and a 9Vbattery. Take the hightest resistance value (1 kΩ = 1000 Ω, colours: brown, black, red) fromthe learning package to be on the safe side in terms of LED current. Figure 2.1 shows the cir-cuit as a circuit diagram.

Use the patch board to set up the circuit. Connect the upper supply rail with the positive terminalof the battery, i.e. with the red connector on the battery clip. Connect he lower supply rail accor-dingly to the black clip connector, i.e. to the negative terminal of the battery. The actual circuit willresemble the circuit diagram so that troubleshooting should not pose any problems. Bend theconnecting wires of the LEDs and the resistors so that they fit into the contacts. Some connectingwires were shortened in this test setup for better illustration. You should, however, leave thewires uncut to ensure that the components can be used for all other experiments as well.

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Figure 2.1: Circuit diagram of LED with series resistor

The first try will probably be successful. The LED lights up brightly. If not, look for the mistake.Any interruption of the circuit prevents current flow. Therefore, check all lines and the positionof the components on the patch board. As another possible problem, the LED might have beeninserted the wrong way, or the battery is empty. You will notice, however, that even very oldbatteries still provide enough power for the LED to light up weakly.

Try a different layout. Swap the LED and the resistor. The current will then flow through the LEDbefore flowing through the resistor. The effect is the same as in the first case, however. Theonly important thing is that all three components are connected in a closed circuit.

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Figure 2.2: Setup on the patch board

Figure 2.3: Swapped components

2.2 Current direction

Turn the LED so that the anode is connected to the negative terminal of the battery. There is nolight! This means that current can only flow through the LED in one direction. The forwarddirection is the current direction from anode to cathode, with the anode connected to the positi-ve terminal of the battery and the cathode to the negative terminal. In reverse direction, the LEDis blocked. A diode is like an electric valve. It only lights up when current is let through. Figure2.5 shows the LED with reverse direction. It cannot light up.

The arrows in the LED circuit diagram in figure 2.6 indicate the direction of current. The directionof current as well as the designation plus and minus was defined arbitrarily in history. Thismeans, current always flows from the positive terminal of the battery through the load to thenegative terminal of the battery. Today, it is common knowledge that negatively charged elect-rons inside the wires move exactly opposite to the direction indicated by the arrows in figure 2.6.There are, however, positive charge carriers as well, as e.g. in fluids, that move with the direc-tion of the current. Even inside the LED itself there are negative and positive charge carriers.

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Figure 2.4: LED and resistor swapped

Figure 2.5: LED in reverse direction

2.3 Amperages

Instead of the 1-kΩ resistor, insert a smaller resistor of 470 Ω (yellow, purple, brown). The LEDlights up noticeably brighter. This indicates higher current. The rule is: The higher the resis-tance, the lower the current. More accurate calculations are stated below.

Test the brightness of all LEDs with resistors of 1 kΩ (brown, black, red), 470 Ω (yellow, purple,brown) and 330 Ω (orange, orange, brown) each. However, do not use a resistance lower than330 Ω, as this might result in a current too high for the circuit with a 9-V battery and consequentlyharm the LED.

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Figure 2.6: Direction of current

Figure 2.7: More brightness with a lower series resistance

The used LEDs are approved for continuous current of 20 mA. The table below shows that theactual current of the used LED depends on the series resistor. In some cases, the approved cur-rent is slightly exceeded. If this happens for a short time, it does not constitute a problem. If over-loaded for a long period of time, the LEDs wear out faster and lose their luminosity.

Table 2.1: LED current at a battery voltage of 9 V

Resistance red LED yellow LED green LED

330 Ω 21.4 mA 21.1 mA 20.8 mA

470 Ω 15.1 mA 14.9 mA 14.7 mA

1.000 Ω 7.2 mA 7.1 mA 7.0 mA

2.4 Signal lamp with pushbutton switch

Make a simple pushbutton switch using stripped jumper wire as illustrated in figure 2.9. Whenopened, the switch represents an interruption of the circuit. When the switch is pressed, the twocontacts are connected and the circuit is closed. The elasticity of the wire disconnects the con-tact when you let go of the switch. Consequently, the LED within the circuit only lights up aslong as the switch is pressed.

