Revised Standard Grade Technological Studies. Applied Electronics

Transcription

1 Revised tandard Grade Technological tudies Applied Electronics

2 Contents Introduction iii tructure iii Resources iv Assessment iv Detailed resource list vi Component Electronic ystems Electricity 3 imple Circuits 4 Integrated Circuits: 555 timer 77 Modular Electronic ystems 8 Introduction: Electronics a systematic approach 85 Modular boards 86 Analogue and digital signals 88 Problem solving in electronics 95 witches 98 Truth tables 0 The comparator 5 NAND and NOR gate boards 7 Logic in Electronics 9 witching logic 2 Binary numbers 30 Combinational logic 32 Pinout diagrams 47 Practical tasks 50 Electronics MiniProject 57 Introduction 57 Application of Technology 58 Example: Remote Controlled Buggy with Light and ound 59 Computer simulation 6 Appendix : Infrared Remote Control 65 Remote controlled toy 69 ii tandard Grade Technological tudies: Applied Electronics

3 Introduction Electronics is a key influence in today s society and is therefore a key area of study in Technological tudies. This component unit should be studied early in the course after pupils have some knowledge of systems. tructure This unit is split into three distinct sections: Component Electronic ystems Modular Electronic ystems Logic in Electronics. These sections can be delivered in any order, with each approach having its own advantages and disadvantages. Obviously there is some overlap between the units. For example, logic appears in the Modular ystems section in the form of the inverter, the OR, AND, NAND and NOR gates and as subsystem boards. imilarly, if the Modular ystems section is delivered first then some areas of Component ystems are mentioned. The advantage of delivering the Modular ystems first is that it gives an easy introduction to electronics; however, there is a danger that pupils will work through this section and solve the problems without picking up a real understanding of electronics. Logic ystems naturally follows on from the Modular ystems section. The advantage of delivering the Component ystems first is that the pupils gain a firm understanding of electronics, components and circuits. This gives them a deeper understanding, which helps in the subsequent sections. The disadvantage is that pupils may find the theory contained in this section difficult early in the course, although as they are General/Credit pupils the level should be well within their ability. By delivering the Component ystems first, pupils will cover the use of multimeters and prototype circuit boards that are used in the other sections. If the Component ystems section is to be delivered last, then it will be necessary to give the pupils some instruction in the use of multimeters, prototype circuit boards, simple electronic theory and components. The contents of this unit are set out comprehensively so that teachers do not require the use of additional notes or textbooks. Pupils can move at their own pace in many areas, but it must be stressed that these unit notes should not be used as an open learning pack and it will be necessary to deliver many important lessons at crucial times. These include an introduction to electrical theory (Ohm s law, Kirchoff s second law, etc.), series circuits, parallel circuits, and an introduction to components (recognition, use and characteristics). tandard Grade Technological tudies: Applied Electronics iii

4 Resources The resources required to run this unit are the same as those being used in Technological tudies at present. ome equipment may differ in type (for example different meters, prototype boards and modular systems) and therefore the notes provided will have to be interpreted differently. The main resources are: a range of components prototype circuit boards (often referred to as breadboards) digital nonauto ranging multimeters a modular electronic system (for example E&L boards) circuit simulation software (for example, Crocodile Clips). Circuit simulation software can be used for many of the activities but pupils must also have experience of building physical circuits. This is necessary to experience the real components as well as the problemsolving attached to building real circuits. Teachers are encouraged to use other resources such as video and the interactive CD ROMs that are available. oftware that simulates Modular ystems is also available and could be used to aid classroom management problems that arise from faulty boards. A detailed list of resources is provided at the end of this introductory section. Assessment External This unit of work and the exercises within will prepare the pupils for any electronics questions that appear in the 90minute exam at the end of the course. It will enable all pupils to gain the knowledge and understanding required and give them suitable practice in reasoning and numerical analysis. Internal The internal assessment of this unit requires pupils to carry out a structured assignment. The assignment should take approximately three hours to complete but in some circumstances this can be extended. The pupils will be expected to: complete a specification from a given brief create appropriate diagrams to illustrate the problem parameters produce a graphical representation of a proposed solution perform a computer simulation of the proposed solution build and test the proposed solution evaluate the solution against the original specification. It must be stressed that this is not like the existing main project report. iv tandard Grade Technological tudies: Applied Electronics

5 Example The pump motor in an automatic heating system is designed to be on all the time, day or night, unless it gets too hot in the daytime. Design a system that drives the pump motor constantly except when it is hot during the day. For this assessment problem, pupils would be expected to produce: a limited specification from the information in the brief a system diagram illustrating the parameters diagram(s) showing a proposed solution using modular boards or a discrete component circuit a parts/component list from classroom resources a computer simulation using Crocodile Clips or other suitable simulation software a physical solution to the problem a small written evaluation comparing the performance of the solution to the specification. Further information on the internal assessment can be found in Appendix of the Arrangement Documents: Guidelines for Internal Assessment. tandard Grade Technological tudies: Applied Electronics v

