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Component Electronic Systems Electricity o Electricity is an important form of energy o Our lives are full of devices that depend upon electricity and have electrical circuits inside them to work. o These circuits change electrical energy into other forms of energy such as heat, light and sound. Switch Electric Circuits An electric circuit is a closed loop or network made up of electrical components such as batteries, bulbs, switches and wires. Battery Lamp Electric current o Negatively charged particles flow through a conductor. o Negatively charged particles are called electrons. o Current is measured in amperes, usually referred to e le c tr o n s as ‘amps’ (A). o Current is the rate of flow (speed) of electrons through a circuit. Voltage o A battery or voltage supply (Vs) provides the energy used to drive the electrons around the circuit. o The force which drives the electrons is called the Electro Motive Force or EMF. o Voltage is measured in volts (V). Standard Grade Technological Studies: Component Electronics Revision Notes 1 Resistance o All materials conduct electricity to some extent. o Materials that conduct electricity well are called conductors. o Poor conductors are called insulators. o Metals are good conductors while rubber and glass are good insulators. o A good conductor has very little resistance to the flow of electrical current. o A good conductor lets currents flow through with very little EMF being applied. o Resistance is a measure of how much voltage (energy) is required to make a current flow. e.g. 1 Volt will force a current of 1 Amp through a resistance of 1 Ohm. o Resistance is measured in ohms (). Electron flow conventional current o Early nineteenth century scientists decided the direction of conventional current flow. o Conventional Current flows from the positive side of power supplies to the negative side. o Twentieth century scientists discovered electrons and the true direction of current flow was proved. o True Current flowed from the negative side of power supplies to the positive side For convenience ‘conventional current’ is used in the circuits and calculations because: o Conventional current flows from positive to negative and this is easy to understand. o Symbols and other data based on conventional current have become standard. + V 2 _ Conventional Current Standard Grade Technological Studies: Component Electronics Revision Notes R Batteries and voltage supplies o Batteries and voltage supplies are the source of power behind all electrical circuits. o Without a power source, electrical circuits will not work. o In most electronic circuits power sources are low-voltage. (up to 12V) o Electronic components normally work on much lower voltages and so the circuits must be designed carefully. Symbols for batteries and voltage supplies: Single battery or cell Positive and negative side of the battery: Multiple batteries or cells -v e +v e 6 volts Voltage supply Direct current (d.c.) o Voltage supplied by batteries or low-voltage supplies is direct current (d.c.). o The normal type of supply to low-voltage circuits is d.c. o Alternating current (a.c.) supplies are high-voltage usually 230 volts. o The normal supply in homes and schools is a.c.. o Many portable electric power tools work from 110 volts for safety. Resistors o Resistors limit the amount of current flowing in circuits or parts of circuits. o Resistors are roughly cylindrical and have coloured stripes. o Resistors have connection wires sticking out of each end. o The stripes indicate the value of the resistors. The colours represent numerical values according to a special code. Standard Grade Technological Studies: Component Electronics Revision Notes 3 Resistor colour code o Resistors are marked with a colour code. o Each band that surrounds the body of the resistor helps identify the value (in ohms) and the tolerance (in per cent). o In most resistors only four colour bands are used. The colour code chart for resistors: First and second colour band Digit Multiplier Black 0 x1 Brown 1 x 10 Red 2 x 100 Orange 3 x 1000 or 1 K Yellow 4 x 10 000 or 10 K Green 5 x 100 000 or 100 K Blue 6 x 1 000 000 or 1 M Violet 7 Silver means divide by 100 Grey 8 Gold means divide by 10 Tolerances: White 9 brown 1% red 2% gold 5% silver 10% none 20% Standard values o Resistors are supplied in a range of standard values: 1.0, 2.2, 3.3, 4.7, 5.6, 6.8, 7.5, 8.2 and 9.1. o Standard values can then be multiplied by 10, 100, 1000, and so on. o Typical values of resistors are 220 R, 100 K, 680 R, etc. o Some other popular sizes are also available, such as 270 R and 390 R. 4 Standard Grade Technological Studies: Component Electronics Revision Notes Resistor Colour Code layout for 4-band Example If the colours on the above resistor are: 4 Band Resistor Colour Code Layout 1st band red 2nd band violet 3rd band brown 4th band gold 1st band 1st digit 4th band tolerance Using the resistor coding table the value of this resistor is 270 and its tolerance is 10 2nd band 2nd digit per cent. 3rd band multiplier This is worked out as: o ‘2’ for the red first band, o ‘7’ for the violet second band and o ‘times 10’ for the brown third band. Symbol for resistance The symbol for ohms is ‘’ it is often shown as a capital R; that is, 270 ohms can be expressed as either 270 or 270 R. Diodes Current can pass this way only Diodes are devices that allow current to flow in one