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Figure 2.8: 470 Ω series resistance

This setup can be used as a signal lamp for various purposes. In principle, this switch can alsobe used to transmit complex messages by Morse code. Admittedly, sending messages in Morsecode is a little old-fashioned and not as comfortable as an e-mail or a phone call. However, usingMorse code with an LED can be a delightful way of communication. With some practice, infor-mation can be exchanged over a distance of up to 100 m with almost nobody else being able tolisten in.

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Figure 2.10: Circuit with switch

Figure 2.9: Setup of a switch using wire

3 LED circuit technologyIt is easy to set up a given circuit using the recommended components. Whoever really wantsto master circuit technology, however, should become familiar with theoretical aspects in orderto be able to calculate required resistances in a circuit. The chapter provides the necessaryaspects and the corresponding experiments. Combine theory with practice. Calculate and testyour own circuits!

3.1 Diode threshold voltage

In comparison with a light bulb, an LED seems to behave strangely. Not only does the currentflow in one direction – as opposed to a light bulb which can be connected with either polarity –,the supply voltage in forward direction is of great importance as well. A small light bulb with 6 V,100 mA shows great tolerance towards the actual supply voltage. Already approx. 1 V isenough to cause a weak, dark red glow. At the rated voltage, a bright yellowish-white light isproduced. If you try a higher voltage for a very short time, the light becomes glaring white. Eventwice the rated current of 12 V does not destroy the light bulb immediately, but after a couple ofseconds or minutes.

LEDs show a completely different behaviour. The normal voltage of a red LED supplied with 10 to20 mA is approximately 1.8 V. If the voltage is raised by only 0.5 V to 2.3 V, the LED inevitablyburns out. On the other hand, the LED does not light up at all if the voltage is reduced by only halfa volt. If a higher voltage is applied, a resistor makes sure that the correct voltage is automaticallyset.

Now try to operate a red LED without a resistor directly on a 1.5-V cell. Only because the voltageis at the lower limit, you can omit the series resistor in this case.

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The red LED will light up but only show a very weak light. Now insert the green LED. Result: Itdoes not light up! As a matter of fact, virtually no current flows through the green LED. The cha-racteristics of the yellow LED are between the red and the green LED. 1.5 V might produce a veryweak light at best.

What current flows with what voltage? This question is answered by the characteristic curve ofa component. Figure 3.3 shows the measured characteristic curve of the red and the greenLED in a common diagram. In both cases, you can see that a noticeable amount of current onlyflows above a minimum voltage or “threshold voltage”. With increasing voltage, the currentincreases as well. Measurements were stopped at the approved maximum of 20 mA. It is,however, easy to images the progression of the curves. Only a small increase in voltage leadsto a significant increase in current, which can easily destroy the LED.

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Figure 3.1: At the lower voltage threshold

LED, red

Figure 3.2: Direct connection of an AA battery

The diagram clearly shows the different threshold voltages of the red and the green LED. Now it isobvious why the red LED lights up weakly at 1.5 V whereas the green LED does not light up at all.When LED circuits are dimensioned, usually series resistors are used to set a specific diode cur-rent. If you assume a normal operating current of 20 mA, the resulting voltages for the differentLED types are as shown in table 3.1.

Table 3.1: Typical LED voltages

LED colour Voltage at 20 mA

Red 1.9 V

Yellow 2.1 V

Green 2.2 V

3.2 Series connection

When the battery voltage is sufficient (e.g. 9 V), two or more LEDs can be connected in series.The forward voltages of the diodes are added, so that less voltage is present at the series resistor.A red and a green LED have a diode current of 10 mA and a voltage of 1.9 V + 2.2 V = 4.1 V. Thevoltage at the series resistor is consequently 9 V - 4.1 V = 4.9 V. To set a current of 10 mA, theresistor must be adjusted accordingly.

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Figure 3.3: LED characteristic curves

redgreen

R = U/I R = 4.9 V/10 mA R = 490 Ω

The calculation often results in a resistance value outside the standard values. In such a case, use the next smallest standard value. In this case, the value is 470 Ω. The currentis increased insignificantly. In fact, the voltage ratio hardly changes due to the steep characteristiccurve of the diode.