6 Detailed resource list Component electronics systems Low voltage power supply Prototype circuit boards Digital multimeter (nonautoranging) Resistors 00 R 220 R 270 R 390 R K Lightdependent (LDR) ORP 2 Thermistor: ntc (R 25602) Potentiometers 0 K (R ) 00 K (R ) Transistors BC 08 Diodes Lightemitting 5mm red N400 (R 2648) Capacitors 00 µf electrolytic µf bead witches Miniature push (R 33758) Miniature slide (R ) Lamp Holder (R 56489) ME lamp 6 V (R 58672) ME lamp 2 V (R 58620) Buzzer Piezo flying lead (R ) Motor 3 6 volt miniature Relay Miniature 5 V DPDT (R 37698) Integrated circuit 555 timer IC Computer simulation software Crocodile Clips Modular electronic systems Modular circuit board systems E&L, or Alpha systems imulation software Control tudio Logic in electronics Modular circuit board system E&L, or Alpha systems Computer simulation software Crocodile Clips Lowvoltage power supply Logic probe Prototype circuit board Resistors 220 R Diodes Lightemitting 5 mm red Integrated circuits (TTL) vi tandard Grade Technological tudies: Applied Electronics

7 Component Electronic ystems tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

8 Contents Electricity 3 imple Circuits 4 Integrated Circuits: 555 timer 77 2 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

9 Electricity Introduction Electricity is one of the most important forms of energy available to man. It affects everyone s lives in many ways. If you take time to think about your everyday life you will realise that our lives are full of devices that depend upon electricity. These devices depend on the electrical circuits inside them to work. The circuits often change the electrical energy into other forms of energy such as heat, light and sound. In this area of study you will learn how these circuits work and about the different components within them. Electric circuits An electric circuit is a closed loop or network made up of electrical components such as batteries, bulbs, switches and wires. witch Battery Lamp Electric current Electric current is the name given to the flow of negatively charged particles called electrons. electrons Current is measured in amperes, usually referred to as amps (A). Current is the rate of flow of electrical charges (called electrons) through a circuit. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 3

10 Voltage In most circuits a battery or voltage supply is used to drive the electrons through the components. Voltage is measured in volts (V). V _ Current Flow R Resistance All materials conduct electricity. The materials that conduct electricity well are called conductors and those that are poor conductors are called insulators. Metals are good conductors while rubber and glass are good insulators. A good conductor offers very little resistance to the flow of electrical current. In other words, it lets currents flow with very little voltage being applied. Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms (Ω). Electron flow conventional current cientists in the early nineteenth century decided the direction of conventional current flow. It seemed to them that current flowed from the positive side of power supplies to the negative side. It was not until the twentieth century that electrons were discovered and the true direction of current flow was proved. As stated earlier, electric current is the flow of electrons but often it is more useful to consider electric current to flow in the opposite direction. This is called conventional current. o although it is technically wrong, for convenience conventional current will be used in the circuits and calculations throughout this work. V _ Conventional Current R Conventional current flows from positive to negative. One of the main reasons for maintaining this convention is that symbols and other data based on conventional current have become standard. 4 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

11 Batteries and voltage supplies Batteries and voltage supplies are the source of power behind all electrical circuits. Without a power source, electrical circuits will not work. In your work (as in most electronic circuits) all power sources will be lowvoltage this normally means everyday batteries or a lowvoltage power supply. The lowvoltage supplies and batteries will normally supply between three and 2 volts. Electronic components normally work on much lower voltages and so the circuits must be designed carefully. The symbols for batteries and voltage supplies are as follows. ingle battery or cell Multiple batteries or cells Note the positive and negative side of the battery: 6 volts Voltage supply ve ve tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 5

12 Direct current (d.c.) The voltage supplied by batteries or lowvoltage supplies is direct current (d.c.). This is the normal type of supply to lowvoltage circuits. Alternating current (a.c.) supplies are highvoltage usually 230 volts. This is the normal supply in homes and schools. Many portable electric power tools work from 0 volts for safety. Resistors Resistors are basic components in electrical and electronic circuits. They limit the amount of current flowing in circuits or parts of circuits. Resistors are roughly cylindrical and have coloured stripes. They also have connection wires sticking out of each end. The stripes indicate the value of the resistors. The colours represent numerical values according to a special code. 6 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

13 Resistor colour code Resistors are marked with what is known as a resistor colour code. Each band that surrounds the body of the resistor helps identify the value (in ohms) and the tolerance (in per cent). In most resistors only four colour bands are used. The colour code chart for resistors is shown below. The colours are used to represent different numbers, and in this way we are able to tell the value for each digit. First and second colour band Digit Black 0 x Multiplier Brown x 0 Red 2 x 00 Orange 3 x 000 or K Yellow 4 x or 0 K Green 5 x or 00 K Blue 6 x or M Violet 7 ilver means divide by 00 Grey 8 Gold means divide by 0 White 9 Tolerances: brown % red 2% gold 5% silver 0% none 20% tandard values Resistors are supplied in a range of standard values:.0, 2.2, 3.3, 4.7, 5.6, 6.8, 7.5, 8.2 and 9.. These standard values can then be multiplied by 0, 00, 000, and so on. Typical values of resistors are 220 R, 00 K, 680 R, etc. ome other popular sizes are also available, such as 270 R and 390 R. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 7