direction only. Current will flow through the Anode Cathode Symbol for Diode diode only when: o The anode (positive side) is connected to the positive side of the circuit o The cathode (negative side) is connected to the negative side of the circuit. Standard Grade Technological Studies: Component Electronics Revision Notes 5 Light-emitting diodes o A light-emitting diode is a special diode that gives out light when current is flowing though it. -ve o LEDs are used as indicators to tell when a circuit is working. o The cathode (-ve) of an LED as it is the short leg and there is a ‘flat’ on the plastic casing. o As with the normal diode, the current can only pass one way. Switches o Switches are useful input devices (or transducers) that have metal contacts inside them to allow current to pass when then they are touching. o The main types are push-button, toggle, key, slide, magnetic (reed) and tilt. o Switches are ‘digital’ input devices as they can only be on or off. The switches shown are all single pole with single or double throws. These are known as SPST and SPDT switches. The symbols are shown below. These can be wired using terminal combinations in another switch called a double –pole Double-throw (DPDT) 6 Toggle Slide Key Tilt Rocker Reed Standard Grade Technological Studies: Component Electronics Revision Notes Microswitches o A small switch useful for detecting motion. o Good as sensors and limit switches. o Typical systems that use microswitches are traffic barriers and lift systems. Description: o Has a roller fixed to a lever that detects movement and throws the switch. o Has three terminals: common, normally open (NO) and normally closed (NC). Wiring: o Microswitches can be wired in three ways. 1. Common (C) and NO: this is a normal on/off switch. 2. Common (C) and NC: this allows current to flow when the switch is not operated. 3. Common (C), NC and NO: when wired like this it acts as a changeover switch. These microswitches are single-pole double-throw (SPDT) switches. Input transducers 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. Strain gauge Analogue input transducers The two most common analogue input transducers are the thermistor and the lightdependent resistor (LDR). Standard Grade Technological Studies: Component Electronics Revision Notes 7 Thermistor A thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types. 1. 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. NTC thermistors are the most commonly used. 8 Standard Grade Technological Studies: Component Electronics Revision Notes Strain gauges Strain gauges are load sensors: A length of resistance wire When stretched their resistance changes. Strain gauges are attached to structural members (beams, etc.) As they are loaded, a reading on a voltmeter can be obtained. Strain gauge Light-dependent 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 1 K). In darkness its resistance is high (usually around 1 M). The circuit symbol and typical characteristics of an LDR are shown above. Standard Grade Technological Studies: Component Electronics Revision Notes 9 Variable resistor (potentiometer) A potentiometer (variable resistor) can be used as a voltage or current control device. It is used in voltage divider circuit to adjust the sensitivity of the input. 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 or less resistive material in series with the circuit. In this way the resistance in a voltage divider circuit is varied. Miniature potentiometers Modern circuits now use miniature potentiometers or variable resistors. Examples of miniature potentiometers (not to scale) 10 Standard Grade Technological Studies: Component Electronics Revision Notes Simple Circuits Switch I Series circuits The diagram shows a circuit diagram. 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). 6V LED Series circuits and Kirchoff’s second law When components are connected end to end (in series) to form a closed loop: o The same current flows through all components o The voltage is divided up amongst them. Kirchoff’s 2nd Law: 6V 6V 6V The sum of voltages dropped across each component is equal to the total voltage supply in the circuit. VT = V1 + V2 + V3 + … 18 V Parallel circuits Parallel circuits are circuits where: o There is more than one path for electricity to flow along. o Each branch receives the supply voltage o You can run a number of devices from one supply voltage. This arrangement ensures that: o If one or two bulbs ‘blow’ then the rest of them will continue to function o You know which are faulty. Note: 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. 12 volts Parallel circuits are normally set out so that you can easily see the parallel ‘branches’. Standard Grade Technological Studies: Component Electronics Revision Notes 11 Practical Electronics Circuits can be physically built on o Prototype boards o Strip board o PCB (Printed Circuit Board) o Computer Simulation Prototype circuit boards Prototype circuit boards (often called breadboards) They have the advantage that: o They are non-permanent o Components can be moved and used again. o Alterations or corrections are easy to make. Once a circuit has been proved on a prototype circuit board it is usually built by a more permanent method on strip board or printed circuit board (PCB). LIGHT EMITTING DIODE There are four mains sections of connection holes. o The two centre areas, separated by a gutter, are where most of the components are