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Figure 3.4: LEDs in series connection

Figure 3.5: Red and green LED in series connection

red

green

3.3 Little energy – a lot of light

Connecting several LEDs in series is often more efficient as less energy is transformed intouseless heat in the series resistor. Thus, you have to try to keep the voltage drop at the seriesresistor as low as possible. Figure 3.6 shows the possible dimensioning with three LEDs (red,yellow, green). The combined diode voltage is 1.8 V + 2.1 V + 2.2 V = 6.1 V. The voltage dropat the series resistor is 2.9 V. For a current of 20 mA, a resistance of 145 Ω is required. Even at220 Ω there is still a good amount of brightness. Instead of 20 mA, the resulting current is 15 mA which results in quite a long operating time with a 9-V battery.

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Figure 3.6: Series connection with three LEDs

Figure 3.7: All colours in series

red

green

yellow

3.4 Parallel connection

If two or more loads are to be operated on a common current source, there are generally two pos-sibilities: a parallel circuit or a serious circuit.

When two loads are connected in series (fig. 3.8, right), the same current flows through them. However, the loads only get part of the battery voltage. This circuit was used in the preceding section. With LEDs in a series circuit, the same current flows throughevery LED. This does not allow you to individually adjust the current. Different LEDs do nothave the brightness at the same current.

If both loads are connected in parallel (fig. 3.8, left), they are supplied the same voltage. The wiringof a vehicle is an example. The battery has a voltage of 12 V, as do all lamps. This means theyhave to be connected in parallel. When connected in parallel, the series connection of LED andseries resistor combined has to be regarded as one load. Due to the differences in LED voltage, itis not possible to use a common series resistor. The differences in brightness can be balancedusing different series resistors.

For every single LED, the maximum current and the lowest admissible series resistance has tobe observed according to the supply voltage. Table 3.2 provides an overview of the minimumresistances.

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Figure 3.8: Parallel and series connection

Table 3.2: Minimum resistances at different supply voltages

LED 3 V 6 V 9 V 12 V

Red, 20 mA, 1.8 V 60 Ω 210 Ω 360 Ω 510 ΩYellow, 20 mA, 2.1 V 45 Ω 195 Ω 345 Ω 495 ΩGreen, 20 mA, 2.2 V 40 Ω 190 Ω 340 Ω 490 Ω

Fig. 3.9 shows the example of a parallel connection of three LEDs with a series resistor each. Theyellow LED should be supplied with more current to balance its brightness which is not perceivedas strong. The circuit diagram shows the measured currents for every LED. The currents add up to a total of almost 30 mA.

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Figure 3.9: Parallel connection with three LEDs

greenyellowred

Figure 3.10: Each LED has an individual resistor

3.5 Plays of colour

Set up a circuit with a 9-V battery, a green LED and a 1-kΩ series resistor, as described inchapter 2. The green LED lights up as expected. Now connect a red LED to the green LED inparallel, i.e. cathode to cathode and anode to anode. Now the red LED lights up but the greenLED goes out. This might be surprising, as a single switch or contact is enough to achieve aswitching function.

The function of the switch is explained by the different characteristic curves of the two LEDs.When connected in parallel, both have the same voltage. At the same voltage, however, signi-ficantly more current flows through the red LED than through the green LED. When the red LEDis connected, the common voltage is reduced to such an extent that almost no current flowsthrough the green LED anymore.

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Figure 3.11: Different LEDs in parallel connection

Figure 3.12: Switching the colour with a pushbutton switch

redgreen

3.6 Flashlight

A capacitor stores electrical energy. The xenon flash lamp of a camera, for example, uses a100 µF electrolyte capacitor which is charged to 400 V and then discharges 8 wattseconds, agreat amount of energy.

An LED flash has to be constructed a bit more modestly since the LED cannot withstand asmuch energy. Charge the 47-µF electrolytic capacitor with a voltage of 9 V. Due to the low vol-tage, the flash energy is only about 2 mWs. Only a very low charging current is required so acharging resistor of 100 kΩ is sufficient. The electrolytic capacitor is sufficiently charged afterabout five seconds. Now press the pushbutton. The LED lights up quickly and then goes outalmost completely. The LED then emits a very weak light because of the low current that stillflows through the charging resistor.

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Figure 3.13: LED flash with electrolytic capacitor

Figure 3.14: Flash

4 Test instruments with LEDsIt is often the small and simple devices that facilitate our work. Simple test instruments with LEDsas indicators save power and work effectively. The advantage of an LED is the brightness reachedwith very low currents and the voltage threshold which can be used as reference voltage.