14 4band resistor colour code layout 4 Band Resistor Colour Code Layout st band st digit 4th band tolerance 2nd band 2nd digit 3rd band multiplier Example If the colours on the above resistor are: st band red 2 nd band violet 3 rd band brown 4 th band gold then using the table on the previous page, the value of this resistor is 270 Ω and its tolerance is 0 per cent. This is worked out as 2 for the red first band, 7 for the violet second band and times 0 for the brown third band. For most purposes you can ignore the tolerance. In the above example the manufacturers guarantee that the resistor will not vary from the marked resistance by more than 0 per cent. ymbol for resistance Although the symbol for ohms is Ω it is often shown as a capital R; that is, 270 ohms can be expressed as either 270 Ω or 270 R. 8 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

15 Using the resistor colour code Using the resistor colour code chart, record the resistance values of the following resistors. Write your answers in your normal report notes/jotter.. 00 R ± 0% blue violet brown silver 2. 3 K9 ± 2% orange white brown gold K ± 0% brown black red gold 4. M2 ± 5% brown black green brown Draw and note the colours of the resistors below. Use colour pencils to show the correct colour bands. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 9

16 Exercises. Using the colourcode chart, determine the colours of the first three bands of the following resistors. No. Value Colour 270 R 2 K K 4 M R 6 50 R 7 82 K R 9 6 K R 390 R 2 2 M 3 82 R R 5 9 K 2. Using the colourcoding code, calculate the values of the following resistors. No. Value First three colour bands red red red 2 yellow violet black 3 grey red red 4 yellow violet orange 5 red red orange 6 orange orange orange 7 green blue brown 8 red violet black 9 grey red brown 0 brown green green brown grey yellow 2 brown black yellow 3 green blue orange 4 brown grey black 5 brown grey green 6 blue grey orange 7 orange orange yellow 8 red red brown 9 grey red black 20 violet brown orange 0 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

17 Diodes Diodes are devices that allow current to flow in one direction only. Current can pass this way only Anode Cathode ymbol for Diode Current will flow through the diode only when the anode (positive side) is connected to the positive side of the circuit and the cathode (negative side) is connected to the negative side of the circuit. Lightemitting diodes A lightemitting diode is a special diode that gives out light when current is flowing though it. LEDs are used as indicators to tell when a circuit (or part or a circuit) is working. You can tell the cathode of an LED as it is the short leg and there is a flat on the plastic casing. ve As with the normal diode, the current can only pass one way. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

18 witches witches are useful input devices (or transducers) that have metal contacts inside them to allow current to pass when then they are touching. There are several ways in which the contacts in mechanical switches can be operated. The main types are pushbutton, toggle, key, slide, magnetic (reed) and tilt. These switches are digital input devices as they can only be on or off. Toggle lide Key Tilt Rocker Reed The switches shown above are all single pole with single or double throws. These are known as PT and PDT switches. The symbols are shown below. 2 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

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20 Microswitches Microswitches are small switches that are useful for detecting motion. They are especially good as sensors and limit switches. Typical systems that use microswitches are traffic barriers and lift systems. The microswitch above has a roller fixed to a lever that detects movement and throws the switch. It has three terminals: common, normally open (NO) and normally closed (NC). The microswitch below is commonly used in schools. 3 NO C 2 NC Like most microswitches, this one can be wired in three ways. C and NO: this is a normal on/off switch. C and NC: this allows current to flow when the switch is not operated. C, NC and NO: when wired like this it acts as a changeover switch. These microswitches are singlepole doublethrow (PDT) switches. 4 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

21 imple Circuits eries circuits The diagram below shows a typical use for an LED circuit, where the LED indicates that the car radio/cassette is on. The diagram also shows a simplified series circuit layout for the LED indicator. The resistor is necessary to protect the LED from drawing too much current and blowing. The diagram below shows the above circuit using the component s symbols. This is called the circuit diagram. witch I 6V LED The components in this circuit are connected in series. This means that they are connected up in a line, one after the other (or end to end). eries circuits are the simplest to deal with as the same current flows through all of the components. The voltage, however, is divided up between the components more of this later. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 5

22 eries circuits and Kirchoff s second law When components are connected end to end (in series) to form a closed loop, the same current flows through all components while the voltage is divided up amongst them. In other words, the sum of voltages dropped across each component (V, V 2 ) is equal to the total voltage supply in the circuit. This is known as Kirchoff s second law: V T = V V 2 V 3 In the example below each bulb is rated at 6 volts and the supply voltage is 8 volts. This shows Kirchoff s second law in practice. Prototype circuit boards 6 V 6 V 6 V 8 V Prototype circuit boards (often called breadboards) are used to build and test circuits. They have the advantage that they are nonpermanent: that is, the components can be moved and used again. This makes it easy to make alterations or corrections to circuits. Once a circuit has been proved on a prototype circuit board it is usually built by a more permanent method on stripboard or printed circuit board (PCB). METALLIC TRIP CONNECTOR tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 5

23 The board above shows four mains sections of connection holes. The two centre areas, separated by a gutter, are where most of the components are placed. The two outer rows are used for the power connections. The uncovered reverse side, seen below, shows how the connection holes are interconnected. The metallic strips connect the middle sections in columns of five, while the two sets of outer rows are connected horizontally. The diagram below shows how most common components can be inserted. Note that the most complicated components are usually connected over the centre gutter. This is especially true for transistors and integrated circuits (ICs). 6 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