placed. o The two outer rows are used for the power connections. VARIABLE RESISTOR SLIDE SWITCH WIRE LINK 1 5 10 15 20 25 555 TRANSISTOR 1 5 10 The diagram shows how some common components can be inserted. The most complicated components are usually connected over the centre gutter. This is especially true for transistors and integrated circuits (ICs). ELECTROLYTIC CAPACITOR 12 A B C D E 15 20 25 F G H I J LDR RESISTOR Standard Grade Technological Studies: Component Electronics Revision Notes Computer simulation Series circuits can be built and simulated in a computer programme such as Crocodile Clips. The computer calculates circuit values for voltage, resistance and current. It makes the components appear to work or not work according to these values. LED circuit As in the prototype circuit, when the switch is ‘pressed’ the LED should light. 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. Making Electrical Measurements 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 To measure d.c. voltage: o o o o o a.c. d.c. Volts 10A 50V 10V 1V 2mA connect the black lead to the ‘COM’ socket connect the red lead to the ‘V’ 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) 100mV 200A 200k 20k 200 2k Ohms 10A mA V COM Method: 1. Place the lead probes on the points at each side of the place where the voltage is to be measured. 2. Voltage is measured across components or parts of circuits as shown in the circuit diagram below. V 6v ‘Across’ means in ‘parallel’ Standard Grade Technological Studies: Component Electronics Revision Notes 13 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. 6V A Method: 1. Current is measured through components or parts of circuits, as shown in the circuit diagram below. 2. It is necessary to ‘break’ the circuit and connect the meter in series with the components. To measure resistance: o o o o o o Connect the black lead to the ‘COM’ socket Connect the red lead to the ‘V’ 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. 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. R1 Resistance in Series and Kirchoff’s 2nd Law: The total resistance measured across both resistors shown in the diagram will equal the sum of their individual resistances. 10A mA V COM R2 Rtotal = R1 + R2 For three resistors in series Rtotal = R1 + R2 + R3 14 Standard Grade Technological Studies: Component Electronics Revision Notes Ohm’s law in Series Circuits The current is proportional to the voltage applied. e.g. If the Voltage increases the Current also increases. 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: Current (A) By applying a voltage to a circuit a current flows through the circuit. Voltage (V) R = V I The formula can also be expressed 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. V I R Ohm’s law in practice For this exercise a simple series circuit is used. The task is to calculate the resistance of the lamp using Ohm’s law. V I 6 R = 0.06 6 volts Lamp R = R = 100 Standard Grade Technological Studies: Component Electronics Revision Notes Current 0.06 amps 15 Ohm’s Law with Resisters in Parallel Special cases: Two resistors in parallel There is a special rule that can be applied when adding two resistors in parallel only: total resistance (RT) = product/sum. R R 1x 2 R T= R + R 1 2 More Than Two Resistors in Parallel The following rule can be used to calculate the total resistance Combined series and parallel circuits It is possible and quite common, to have series and parallel connections in the same circuit. You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the parallel combination. 16 Standard Grade Technological Studies: Component Electronics Revision Notes Some points to remember with combined series and parallel circuits o The voltage drop across R2 is the same as the voltage drop across R3 o The current through R2 added to the current through R3 is the same as the current through R1 o The voltage drop across R1 added to the voltage drop across R2 (which is the same as across R3) would equal the supply voltage Vs. Power in electric circuits o Electrical power is measured in watts (W). o Electrical power can be converted into other forms of power using electric circuits. E.g. the power used in overcoming electrical resistance can be converted into heat – this is the principle of an electric fire. o The power in an electric circuit depends both on the amount of current (I) flowing and the voltage (V) applied. o The formula for power in electric circuits is: Power = Voltage x Current (watts) P = V x I (W) Standard Grade Technological Studies: Component Electronics Revision Notes 17 Voltage divider circuits If two or more resistors are connected in series the voltage drop over each resistor will depend on the supply voltage and the ratio of resistances. the Voltage divider circuits work on the basic principle that: If two resistors are connected in series across a supply, the voltage drop across each of the resistors will be proportional to the value of the resistors. The formulae shown on the left illustrate how the calculations can be made to find the voltage drop over each resistor. Note that the voltage drop over R2 will have the same value as Vout. For the circuit shown above, two resistors connected in series. If you change the value of R1, the voltage drop across it will change. A corresponding voltage drop change will occur in R2. The sum of the voltage drops over R1 and R2 will equal the supply voltage. In other words, the resistors divide the voltage up between them. 18 Standard Grade Technological Studies: Component Electronics Revision Notes A voltage divider circuit can be represented in a number of different ways. Some of these are shown below. 