Electronic Start Learning Package

Transcription

1 O P E R A T I N G I N S T R U C T I O N S Version 07/09 Electronic Start Learning Package Item-No These Operating Instructions accompany this product. They contain important information on setting up and using it. You should refer to these instructions, even if you are buying this product for someone else. 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, on page 2.

2 Table of contents 1 Getting started First tries with LEDs LED with series resistor Current direction Amperages Signal lamp with pushbutton switch LED circuit technology Diode threshold voltage Series connection Little energy a lot of light Parallel connection Plays of colour Flashlight Test instruments with LEDs Cable tester Water detector Alarm device Polarity tester Battery tester LED as temperature sensor Transistor circuits Amplification Follow-up control Touch sensor LED as light sensor Constant brightness Temperature sensor On and off LED blinker

3 1 Getting started This learning package is an easy introduction to electronics. The following is a presentation of the components. Patch board All 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 the integrated circuits (ICs) and the individual components. Figure 1.1: The experimenting board 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 the upper 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. Figure 1.2: The internal contact rows 3

4 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 tweezers or a small pair of pliers. Hold the connecting wires as closely as possible to the patch board and press them down in a vertical movement. Proceed in the same way to insert sensitive connecting wires such as the tinned ends of battery clips. For your experiments, you require different lengths of wire which must be cut off from the provided jumper wire. To strip the wire ends, it is a proven method to first cut into the insulation around the wire using a sharp knife. Battery The following overview shows the components as they really look and the symbols used in circuit diagrams. The battery can be replaced by e.g. a power supply. Figure 1.3: Battery and battery diagram symbol 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 or the battery itself to heat up considerably. The current supplied by a zinc-carbon battery during a 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 and tinned. 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 battery wires connected and just removing the clip from the battery. 4

5 A single zinc-carbon or alkali cell has a voltage of 1.5 V. Several cells are connected in series in one battery. Accordingly, the symbols show the number of cells in a battery. For higher voltages, it is common practice to indicate the middle cells by a dotted line. Figure 1.4: Diagram symbols for different batteries LED The learning package includes two red LEDs, one green LED and one yellow LED. The polarity of all LEDs must always be observed. The negative connection is called cathode. It is at the shorter connecting wire. The positive connection is called anode. The cup-shaped holder that holds the LED crystal at the cathode is visible inside the LED. The anode connection is connected with an extremely thin wire to a contact at the top of the crystal. Caution! Unlike light bulbs, LEDs must never be directly connected to a battery. A series resistor is always required. - Cathode + Anode Figure 1.5: LED 5

6 Resistors The 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 of different colours indicate the resistor type. The resistance value and the accuracy class are indicated. Figure 1.6: Resistor Resistors with a tolerance of ±5 % are in the E24 list. Every decade includes 24 values with about 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 two rings represent digits whereas the third ring is a multiplier for the resistance value in ohms. The fourth ring represents the tolerance. 6

7 Table 1.2: Resistance colour codes Colour Ring 1 Ring 2 Ring 3 Ring 4 1. digit 2. digit Multiplier Tolerance Black 0 1 Brown % Red % Orange Yellow Green Blue Purple 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 tolerance of 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 7

8 Transistors Transistors are components used for the amplification of small currents. The used BC547 resistors are silicon NPN transistors. Figure 1.7: Transistors The connections on the transistor are called emitter (E), basis (B) and collector (C). The basis connection for both transistors is in the middle. Looking at the label with the connection pointing downwards, the emitter is on the right. Capacitor The capacitor is another important electronic component. A capacitor consists of two metal surfaces and an insulating layer. When electric voltage is applied, an electric field in which energy is stored builds up between the two capacitor plates. A capacitor with a big plate surface and a small distance between the plates has a high capacity, i.e. it stores a lot of energy when voltage is 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 aluminium oxide. The electrolytic capacitor contains a fluid electrolyte and aluminium foil with a big surface. Voltage must only be applied in one direction. In the wrong direction, leakage current will 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. Figure 1.8: Electrolyte capacitor 8