24 LIGHT EMITTING DIODE LIDE WITCH VARIABLE REITOR WIRE LINK TRANITOR A B C D E F G H I J LDR ELECTROLYTIC CAPACITOR REITOR Example Build the LED series circuit for the car radio/cassette. It can be built on a prototype circuit board or simulated on computer software such as Crocodile Clips. Circuit diagram witch I 6V 390R or 390 LED tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 7

25 Prototype board circuit layout diagram 6 volts 390 ohms Push witch 0 volts LED Remember to connect the LED the right way round ; that is the short lead (cathode) is connected to the zero volt line or negative battery terminal. The LED should light when the switch is pressed. Example 2 Build the lamp circuit below. It can be built on a prototype circuit board or simulated or computer software such as Crocodile Clips. Circuit diagram lide witch 6V I Lamp 8 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

26 Layout diagram 6 volts lide witc h 0 volts Diode Lamp Remember to connect the diode the right way round ; that is, the negative lead (cathode) is connected to the zero volt line or negative battery terminal. The lamp should light when the switch is moved to the right. Try connecting the diode the other way round to confirm its operation. Computer simulation The two series circuits can be built and simulated in a computer programme such as Crocodile Clips. LED circuit As in the prototype circuit, when the switch is pressed the LED should light. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 9

27 Lamp circuit As in the prototype circuit, the lamp should light when the switch is pressed/moved and it will not light when the diode is reversed. Note: Crocodile Clips uses a rocker switch to represent the action of a slide switch. Digital multimeters The digital multimeter is used to measure voltage, current and resistance. It is very simple to use and easy to read. On/Off Amps 20mA 0A 5 a.c. d.c. Volts 2mA V 200µA 00mV 200k 20k 2k 200Ω Ohms 0A ma VΩ COM To measure d.c. voltage: connect the black lead to the COM socket connect the red lead to the VΩ socket make sure that d.c. is selected move the dial into the voltage (volts) range select a suitable range (always slightly higher than the expected measurement) place the lead probes on the points where the voltage is to be measured. 20 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

28 To measure direct current: connect the black lead to the COM socket connect the red lead to the ma socket make sure d.c. is selected move the dial into the current (amps) range select a suitable range (always slightly higher than the expected measurement) connect the probes to the wire in which the current is to be measured. To measure resistance: connect the black lead to the COM socket connect the red lead to the VΩ socket make sure d.c. is selected move the dial into the resistance (ohms) range select the range (always slightly higher than the expected measurement) connect the probes to the ends of the component being measured. Measuring d.c. voltage Voltage is measured across components or parts of circuits as shown in the circuit diagram below. 6v V This can be done in actual circuits or simulated with Crocodile Clips. Across means in parallel as opposed to series. Parallel circuits will be dealt with later. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 2

29 Practical task Using circuit simulation measure the voltage across all three components in the LED circuit. The voltage across each component is known as the voltage drop across the component. This is the amount of voltage used up or dropped by each. The total voltage dropped in the circuit should equal the total supply voltage as stated in Kirchoff s second law. Record your results. Measuring direct current Current is measured through components or parts of circuits, as shown in the circuit diagram below. Note that it is necessary to break the circuit and connect the meter in series with the components. 6V A 22 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

30 Practical tasks. Using circuit simulation, measure the current flowing through all three components in the LED circuit. In a series circuit the current flowing through all components is the same. Try placing the meter at different parts of the circuit to prove this. In parallel circuits the same current does not always flow through each component you will find out about this later. Record the current flowing in this circuit. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 23

31 2. Using the prototype LED circuit, measure the voltages (potential difference, p.d.) across each of the components. 6 volts Push witc h 390 ohms volts 0 volts LED 0A ma V COM Alter the position of the zero volt connection and measure the current flowing in the circuit. et the meter to ma. 6 volts Push witc h 390 ohms ma 0 volts LED 0A ma V COM Record all results. Measuring resistance When measuring resistance make sure that your circuit is disconnected from the supply, otherwise this will affect the reading. Do not touch the meter probes or the components when measuring, as your own electrical resistance will then be included. 24 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

32 Resistors connected in series As resistors come in standard sizes, they are often connected in series to obtain a specific size that is otherwise unavailable. Practical tasks. Connect two resistors in series on a prototype circuit board and measure the overall resistance. R 0A ma V COM R2 You should find that if R total (or R T ) is the total resistance measured across both resistors then the equation for adding resistances in a series circuit is For three resistors in series R total = R R 2 and so on. R total = R R 2 R 3 2. Using two unknown resistors, measure the resistance of each and calculate R total. Check your answer by measuring R total as shown in the above diagram. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 25

33 Ohm s law You have already found that applying a voltage to a circuit results in a current flowing through the circuit. In the simple Crocodile Clips circuit below, double the voltage and you will see that the current doubles as well. In other words if you double the voltage across a component, the current flow through that component will also double. Thus we can say that the current is proportional to the voltage drop across a resistor. This rule is known as Ohm s law. The rule applies to all metals, provided that their temperature does not change. This relationship gives rise to the Ohm s law formula: R= V I Current (A) Voltage (V) which is more easily remembered as: V = I x R We can use the triangle trick to help transpose this formula. Cover up the quantity that you are trying to find and the other two will be in the form that is needed. I V R 26 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