4.1 Cable tester

When checks of electrical devices or installations are carried out, it is often necessary to checkthe individual connections. The following test instrument lets a test current flow through the line.The LED lights up when there is a connection. This way, you can look for bad contacts or interrupted lines. Set up a continuity tester on the patch board and have two long cable protrude from it as testcables.

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Figure 4.1: Continuity tester with LED

Figure 4.2: Test instrument with test cables

The LED does not only light up in case of full continuity, but also when loads with a certainresistance close the circuit. Thus, you can use this tester to check light bulbs etc. The DC resis-tance of a transformer is also low enough to make the LED light up brightly. With faulty powersupplies, usually the internal thermal fuse is interrupted. In such a case, there is no continuitybetween the two pins of the power plug. Also check other components such as LEDs or resis-tors. LEDs only show continuity in one direction and then light up by themselves. Resistorsshow less brightness depending on the resistance value.

4.2 Water detector

The continuity tester described in the previous section can also be used to measure the conducti-vity of water or other fluids. If you hold the two wires into water, the LED should also light up dimly. The conductivity is increased significantly if you add some salt. Lemon juice or another acidwill cause the same effect. As soon as there is a flowing current, little gas bubbles are generatedaround the wires. The chemical electrolytic reaction also corrodes the wire surface. For moreextended experiments, electrodes made of carbon or graphite are suitable as they do not corrode.Use e.g. pencil leads or carbon sticks from old batteries.

Apart from interesting conductivity experiments in fluids, there are also practical applications.For example, you can construct warning devices for leaking water, or rain detectors. The circuitis also suitable as a moisture sensor for flower pots. If you stick the test wires into the soil, theLED brightness indicates the degree of humidity.

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Figure 4.3: Water in the circuit

Water

4.3 Alarm device

To set up a theft or burglary alarm, mechanically or magnetically activated contacts on doors orwindows are used. The alarm is triggered, for example, when a window is opened. The simplestway is to install a thin wire which is severed in case of an alarm. If someone tries to disable thealarm by cutting the wire, the alarm is triggered as well.

In the simplest case, the current loop can be monitored by an LED. In idle state however, theLED should be off in order not to attract additional attention. The LED should only light up whenthe wire is severed. Figure 4.4 shows the circuit. As long as the monitoring circuit is closed, theLED current is diverted because the LED is short-circuited.

The disadvantage of this circuit is that even without the alarm a steady current of about 9 mA isflowing. A battery would be exhausted fairly quickly. Therefore, a power adapter should beused.

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Figure 4.4: LED is short-circuited

Figure 4.5: Alarm loop

Current loop

4.4 Polarity tester

Especially with power adapters, it is often difficult to know the polarity. A simple test with twoLEDs provides clarity. If a voltage source as in figure 4.6 is connected, the red LED lights up.When the polarity is inverted, the green LED lights up.

The tester can also be used for alternating voltage. In that case, both LEDs light up. Thus, youhave a complete test instrument for small power adapters and transformers up to 12 V.

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Figure 4.6: Current direction indicator

Figure 4.7: Polarity tester with test cables

redgreen

4.5 Battery tester

You can make a simple battery tester using LEDs. The LEDs can give you a rough idea of thebattery state based on the voltage. All LED circuits presented so far mostly work within a widevoltage range and only show little differences in brightness when the battery is almost used up.One exception is the direct connection of a red LED to a 1.5-V cell (see section 3.1). As 1.5 V isexactly at the diode threshold voltage, the LED only lights up at full voltage.

Using a voltage divider made up of two resistors, the threshold voltage of an LED circuit can beraised as desired and adjusted to different needs. The dimensioning shown in figure 4.8 raises the threshold to about 9 V. At exactly 9 V, the unloaded voltage divider has a voltage of 1.62 V which is just slightly abovethe voltage threshold of the red LED.

U = Utotal x R1/(R1 + R2) U = 9 Vx 220 Ω/1220 ΩU = 1.62 V

Under normal conditions, the LED lights up very dimly at a battery voltage of 9 V. As soon asthe voltage falls only slightly, the LED goes out. Thus, the test is unrealistically tight. If partialresistance R1 is increased to 330 Ω, the battery state is represented well. At 9 V, the LED lightsup brightly, at 8 V or 7 V it is less bright. The LED goes out completely at 6V.