9 2 First tries with LEDs You can take a battery and a small light bulb and try different things until the bulb lights up. You should 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, and use a suitable series resistor. It is not really difficult. Try out the circuits described below to become familiar 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 9V battery. Take the hightest resistance value (1 kω = 1000 Ω, colours: brown, black, red) from the learning package to be on the safe side in terms of LED current. Figure 2.1 shows the circuit as a circuit diagram. Figure 2.1: Circuit diagram of LED with series resistor Use the patch board to set up the circuit. Connect the upper supply rail with the positive terminal of the battery, i.e. with the red connector on the battery clip. Connect he lower supply rail accordingly to the black clip connector, i.e. to the negative terminal of the battery. The actual circuit will resemble the circuit diagram so that troubleshooting should not pose any problems. Bend the connecting wires of the LEDs and the resistors so that they fit into the contacts. Some connecting wires were shortened in this test setup for better illustration. You should, however, leave the wires uncut to ensure that the components can be used for all other experiments as well. 9

10 Figure 2.2: Setup on the patch board 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 position of the components on the patch board. As another possible problem, the LED might have been inserted the wrong way, or the battery is empty. You will notice, however, that even very old batteries 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 LED before flowing through the resistor. The effect is the same as in the first case, however. The only important thing is that all three components are connected in a closed circuit. Figure 2.3: Swapped components 10

11 Figure 2.4: LED and resistor swapped 2.2 Current direction Turn the LED so that the anode is connected to the negative terminal of the battery. There is no light! This means that current can only flow through the LED in one direction. The forward direction is the current direction from anode to cathode, with the anode connected to the positive terminal of the battery and the cathode to the negative terminal. In reverse direction, the LED is blocked. A diode is like an electric valve. It only lights up when current is let through. Figure 2.5 shows the LED with reverse direction. It cannot light up. Figure 2.5: LED in reverse direction The arrows in the LED circuit diagram in figure 2.6 indicate the direction of current. The direction of current as well as the designation plus and minus was defined arbitrarily in history. This means, current always flows from the positive terminal of the battery through the load to the negative terminal of the battery. Today, it is common knowledge that negatively charged electrons 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 direction of the current. Even inside the LED itself there are negative and positive charge carriers. 11

12 Figure 2.6: Direction of current 2.3 Amperages Instead of the 1-kΩ resistor, insert a smaller resistor of 470 Ω (yellow, purple, brown). The LED lights up noticeably brighter. This indicates higher current. The rule is: The higher the resistance, the lower the current. More accurate calculations are stated below. Figure 2.7: More brightness with a lower series resistance 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 than 330 Ω, as this might result in a current too high for the circuit with a 9-V battery and consequently harm the LED. 12

13 Figure 2.8: 470 Ω series resistance The used LEDs are approved for continuous current of 20 ma. The table below shows that the actual current of the used LED depends on the series resistor. In some cases, the approved current is slightly exceeded. If this happens for a short time, it does not constitute a problem. If overloaded 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 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. When opened, the switch represents an interruption of the circuit. When the switch is pressed, the two contacts are connected and the circuit is closed. The elasticity of the wire disconnects the contact when you let go of the switch. Consequently, the LED within the circuit only lights up as long as the switch is pressed. 13

14 Figure 2.9: Setup of a switch using wire Figure 2.10: Circuit with switch This setup can be used as a signal lamp for various purposes. In principle, this switch can also be used to transmit complex messages by Morse code. Admittedly, sending messages in Morse code is a little old-fashioned and not as comfortable as an or a phone call. However, using Morse code with an LED can be a delightful way of communication. With some practice, information can be exchanged over a distance of up to 100 m with almost nobody else being able to listen in. 14