34 Ohm s law in practice For this exercise a simple series circuit is used. I = V R 6 volts La mp Current 0.06 amps The task is to calculate the resistance of the lamp using Ohm s law. R = V I R = R = 00Ω Tasks. Calculate the total resistance (R total = R R 2 ) and the current flowing through the circuit. You can verify your answer by physical measurement or with Crocodile Clips. witc h 6v 390 ohms LED tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 27

35 2. Using Crocodile Clips measure the current flowing in the LED circuit (you should have done this earlier) and calculate the resistance of the LED. 9 volts 220 ohms Buzzer 240 ohms 0 volts Check your answer by physically measuring the resistance of the resistor and LED with a multimeter. Ω REITOR 0A ma VΩ COM The diagram above shows how to check the resistance of a resistor. A similar technique is used to measure the resistance of the LED. 28 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

36 Worked example: series circuit For the series circuit shown, calculate: (a) the total resistance (R T ) (b) the circuit current (I C ) (c) the potential difference across both resistors (V and V 2 ) c (a) R = R R T 2 = 6 8 R = 24Ω T (b) V = I R C T I = C V R T 2 = 24 I = 0.5 A C (c) We can use Kirchoff s second law to check the answers calculated for the potential difference across the resistors: V = I R 2 C 2 = V = 9 V 2 V = V V V T 2 T = 3 9 = 2 V tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 29

37 Exercises: resistors in series. For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current. 2V 2. For the circuit shown below calculate: (a) the total resistance (b) the circuit current (c) the voltage drop across each resistor. (d) Use Kirchoff s second law to verify your answers to (c). 3. For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current. 6V 24V 4. A circuit has three resistors in series. Their values are 5 R, 24 R and 60 R. Calculate the total resistance of the circuit. 5. Two resistors are connected in series. Their values are 25 R and 75 R. If the voltage drop across the 25 R resistor is 4 volts, determine the circuit current and the supply voltage. 30 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

38 Parallel circuits Parallel circuits are circuits where there is more than one path for electricity to flow along or that have more than one branch. Each branch receives the supply voltage, which means that you can run a number of devices from one supply voltage. A good example of a simple parallel circuit is a set of Christmastree lights where all the bulbs require a 230 volt supply. This arrangement ensures that if one or two bulbs blow then the rest of them continue to function and, importantly, you know which are faulty. In a series circuit if one bulb blew then all the bulbs would go out and you would have to test them all to see which one was faulty. Parallel circuits can be arranged in many ways, but are normally set out so that you can easily see the parallel branches. A simple parallel caralarm circuit is shown below with the switches wired up in parallel. 2 volts The two switches in parallel represent the sensor switches connected to the doors. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 3

39 Resistors in parallel As resistors come in standard sizes, they are often connected in parallel to obtain a specific size that is unavailable. This practice of combining resistors has already been seen in series circuits. Practical tasks. Connect two resistors in parallel on a prototype circuit board and measure the overall resistance. R 0A ma V COM You should find that if R total (or R T ) is the total resistance measured across both resistors then the equation for adding resistances in a parallel circuit is R = R R T 2 For three or more resistors, the equation can be extended: R = R R R T Using two unknown resistors, measure the resistance of each and calculate R total when the resistors are connected in parallel. Check your answer by measuring the total resistance as shown in the above diagram. 32 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

40 As stated earlier, each branch of a parallel circuit receives the supply voltage. Each branch has its own current; that is, when the current reaches a junction it splits up, with some current flowing into each branch. The total supply current is therefore the sum of the currents flowing in the branches. When resistors or resistive components are connected in parallel, the effect is to reduce the resistance in the circuit. I I T I T I 2 There are two important points to remember about resistors in parallel. (a) The voltage drop across each resistor is the same. (b) The sum of the currents through each resistor is equal to the current flowing from the voltage source. pecial case: two resistors in parallel There is a special rule that can be applied when adding two resistors in parallel only: total resistance (R T ) = product/sum. R = T R R x R R 2 2 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 33

41 Worked examples: resistors in parallel. For the circuit below, calculate the total resistance of the parallel part of the circuit and the total resistance in the circuit. R R 2 2 volts The resistance values are R = 270 R, R 2 = 00 R and for the buzzer 240 R. 34 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

42 R R R For the circuit below, calculate the total resistance of the resistors in parallel. R = R R T 2 R = T R T = x 00 For the series part R = T R = T T R = 73Ω R = R R T 2 R = T R = 33Ω T The resistance values are R = 220 R, R 2 = 00 R and R 3 = 330 R. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 35

43 3. For the parallel circuit shown calculate: R = R R R T 2 3 R = T R T = R = T R = T R = 57Ω T (a) the total resistance (R T ) (b) the circuit current (I C ) (c) the current in each resistor (I and I 2 ). c s (a) R = R R T R R = = 20 R = 4.8Ω T 2 2 (b) V = I R C T V I C = R T = I = 2.5 A C 36 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

44 (c) I = V R 2 = 8 I =.5A We can use Kirchoff s second law to check the answers calculated for the current in each branch. I = 2 V R 2 = 2 I = A 2 I = I I I C 2 C =.5 = 2.5 A tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 37