0V 0V Simple 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 on the right 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. Digital switch types A switch with its contacts apart when it is not operating is called normally open. Double-pole switch symbols Standard Grade Technological Studies: Component Electronics Revision Notes 19 Using Voltage Divider circuits With Transducers The main purpose of the voltage divider circuits is to sense and process inputs from analogue sensors. VS = 9 volts 1 In this example the resistor R2 of the previous circuit has been replaced by an NTC thermistor. -t VO 0 volts Voltage Divider circuits using LDR’s The LDR voltage divider circuit can be set up to detect when it is light or when it is dark. VS = 9 volts VS = 9 volts ORP12 10K 10K ORP12 VO VO 0 volts Detects when dark Detects when light Adjusting Sensitivity in Potential Divider circuits VS = 9 volts 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). ORP12 47K VO 0 volts 20 Standard Grade Technological Studies: Component Electronics Revision Notes Practical Example: voltage divider circuits The picture below shows a typical situation where a light sensor circuit could be useful. Residents want an outside light to come on when it gets dark. They also want to be able to adjust the sensitivity from summer to winter nights. Transistors There have been generations of electronic development: Diode Valves (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 bulb-like glass container. When the rod was heated, electrons were released but, as in any diode, the electrons could only go in one direction. Triode Valve The triode valve allowed the current flow to be controlled. These valves could act as electronic switches or amplifiers. Radio and television were developed using these amplifier In the 1940s the first computer was built using valves it contained over 20,000 valves and filled a large room. valves. Transistors (1947) The transistor had many advantages over valves: Small size Efficiency Durability Cost. Integrated Circuits (1958 ……) Two transistors were fitted on a silicon chip. The developments since then have been rapid and chips now contain over a million transistors. Standard Grade Technological Studies: Component Electronics Revision Notes 21 Transistors (bipolar) The bipolar transistor is a semiconductor device. This means that it is sometimes a good conductor of electricity and sometimes a poor one. A transistor is made up of three layers of semiconductor materials that are either ‘n type’ or ‘p type’. There are two types of bipolar transistor available: pnp or npn. The voltages and currents should be reversed for a pnp transistor. Action of a Transistor: A small voltage of about 0.6v is applied across the base and emitter. The resistance between the collector and the emitter of the transistor drops from very high to very low. The transistor changes from being a very poor to a very good conductor. When the voltage reaches 0.7 volts the transistor is fully ‘switched on’. The transistor is said to be fully ‘saturated’. General-purpose transistor The BC 108 is common general-purpose transistor. The diagram below shows the position of the legs when viewed from underneath the case. The transistor has to be connected into circuits correctly. The arrowhead on the emitter indicates the direction of ‘conventional’ current flow. 22 Standard Grade Technological Studies: Component Electronics Revision Notes How does the transistor work? Consider the circuit shown. When the switch S1 is open, no current can flow in any part of the circuit. The transistor is acting like a barrier to current flow (insulator). 9V When switch S1 is closed, a very small voltage is applied to the base of the transistor. When this happens the transistor allows current to flow through it and the bulb will light; the transistor is said to ’switch on’. Bipolar transistors amplify current. A small current flowing through the base of a transistor causes a much larger current to flow from the collector to the emitter. The transistor as a switch One of the main uses of a transistor is that of a very sensitive switch. The example shows the transistor used as an electronic switch activated by changes in light. The transistor switches on and in turn activates the relay. Standard Grade Technological Studies: Component Electronics Revision Notes 23 Transistor circuits calculations + 6 volts The transistor circuit on the right is basically a parallel circuit. If the circuit is rearranged slightly this becomes obvious. 