15 3 LED circuit technology It is easy to set up a given circuit using the recommended components. Whoever really wants to master circuit technology, however, should become familiar with theoretical aspects in order to be able to calculate required resistances in a circuit. The chapter provides the necessary aspects and the corresponding experiments. Combine theory with practice. Calculate and test your own circuits! 3.1 Diode threshold voltage In comparison with a light bulb, an LED seems to behave strangely. Not only does the current flow 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 is enough to cause a weak, dark red glow. At the rated voltage, a bright yellowish-white light is produced. If you try a higher voltage for a very short time, the light becomes glaring white. Even twice the rated current of 12 V does not destroy the light bulb immediately, but after a couple of seconds or minutes. LEDs show a completely different behaviour. The normal voltage of a red LED supplied with 10 to 20 ma is approximately 1.8 V. If the voltage is raised by only 0.5 V to 2.3 V, the LED inevitably burns out. On the other hand, the LED does not light up at all if the voltage is reduced by only half a volt. If a higher voltage is applied, a resistor makes sure that the correct voltage is automatically set. Now try to operate a red LED without a resistor directly on a 1.5-V cell. Only because the voltage is at the lower limit, you can omit the series resistor in this case. 15

16 LED, red Figure 3.1: At the lower voltage threshold Figure 3.2: Direct connection of an AA battery The red LED will light up but only show a very weak light. Now insert the green LED. Result: It does not light up! As a matter of fact, virtually no current flows through the green LED. The characteristics of the yellow LED are between the red and the green LED. 1.5 V might produce a very weak light at best. What current flows with what voltage? This question is answered by the characteristic curve of a component. Figure 3.3 shows the measured characteristic curve of the red and the green LED in a common diagram. In both cases, you can see that a noticeable amount of current only flows above a minimum voltage or threshold voltage. With increasing voltage, the current increases 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 leads to a significant increase in current, which can easily destroy the LED. 16

17 red green Figure 3.3: LED characteristic curves The diagram clearly shows the different threshold voltages of the red and the green LED. Now it is obvious 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 current. If you assume a normal operating current of 20 ma, the resulting voltages for the different LED types are as shown in table 3.1. Table 3.1: Typical LED voltages LED colour Red Yellow Green Voltage at 20 ma 1.9 V 2.1 V 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 V = 4.1 V. The voltage at the series resistor is consequently 9 V V = 4.9 V. To set a current of 10 ma, the resistor must be adjusted accordingly. 17

18 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 current is increased insignificantly. In fact, the voltage ratio hardly changes due to the steep characteristic curve of the diode. green red Figure 3.4: LEDs in series connection Figure 3.5: Red and green LED in series connection 18

19 3.3 Little energy a lot of light Connecting several LEDs in series is often more efficient as less energy is transformed into useless heat in the series resistor. Thus, you have to try to keep the voltage drop at the series resistor 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 V V = 6.1 V. The voltage drop at the series resistor is 2.9 V. For a current of 20 ma, a resistance of 145 Ω is required. Even at 220 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. green yellow red Figure 3.6: Series connection with three LEDs Figure 3.7: All colours in series 19

20 3.4 Parallel connection If two or more loads are to be operated on a common current source, there are generally two possibilities: a parallel circuit or a serious circuit. Figure 3.8: Parallel and series connection 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 through every LED. This does not allow you to individually adjust the current. Different LEDs do not have the brightness at the same current. If both loads are connected in parallel (fig. 3.8, left), they are supplied the same voltage. The wiring of a vehicle is an example. The battery has a voltage of 12 V, as do all lamps. This means they have to be connected in parallel. When connected in parallel, the series connection of LED and series resistor combined has to be regarded as one load. Due to the differences in LED voltage, it is not possible to use a common series resistor. The differences in brightness can be balanced using different series resistors. For every single LED, the maximum current and the lowest admissible series resistance has to be observed according to the supply voltage. Table 3.2 provides an overview of the minimum resistances. 20

21 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 Ω Yellow, 20 ma, 2.1 V 45 Ω 195 Ω 345 Ω 495 Ω Green, 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. The yellow LED should be supplied with more current to balance its brightness which is not perceived as strong. The circuit diagram shows the measured currents for every LED. The currents add up to a total of almost 30 ma. red yellow green Figure 3.9: Parallel connection with three LEDs Figure 3.10: Each LED has an individual resistor 21