45 Exercises: resistors in parallel. For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current. 9V 2. For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current (c) the current flowing though R (0 R) (d) the current flowing through R 2 (24 R). 3. For the circuit shown below calculate: (a) the total resistance of the circuit (b) the circuit current (c) the current flowing through R (660 R)4 (d) the current flowing through R 2 (470 R). Use Kirchoff s second law to verify your answers to parts (c) and (d). 240 V 4. A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 2 V. Calculate the circuit current. 38 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

46 5. A 440 R resistor is connected in parallel with a 330 R resistor. The current through the 440 R resistor is 300 ma. Find the current through the 330 R resistor. Combined series and parallel circuits Until now we have been looking at series or parallel circuits individually. It is possible, and quite common, to have series and parallel connections in the same circuit. Consider the combined series and parallel circuit shown in the figure below. You can see that R 2 and R 3 are connected in parallel and that R is connected in series with the parallel combination. ome points to remember when you are dealing with combined series and parallel circuits are: the voltage drop across R 2 is the same as the voltage drop across R 3 the current through R 2 added to the current through R 3 is the same as the current through R the voltage drop across R added to the voltage drop across R 2 (which is the same as across R 3 ) would equal the supply voltage V s. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 39

47 Worked example: combined series and parallel circuits. For the combined series and parallel circuit shown, calculate: (a) the total circuit resistance (R T ) (b) the circuit current (I C ) (c) the voltage drop across resistor R (V R ) (d) the current through resistor R 2 (I 2 ). 24R 48R 0R 2V (a) In the first instance you must calculate the equivalent resistance of the parallel arrangement (R P ) of R 2 and R 3. It is possible to use the special case formula for two resistors in parallel: R P = R R 2 2 R R 3 3 R P 0 48 = 0 48 R P = R P = 8.28Ω The total circuit resistance (R T ) is then found by adding R P to R : R T = R R P R T = R T = 32.28Ω 40 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

48 (b) It is now possible to calculate the circuit current: V = I C R T I C V = R T I C = I C = 0.37A (b) The voltage drop across R is found by using the resistance across and the current through R. V = I R V R = I C R V R = V R = 8.88V (c) The current through R 2 is found by using the resistance of R 2 and the voltage drop across R 2. By using Kirchoff s second law we know that the voltage drop across the parallel arrangement must be: V = V R V P V P = V V R V P = V P = 3.2V tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 4

49 By using Kirchoff s first law we know that the circuit current I C will split or divide between the two resistors R 2 and R 3. In order to find the current through R 2 we use: V = I R V P = I 2 R 2 I 2 = V R P 2 I 2 = I 2 = 0.32A 42 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

50 Exercises: combined series and parallel circuits. For the circuit shown calculate: (a) the resistance of the parallel combination (b) the total circuit resistance. 7.5 V 2. For the circuit shown calculate: (a) the total resistance (b) the circuit current (c) the voltage drop across each resistor. 24 V 3. For the circuit shown calculate: (a) the total resistance of the circuit (b) the circuit current (c) the current through each resistor (d) the voltage drop across each resistor. 0 V tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 43

51 Power in electric circuits Electrical power is measured in watts (W). Electrical power can be converted into other forms of power using electric circuits. For example the power used in overcoming electrical resistance can be converted into heat this is the principle of an electric fire. The power in an electric circuit depends both on the amount of current (I) flowing and the voltage (V) applied. The formula for power in electric circuits is: Power = Voltage x Current (watts) P = V x I (W) Worked example An electric household lamp consumes 60 watts from a 230 volt supply. Calculate the current drawn by the lamp and the resistance of the lamp. P = V I I = P V I = I = A (or 250 ma) V = I R R = V I R = R = 960Ω 44 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

52 Exercises: power in electric circuits. In the following simplified circuit for a vacuum cleaner motor, calculate: (a) the power consumption of the motor (b) the voltage of the lamp (c) the total power drawn from the power supply. 3 amps 240 volts M 200 volts 2. The torch circuit below is supplied with two 4.5 volt batteries connected in series, with the current being 20 ma. Determine: (a) the resistance of the bulb (b) the voltage across the bulb (c) the total power drawn from the supply (d) the power drawn by the bulb. witc h 00 ohms Bulb tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 45

53 For many of the following exercises it would be useful to draw the circuit diagram. 3. An electric iron rated at 800 W is connected to a 230 V supply. Calculate the maximum current drawn by the iron. What is the power used by the iron at halfheat setting? 4. A kettle and a toaster use the same double socket. If the kettle draws a current of 0 A and the toaster 3 A, find the power used by each of the appliances. The two sockets are wired in parallel to a 230 V supply. 5. An electric drill draws a current of.5 amps from a 0 volt supply. Calculate the power rating of the drill. 6. An emergency power generator has to drive 80 lamps. Each lamp takes 60 W at 230 V. Calculate the current through each bulb if: (a) they are connected in series (b) they are connected in parallel. 7. How many 50 W lamps can be connected in parallel to a 250 V supply through a 5 A fuse? 8. In a stereo system each of the speakers is rated as 5 W per channel. If the supply voltage is 230 V, find the current drawn by each speaker when the system is fully on. Assume that the speakers are connected in parallel. If the system uses 2 W in wasted heat energy in normal conditions at full power, what is the current drawn from the supply and what fuse would you recommend for the mains plug? 9. The power ratings for the lamp bulbs on a car with a 2 V battery are as follows: headlamps 60 W indicators 24 W sidelamps 6 W. Find the current drawn by each lamp and the resistance of each bulb. 0. The rear screen heater in a car is connected to the 2 V system and draws a current of 2 A. Find the resistance of the circuit. In reality the 2 V, 0.5 A interior light is on the same circuit. tate whether this is a parallel or series circuit and calculate the power and current when both lamp and heater are on. 46 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