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Figure 4.8: 9-V battery tester

red

4.6 LED as temperature sensor

At a steady current, the voltage at an LED changes by about -2 mV per degree. The tempera-ture dependence of the characteristic curve of the diode can be used to compare two tempera-tures. When the two LEDs are connected in parallel as shown in figure 4.11, the warmer LED isbrighter than the colder one.

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Figure 4.9: Voltage test between 6 V and 9 V

red

Figure 4.10: 9-V battery tester

Figure 4.11: Comparing the temperature with two LEDs

redred

cold hot

Temperature differences of 10 degrees are clearly visible. The warmth of the hand is enoughfor a visible effect.

At a temperature difference of more that 50 degrees, the colder LED goes out almost comple-tely. One of the LEDs can be heated up with a flame or a soldering iron. Avoid direct contactwith the flame, however, in order not to damage the plastic coating. Wind a piece of wire aroundthe cathode connection of the LED to be heated up. Then use a lighter to apply heat to the endof the wire in measured doses. The cathode connection transfers heat well as it leads to theLED crystal holder. It constitutes a good thermal contact. The anode, on the other hand, is con-nected to the crystal via a thin wire.

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Figure 4.12: Same temperature and brightness?

Figure 4.13: Transferring heat with a wire

5 Transistor circuitsAll experiments so far only needed LEDs and resistors. However, LEDs are also used in com-plex electronic circuits with transistors. The following experiments are designed to give you aquick overview of the functions of the transistor.

5.1 Amplification

The circuit in figure 5.1 shows the basic function of an NPN transistor. There are two circuits. Alow base current flows through the control circuit. A higher collector current flows through theload circuit. Both currents flow through the emitter together. As the emitter is positioned at thecommon point of reference, this circuit is also called emitter circuit. As soon as the base circuitis opened, the load current does not flow anymore. It is crucial that the base current is signifi-cantly smaller than the collector current. The low base current is thus amplified to a higher col-lector current. In the present case, the current amplification factor is about 100. The base resis-tance of 100 kΩ is 100 times higher than the series resistance in the load circuit. In this circuit,the transistor works like a switch. Only a small voltage drop remains between the collector andthe emitter. The collector current is already limited by the load and cannot rise. The collectorcurrent is saturated. Consequently, the transistor is driven to full power.

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Figure 5.1: NPN transistor in emitter circuit

red

green

The LEDs serve to indicate the currents. The red LED lights up brightly, the green LED dimly.The dim green LED indicating the base current can only be recognised in a completely darkenedroom. The difference indicates the great current amplification.

5.2 Follow-up control

The current amplification of a transistor can be used to extend the discharge time of a capacitor.The circuit shown in figure 5.3 uses an electrolyte capacitor with 47 µF as charging capacitor.When you press the pushbutton, it is charged and provides the base current of the emitter circuitfor a long period of time.

The discharge time is extended considerably by the high base resistance. The time constant inthis case is about five seconds. After this time, the base current is still sufficient to fully drive thetransistor.

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Figure 5.2: Current amplification

Figure 5.3: Delayed cut-off

red

The implementation of the circuit only requires you to quickly push the button to switch on theLED. Then the LED is lit for about five seconds and then gradually fades away. After one minute, there is still a very dim light. In fact even after a long period of time, the LED still doesnot go out completely. However, the current sinks to values so low that there is no visible effectanymore.

5.3 Touch sensor

The current amplification factors of two transistors can be multiplied if the amplified current ofthe first transistor is amplified again as the base current of the second transistor. The Darling-ton circuit in figure 5.5 combines both collectors resulting in a component with three connec-tions, which is also called Darlington transistor.

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Figure 5.4: Minute light

Figure 5.5: Darlington circuit

With an amplification factor of 300 for each transistor, the Darlington circuit has an amplificationof 90000. Now a base resistance of 10 MΩ conducts enough to activate the LED. In an experi-ment, a touch contact can be used instead of the extremely high resistance. Due to the highamplification, a light tough with a dry finger is already enough. The additional protective resistorin the feed line of the battery protects the transistors in case the touch contacts are connecteddirectly by mistake.