22 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 in chapter 2. The green LED lights up as expected. Now connect a red LED to the green LED in parallel, i.e. cathode to cathode and anode to anode. Now the red LED lights up but the green LED goes out. This might be surprising, as a single switch or contact is enough to achieve a switching function. green red Figure 3.11: Different LEDs in parallel connection Figure 3.12: Switching the colour with a pushbutton switch 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, significantly more current flows through the red LED than through the green LED. When the red LED is connected, the common voltage is reduced to such an extent that almost no current flows through the green LED anymore. 22

23 3.6 Flashlight A capacitor stores electrical energy. The xenon flash lamp of a camera, for example, uses a 100 µf electrolyte capacitor which is charged to 400 V and then discharges 8 wattseconds, a great amount of energy. Figure 3.13: LED flash with electrolytic capacitor An LED flash has to be constructed a bit more modestly since the LED cannot withstand as much energy. Charge the 47-µF electrolytic capacitor with a voltage of 9 V. Due to the low voltage, the flash energy is only about 2 mws. Only a very low charging current is required so a charging resistor of 100 kω is sufficient. The electrolytic capacitor is sufficiently charged after about five seconds. Now press the pushbutton. The LED lights up quickly and then goes out almost completely. The LED then emits a very weak light because of the low current that still flows through the charging resistor. Figure 3.14: Flash 23

24 4 Test instruments with LEDs It is often the small and simple devices that facilitate our work. Simple test instruments with LEDs as indicators save power and work effectively. The advantage of an LED is the brightness reached with 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 check the 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 test cables. Figure 4.1: Continuity tester with LED Figure 4.2: Test instrument with test cables 24

25 The LED does not only light up in case of full continuity, but also when loads with a certain resistance close the circuit. Thus, you can use this tester to check light bulbs etc. The DC resistance of a transformer is also low enough to make the LED light up brightly. With faulty power supplies, usually the internal thermal fuse is interrupted. In such a case, there is no continuity between the two pins of the power plug. Also check other components such as LEDs or resistors. LEDs only show continuity in one direction and then light up by themselves. Resistors show 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 conductivity 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 acid will cause the same effect. As soon as there is a flowing current, little gas bubbles are generated around the wires. The chemical electrolytic reaction also corrodes the wire surface. For more extended 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. Water Figure 4.3: Water in the circuit 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 circuit is also suitable as a moisture sensor for flower pots. If you stick the test wires into the soil, the LED brightness indicates the degree of humidity. 25

26 4.3 Alarm device To set up a theft or burglary alarm, mechanically or magnetically activated contacts on doors or windows are used. The alarm is triggered, for example, when a window is opened. The simplest way is to install a thin wire which is severed in case of an alarm. If someone tries to disable the alarm by cutting the wire, the alarm is triggered as well. Current loop Figure 4.4: LED is short-circuited In the simplest case, the current loop can be monitored by an LED. In idle state however, the LED should be off in order not to attract additional attention. The LED should only light up when the wire is severed. Figure 4.4 shows the circuit. As long as the monitoring circuit is closed, the LED current is diverted because the LED is short-circuited. Figure 4.5: Alarm loop The disadvantage of this circuit is that even without the alarm a steady current of about 9 ma is flowing. A battery would be exhausted fairly quickly. Therefore, a power adapter should be used. 26

27 4.4 Polarity tester Especially with power adapters, it is often difficult to know the polarity. A simple test with two LEDs 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. red green Figure 4.6: Current direction indicator The tester can also be used for alternating voltage. In that case, both LEDs light up. Thus, you have a complete test instrument for small power adapters and transformers up to 12 V. Figure 4.7: Polarity tester with test cables 27