54 Voltage divider circuits Input transducers 0 V Input transducers are devices that convert a change in physical conditions (for example, temperature) into a change in resistance and/or voltage. This can then be processed in an electrical network based on a voltage divider circuit. If two or more resistors are connected in series (see diagram below), the voltage over each resistor will depend on the supply voltage and the ratio of the resistances. Voltage divider circuits work on the basic electrical principle that if two resistors are connected in series across a supply, the voltage load across each of the resistors will be proportional to the value of the resistors. The layouts of voltage divider circuits are conventionally represented as shown above. A voltage divider circuit can be represented in a number of different ways. ome of these are shown below. 0 V 0 V tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 47

55 Voltage divider circuit If an input transducer changes its resistance as the physical conditions change, then the resistance change has to be converted into a voltage change so that the signal can be processed. This is normally done using a voltage divider circuit. A typical voltage divider circuit is shown below. V R R 2 V 0 volts As you can see, this circuit consists basically of two resistors connected in series. As you already know, if you change the value of R, the voltage across it will change, as will the voltage across R 2. In other words, the resistors divide the voltage up between them. Practical task Use a prototype board to build the voltage divider circuit shown below. If the supply voltage is 6 volts, R = 220 Ω and R 2 = 330 Ω, what is the voltage across R 2? V Volts R 0A ma V COM R 2 0 V The voltage across R 2 is normally called the output or V o. You should find that the voltage is divided up according to the formula V = O R 2 R R 2 V 48 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

56 Worked example: voltage divider circuit Calculate the voltage signal V 2 across the resistor R 2 in the voltage divider circuit below. V = 2 volts R = 80k R = 40k 2 V 2 0 volts Applying the voltage proportion formula: V = V 2 V = 2 2 V = 4 volts 2 R 2 R The voltage over the 80 K resistor could be calculated in the same way, but this is unnecessary for this circuit since we can use Kirchoff s second law to confirm the answer. The voltages over each of the components in a series circuit must add up to the supply voltage, hence the voltage over the 80 K resistor is 2 V 4 V = 8 V. It is also possible to use Ohm s law to solve these voltage divider problems. R tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 49

57 Exercises: Voltage divider circuits. Calculate the voltage signal V 2 across the resistor R 2 in the voltage divider circuit below. V = 2 volts R = 270R R = 80R 2 V 2 0 volts 2. Calculate the voltage signal V 2 across the resistor R 2 in the voltage divider circuit below. V = 2 volts R = 390R R = 0K 2 V 2 0 volts 50 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

58 3. Calculate the voltage signal V 2 across the resistor R 2 in the voltage divider circuit below. V = 6 volts R = 0K R = 47K 2 V 2 0 volts 4. Calculate the voltage signal V 2 across the resistor R 2 in the voltage divider circuit below. V = 9 volts R = 0K R = 2.2K 2 V 2 0 volts tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 5

59 imple switches can be used in voltage divider circuits to give a digital signal (that is definitely ON or OFF) to another part of a circuit. In the example below a normally closed switch is used. When the switch is pressed, the voltage divider comes into use and power is supplied to the LED to give a definitely ON signal. Build the circuit in Crocodile Clips to test this. Digital switch types Different types of switch were described earlier but they can be wired up to suit their application. A switch with its contacts apart when it is not operating is called normally open. Doublepole switch symbols Doublepole singlethrow switch (DPT) Doublepole doublethrow switch (DPDT) 52 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

60 Analogue input transducers The two most common analogue input transducers are the thermistor and the lightdependent resistor (LDR). Thermistor A thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types.. Negative temperature coefficient ( t or NTC) where resistance decreases as temperature increases. 2. Positive temperature coefficient (t or PTC) where resistance increases as temperature increases. The circuit symbols for and typical characteristics of the two types of resistor are shown below. NTC thermistors are the most commonly used. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 53

61 Graph of temperature versus resistance The graph below shows accurately how the resistance varies with temperature for an NTC thermistor. Thermistor types train gauges train gauges are really load sensors. They consist of a length of resistance wire and when stretched their resistance changes. train gauges are attached to structural members (beams, etc.) and as they are loaded, a reading on a voltmeter can be obtained. train gauge Lightdependent resistor (LDR) The LDR (sometimes called a photoresistor) is a component whose resistance depends on the amount of light falling on it. Its resistance changes with light level. In bright light its resistance is low (usually around K). In darkness its resistance is high (usually around M). 54 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