5.4 LED as light sensor

Virtually no current flows through a diode if the diode is connected to the voltage supply inreverse direction. In fact, there is a very low reverse current in the range of a few nanoamperes,which can normally be neglected. The high amplification of the Darlington circuit allows you toconduct experiments with extremely small currents. Therefore, the reverse current of an LEDeven depends on the lighting. Consequently, an LED is a photo diode at the same time. Theextremely low photocurrent is amplified by the two transistors to such an extent that the secondLED lights up.

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Figure 5.6: Touch sensor

Figure 5.7: Amplification of LED reverse current

In the experiment, the right LED is already clearly lit in normal ambient light. The brightness ofthe indicator LED is influenced if the sensor LED is covered with one hand.

5.5 Constant brightness

Sometimes, a constant current is required which is as independent as possible from voltagefluctuations. A lit LED would have the same brightness, even if the battery already has a lowervoltage. Figure 5.9 shows a simple stabiliser circuit. A red LED at the input stabilises the basevoltage at about 1.6 V. As the base emitter voltage is always around 0.6 V, the voltage at theemitter resistor is about 1 V. The resistance determines the emitter current. The collector cur-rent is almost identical with the emitter current, which is only higher by the much smaller basecurrent. The LED in the collector circuit does not require a series resistor, because the LED cur-rent is regulated by the transistor.

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Figure 5.8: LED light sensor

Figure 5.9: Stabilised current source

green

Check the results with a new and an intensely used battery. As long as there is a certainamount of residual voltage, the brightness of the LED remains almost unchanged.

5.6 Temperature sensor

The circuit in figure 5.11 is a so-called current mirror. The current flowing through the 1-kΩresistor is mirrored in the two transistors and appears as collector current of the right transistorat almost the same amperage. As the base and the emitter are interconnected in the left tran-sistor, a base emitter voltage resulting in the given collector current appears. In theory, thesecond transistor should – with exactly the same values and at the same base emitter voltage– show the same collector current. In a real-life setting however, there are often slight diffe-rences.

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Figure 5.10: Stabilisation of LED brightness

Figure 5.11: Current mirror

red

In practice, it is very difficult to achieve identical transistor values. This circuit is mainly used inintegrated circuits where a number of transistors on a chip have the same values. It is alsoimportant to note that both transistors should have the same temperature as the transmissioncharacteristic curve changes with the temperature.

The implementation of the current mirror can be used as a temperature sensor. Touch one ofthe transistors with your finger. The increase in temperature changes the output current. It isvisible by the change of brightness of the LED. Depending on which of the two transistors youtouch, you can slightly increase or decrease the brightness.

5.7 On and off

A circuit with two stable states is called trigger circuit or flip-flop. An LED is either lit or not lit. Itis never half-lit. Figure 5.13 shows the typical wiring of an ordinary flip-flop.

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Figure 5.12: Transistor used as temperature sensor

Figure 5.13: Bistable flip-flop

green

red

The circuit is flipped into one of two possible states: When the right transistor conducts, the lefttransistor is blocked and vice versa. The conducting transistor has a low collector voltage whichit applies to switch off the base current of the other transistor. Therefore, a switching state onceassumed remains stable until it is changed by one of the pushbutton switches.

Switch on the power supply. You should see one of the two LEDs light up. You cannot predict whichside will be activated. In most cases, the different current amplification values of the transistorsdetermine what side of the circuit is activated.

Now use a jumper to block one of the two transistors. The current state remains active when thejumper is removed. The two states are also called “set” (S) or “reset” (R) which is where thename RS flip-flop comes from.

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Figure 5.14: A simple flip-flop

5.8 LED blinkerSet up a flip-flop that automatically switches between the two states. Like the RS flip-flop, thecircuit requires two transistors with an emitter circuit. The feedback from the output to the inputis done via a capacitor that is continually charged and discharged.

A central operating point without feedback is a precondition for the safe oscillation build-up inthe circuit. Otherwise, the output transistor is either completely blocked or driven to full power.The whole circuit would then not have sufficient amplification for oscillation build-up. A strongnegative feedback at the first transistor provides a central operating point. However, the feed-back via an RC element predominates and finally results in the output transistor being blockingor driven to full power alternately.

First set up the circuit without the feedback capacitor. The LED should light up weakly as the output transistor is not driven to full power. When acapacitor is inserted, the LED lights up and goes out completely alternately. With the 47-µFcapacitor, the LED flashes about once a second.

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Figure 5.15: Multivibrator

Figure 5.16: LED blinker

red

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