28 4.5 Battery tester You can make a simple battery tester using LEDs. The LEDs can give you a rough idea of the battery state based on the voltage. All LED circuits presented so far mostly work within a wide voltage 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 is exactly at the diode threshold voltage, the LED only lights up at full voltage. red Figure 4.8: 9-V battery tester Using a voltage divider made up of two resistors, the threshold voltage of an LED circuit can be raised 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 above the voltage threshold of the red LED. U = Utotal x R1/(R1 + R2) U = 9 Vx 220 Ω/1220 Ω U = 1.62 V Under normal conditions, the LED lights up very dimly at a battery voltage of 9 V. As soon as the voltage falls only slightly, the LED goes out. Thus, the test is unrealistically tight. If partial resistance R1 is increased to 330 Ω, the battery state is represented well. At 9 V, the LED lights up brightly, at 8 V or 7 V it is less bright. The LED goes out completely at 6V. 28

29 red Figure 4.9: Voltage test between 6 V and 9 V Figure 4.10: 9-V battery tester 4.6 LED as temperature sensor At a steady current, the voltage at an LED changes by about -2 mv per degree. The temperature dependence of the characteristic curve of the diode can be used to compare two temperatures. When the two LEDs are connected in parallel as shown in figure 4.11, the warmer LED is brighter than the colder one. cold red red hot Figure 4.11: Comparing the temperature with two LEDs 29

30 Temperature differences of 10 degrees are clearly visible. The warmth of the hand is enough for a visible effect. Figure 4.12: Same temperature and brightness? At a temperature difference of more that 50 degrees, the colder LED goes out almost completely. One of the LEDs can be heated up with a flame or a soldering iron. Avoid direct contact with the flame, however, in order not to damage the plastic coating. Wind a piece of wire around the cathode connection of the LED to be heated up. Then use a lighter to apply heat to the end of the wire in measured doses. The cathode connection transfers heat well as it leads to the LED crystal holder. It constitutes a good thermal contact. The anode, on the other hand, is connected to the crystal via a thin wire. Figure 4.13: Transferring heat with a wire 30

31 5 Transistor circuits All experiments so far only needed LEDs and resistors. However, LEDs are also used in complex electronic circuits with transistors. The following experiments are designed to give you a quick 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. A low base current flows through the control circuit. A higher collector current flows through the load circuit. Both currents flow through the emitter together. As the emitter is positioned at the common point of reference, this circuit is also called emitter circuit. As soon as the base circuit is opened, the load current does not flow anymore. It is crucial that the base current is significantly smaller than the collector current. The low base current is thus amplified to a higher collector current. In the present case, the current amplification factor is about 100. The base resistance 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 and the emitter. The collector current is already limited by the load and cannot rise. The collector current is saturated. Consequently, the transistor is driven to full power. green red Figure 5.1: NPN transistor in emitter circuit 31

32 Figure 5.2: Current amplification 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 darkened room. 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 circuit for a long period of time. red Figure 5.3: Delayed cut-off The discharge time is extended considerably by the high base resistance. The time constant in this case is about five seconds. After this time, the base current is still sufficient to fully drive the transistor. 32

33 Figure 5.4: Minute light The implementation of the circuit only requires you to quickly push the button to switch on the LED. 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 does not go out completely. However, the current sinks to values so low that there is no visible effect anymore. 5.3 Touch sensor The current amplification factors of two transistors can be multiplied if the amplified current of the first transistor is amplified again as the base current of the second transistor. The Darlington circuit in figure 5.5 combines both collectors resulting in a component with three connections, which is also called Darlington transistor. Figure 5.5: Darlington circuit 33

34 With an amplification factor of 300 for each transistor, the Darlington circuit has an amplification of Now a base resistance of 10 MΩ conducts enough to activate the LED. In an experiment, a touch contact can be used instead of the extremely high resistance. Due to the high amplification, a light tough with a dry finger is already enough. The additional protective resistor in the feed line of the battery protects the transistors in case the touch contacts are connected directly by mistake. Figure 5.6: Touch sensor 5.4 LED as light sensor Virtually no current flows through a diode if the diode is connected to the voltage supply in reverse 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 to conduct experiments with extremely small currents. Therefore, the reverse current of an LED even depends on the lighting. Consequently, an LED is a photo diode at the same time. The extremely low photocurrent is amplified by the two transistors to such an extent that the second LED lights up. Figure 5.7: Amplification of LED reverse current 34