62 The circuit symbol and typical characteristics of an LDR are shown above. Graph of illumination versus resistance The graph below shows accurately how the resistance varies as the amount of illumination falling on an LDR varies. Voltage divider circuits One of the main purposes of the voltage divider circuits is to sense and process inputs from analogue sensors. In this example a thermistor will be used. The resistor R 2 of the previous circuit has been replaced by an NTC thermistor. V = 9 volts t V O 0 volts tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 55

63 Practical task Build the above circuit and measure and record the output voltage (V o ) at room temperature and at a higher temperature (use your fingers to warm up the thermistor). V Volts R 0A ma V COM NTC Thermistor 0 V Measure the voltage drop and the resistance of the thermistor at both these temperatures. Check your results by calculation and record them in a simple table. NTC thermistor Voltage Resistance Low temperature High temperature This voltage divider circuit uses a lightdependent resistor or LDR. The LDR replaces R 2 from the basic voltage divider circuit. V = 9 volts 0K ORP2 V O 0 volts 56 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

64 V Volts R 0A ma V COM LDR 0 V Practical task 2 Build the above circuit and measure and record the output voltage (V o ) at normal room light conditions and when the LDR is covered (use your finger or a coin to do this). Measure the voltage drop and resistance of the LDR at both light states. Check your results by calculation and record these in a simple table. LDR Voltage Resistance Light Dark Variable resistor (potentiometer) A potentiometer configured as a variable resistor can be used in a circuit as a voltage or current control device. They are often used in voltage divider circuits to adjust the sensitivity of the input. In tandard Grade Technological tudies the majority of applications will use a variable resistor or a potentiometer configured as a variable resistor. Potentiometers normally have three tags or terminals. The outer ones are connected to the ends of the resistive material and the centre one is connected to the wiper. The spindle of the potentiometer is connected to the wiper, which is able to traverse the whole of the resistive material. As the spindle rotates, a sliding contact puts more tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 57

65 or less resistive material in series with the circuit. In this way the resistance in a voltage divider circuit is varied. Miniature potentiometers Most modern circuits now use miniature potentiometers or variable resistors. Examples of miniature potentiometers (not to scale) Voltage divider circuits The LDR voltage divider circuit can be set up to detect when it is light or when it is dark. V = 9 volts V = 9 volts 0K ORP2 ORP2 V O 0K V O 0 volts Detects when dark Detects when light The above circuits should be simulated on Crocodile Clips to confirm their operation. 58 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

66 ensitivity With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit. Rather than using a fixed resistor we can replace it with a variable resistor (or potentiometer). V = 9 volts ORP2 47K V O 0 volts Practical task: voltage divider circuits The picture below shows a typical situation where a light sensor circuit could be useful. To save money and inconvenience the residents want the outside light to come on when it gets dark. They also want to be able to adjust the sensitivity from summer to winter nights. Build the following circuit using a prototype circuit board. The variable resistor is rated at 0 kω. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 59

67 9 volts VR 0 volts LDR Adjust the sensitivity so that the output voltage (V o ) goes higher when your hand is moved across the LDR at a distance of approximately 00 mm. You will have to attach a multimeter to the circuit to see when this is happening. Check this out using Crocodile Clips. V = 9 volts 0K ORP2 V 0 volts 60 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

68 Exercises: voltage divider circuits. Calculate the voltages that would appear across each of the resistors marked X in the circuits below. 9 V 5 V 0 V 0 V 2. In each of the following voltage divider circuits determine the unknown quantity. 2 V 6 V 2 V 0 V 0 V 0 V 5 V 20 V 220R 0 V 0 V 0 V tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 6

69 3. An NTC (negative temperature coefficient) thermistor is used in a voltage divider circuit as shown below. Using information from the graph shown, determine the resistance of the thermistor and hence calculate the voltage that would appear across it when it is at a temperature of: (a) 80 C (b) 20 C. 4. What would happen to the voltage across the thermistor in the circuit shown above as the temperature increased? 5. What would happen to the voltage across the resistor in the circuit shown above as the temperature increased? 6. A thermistor (type 5) is used in a voltage divider circuit as shown below. The characteristics of the thermistor are shown in the graph. If the voltage V 2 is to be 4.5 V at 00 C, determine a suitable value for R. tate whether V 2 will increase or decrease as the temperature drops. Explain your answer. 62 tandard Grade Technological tudies: Applied Electronics Component Electronic ystems

70 Transistors The first major breakthrough in electronics came with the invention of the diode valve at the beginning of the twentieth century. This was the first real electronic component and was to lead to the modern diode and transistor. A diode valve consisted of a heater inside a hollow rod that had been coated with a substance which released electrons when heated. This was surrounded by a thin metal cylinder, with all of this being contained in a bulblike glass container. When the rod was heated, electrons were released but, as in any diode, the electrons could only go in one direction. The diode was followed by the triode, which allowed the current flow to be controlled. These valves could act as electronic switches or amplifiers. Radio and television were developed using these amplifier valves. In the 940s the first computer was built using valves it contained over 20,000 valves and filled a large room. In 947 the transistor was invented. The transistor had many advantages over valves, the main ones being size, efficiency, durability and cost. The next big advance in electronics was the integrated circuit in 958: two transistors were fitted on a silicon chip. The developments since then have been rapid and chips now contain over a million transistors. tandard Grade Technological tudies: Applied Electronics Component Electronic ystems 63