6V 60mA Lamp c Ic b 1K BC 108 e c 6 volts T x e 0V Ie b Rb 0V Ib The transistor (marked T) is at the junction of the parallel circuit. Assume that no voltage drops across the collector/emitter in the transistor then Vxe = 6 volts (in the bulb branch) As the two branches between x and e are in parallel then Vxe across the resistor branch must also be 6 volts. VRb + Vbe = 6 volts We know that Vbe must be 0.7 volts to switch the transistor on; VRb = Vxe –Vbe VRb = 6 – 0.7 VRb = 5.3 volts It is now possible to calculate all other currents and resistance values in other components which may be in the circuit. Relays Often considered to be output devices. They are really output switches for electric or electronic control circuits. These output switches are used as inputs for other circuits. In practice you can hear relays clicking on and off as the control circuit is switched on and off. 24 Standard Grade Technological Studies: Component Electronics Revision Notes How the relay works An electric current flows into the relay coil The coil becomes an electromagnet The electromagnet attracts the armature and moves the contacts. This movement provides the switching, just as the contacts in any other switch do. The relay is very useful because it is a vital link between low energy microelectronics and high-energy systems that require large amounts of current. Relays connections The connections for a typical miniature relay are shown in the diagram below. Connections 1 and 16 are those from the sensing or input circuit. Connections 4 and 13 are the supply voltage to run the output. Connections 6 and 11 are the normally closed output terminals. Connections 8 and 9 are the normally open output terminals. 1 + 16 - 4 6 13 11 8 9 Standard Grade Technological Studies: Component Electronics Revision Notes 25 Relays and protective diodes Relays have an electro magnetic coil that is energised and de-energised as the relay switches on and off. During this process of switching, the coil can generate a large reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable damage to components, especially transistors. Transistors and other sensitive components can be protected by the inclusion of a diode. The diode provides a path for the current caused by the reverse voltage to escape. A solenoid is another output transducer that has an electromagnetic coil inside. Circuits containing a solenoid require a protective diode as well. Relay in a Transistor Sensing Circuit The circuit below shows a typical transistor temperature sensing circuit with a relay as an output. + 6 volts -t Relays can also be used to switch on (and off) solenoidactuated pneumatics valves. This is a method of controlling pneumatic circuits and systems with microelectronics. Relay c b 1K BC 108 e 10K 0 volts 26 Standard Grade Technological Studies: Component Electronics Revision Notes Frequency Frequency is the regular rate at which a physical event repeats itself. It may be the rate at which an LED flashes. In electronic circuits the common events are: o Flashing of optical devices (LEDs and lamps) o Sounding of buzzers/speakers. These outputs are driven by an electrical pulse from the electronic system or circuit. Capacitors can be used to control the frequency of these events. Capacitance Capacitance in measured in farads, this is a very large quantity. Because this is a very large measurement most capacitors are rated in F (microfarads) or in nF (nanofarads). Capacitor Construction INSULATOR Capacitors are electronic components that store electricity for short periods of time within electronic circuits or networks. They are made from two metal plates or films separated by an insulator. In many capacitors, film is used so that the layers of metal film and insulator can be wound into a cylinder. METAL PLATE OR FILM Capacitors are especially useful in timer circuits with the 555-timer chip. There are two basic types of capacitor normally used in timer circuits: o electrolytic and ELECTROLYTIC o polyester. Electrolytic capacitors AXIAL CAPACITATOR o These are polarity conscious: RADIAL CAPACITATOR o This means that they must be connected ‘the right way round’ o The negative lead must be connected to zero volts o The positive terminal towards the higher voltage side of the circuit It is very dangerous to reverse connect capacitors. Polyester Capacitors Standard Grade Technological Studies: Component Electronics Revision Notes 27 o These are for small-value uses o Can be connected without regard to polarity POLYESTER Capacitors and Resistor Networks o Capacitors can be put in series with resistors in a similar way as a voltage divider circuit. o The output Vout will be low until the capacitor is fully charged. o Once full charge is achieved Vout will rise and a current will flow. o The time taken will depend on the value of the resistor as it will slow down the current passing to the capacitor. o The discharge switch will discharge the capacitor when pressed and can be used to restart the charging process. This effectively creates a time delay in achieving an output. This can be seen in the circuit on the right. The transistor will not switch on until the capacitor is fully charged. The timing of this will vary depending on the value of the variable resistor. 28 Standard Grade Technological Studies: Component Electronics Revision Notes Integrated Circuits: 555 timer An integrated circuit (or IC) is simply an electronic package that contains a number of components on a silicon ‘chip’. The 555-timer IC that you are going to use is a very versatile chip that has many applications. The 555 chip has eight pins. The pin functions are shown below. 1 8 +V (3 -15) TRIGGER 2 7 OUTPUT 3 6 RESET 4 CONTROL 5 VOLTAGE (LEAVE UNCONNECTED) NE555 0V TIMING PERIOD CONTROL Flashing LED using a 555 Timer Integrated Circuit The circuit shown on the right uses capacitor resistor network C1 and VR to control the timing period of the 555 timer. The capacitor will charge and discharge at a predicted frequency set by the value of its capacitance and the resistance of VR. Standard Grade Technological Studies: Component Electronics Revision Notes 29