35 In the experiment, the right LED is already clearly lit in normal ambient light. The brightness of the indicator LED is influenced if the sensor LED is covered with one hand. Figure 5.8: LED light sensor 5.5 Constant brightness Sometimes, a constant current is required which is as independent as possible from voltage fluctuations. A lit LED would have the same brightness, even if the battery already has a lower voltage. Figure 5.9 shows a simple stabiliser circuit. A red LED at the input stabilises the base voltage at about 1.6 V. As the base emitter voltage is always around 0.6 V, the voltage at the emitter resistor is about 1 V. The resistance determines the emitter current. The collector current is almost identical with the emitter current, which is only higher by the much smaller base current. The LED in the collector circuit does not require a series resistor, because the LED current is regulated by the transistor. green Figure 5.9: Stabilised current source 35

36 Figure 5.10: Stabilisation of LED brightness Check the results with a new and an intensely used battery. As long as there is a certain amount 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-kΩ resistor is mirrored in the two transistors and appears as collector current of the right transistor at almost the same amperage. As the base and the emitter are interconnected in the left transistor, a base emitter voltage resulting in the given collector current appears. In theory, the second 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 differences. red Figure 5.11: Current mirror 36

37 In practice, it is very difficult to achieve identical transistor values. This circuit is mainly used in integrated circuits where a number of transistors on a chip have the same values. It is also important to note that both transistors should have the same temperature as the transmission characteristic curve changes with the temperature. Figure 5.12: Transistor used as temperature sensor The implementation of the current mirror can be used as a temperature sensor. Touch one of the transistors with your finger. The increase in temperature changes the output current. It is visible by the change of brightness of the LED. Depending on which of the two transistors you touch, 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. It is never half-lit. Figure 5.13 shows the typical wiring of an ordinary flip-flop. green red Figure 5.13: Bistable flip-flop 37

38 The circuit is flipped into one of two possible states: When the right transistor conducts, the left transistor is blocked and vice versa. The conducting transistor has a low collector voltage which it applies to switch off the base current of the other transistor. Therefore, a switching state once assumed remains stable until it is changed by one of the pushbutton switches. Figure 5.14: A simple flip-flop Switch on the power supply. You should see one of the two LEDs light up. You cannot predict which side will be activated. In most cases, the different current amplification values of the transistors determine what side of the circuit is activated. Now use a jumper to block one of the two transistors. The current state remains active when the jumper is removed. The two states are also called set (S) or reset (R) which is where the name RS flip-flop comes from. 38

39 5.8 LED blinker Set up a flip-flop that automatically switches between the two states. Like the RS flip-flop, the circuit requires two transistors with an emitter circuit. The feedback from the output to the input is done via a capacitor that is continually charged and discharged. red Figure 5.15: Multivibrator A central operating point without feedback is a precondition for the safe oscillation build-up in the 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 strong negative feedback at the first transistor provides a central operating point. However, the feedback via an RC element predominates and finally results in the output transistor being blocking or driven to full power alternately. Figure 5.16: LED blinker 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 a capacitor is inserted, the LED lights up and goes out completely alternately. With the 47-µF capacitor, the LED flashes about once a second. 39

40 CONRAD IM INTERNET Legal notice These operating instructions are a publication by Conrad Electronic SE, Klaus-Conrad-Str. 1, D Hirschau ( All rights including translation reserved. Reproduction by any method, e.g. photocopy, microfilming, or the capture in electronic data processing systems require the prior written approval by the editor. Reprinting, also in part, is prohibited. These operating instructions represent the technical status at the time of printing. Changes in technology and equipment reserved. Copyright 2009 by Conrad Electronic SE. 01_0709_01/AB