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**Electrical Circuits / Electronics**

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. Some important terms: Electric current Electric current is the name given to the flow of negatively charged particles called 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.

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**Electrical Circuits Voltage**

In most circuits a battery or voltage supply is used to drive the electrons through the components. Voltage is measured in volts (V). 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. Resistance is therefore a measure of how much voltage is required to let a current flow. Resistance is measured in ohms ().

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**Batteries & Voltage Supplies**

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**Components - Resistors**

Fixed Resistor Symbol 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. Although 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.

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**First and second colour band**

Resistor Colour Code First and second colour band Digit Multiplier Black x 1 Brown 1 x 10 Red 2 x 100 Orange 3 x 1000 or 1 K Yellow 4 x or 10 K Green 5 x or 100 K Blue 6 x or 1 M Violet 7 Silver means divide by 100 Grey 8 Gold means divide by 10 White 9 Tolerances: brown 1% red 2% gold 5% silver 10% none 20%

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**Resistor Value Calculation**

If the colours on the resistor are: 1st band red 2nd band violet 3rd band brown 4th band gold Then its value is: 2(red) 7(Violet) x 10(Brown) with a 5% tolerance (Gold) i.e. 270ohms 5% tolerance.

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**Pupil Assignment Calculate the value of the following resistors:**

blue – violet – brown – silver orange – white – brown – gold brown – black – red – gold brown – black – green – brown What colours would the following resistors have? 270 R 1K5 33 K

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Diodes Diodes are devices that allow current to flow in one direction only. 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.

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Light Emitting Diode A light-emitting 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.

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Switches Switches 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 push-button, toggle, key, slide, magnetic (reed) and tilt. These switches are ‘digital’ input devices as they can only be on or off.

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Switch Contacts DPDT SPST SPDT DPST

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Pupil Activity We have now seen a number of common electronic components. Lets now try and combine some of these into a working circuit. Copy the circuit into your workbook Build the circuit using the modular boards Your teacher will demonstrate how to connect the boards

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Series Circuits When components are connected end to end, as in the last activity, we say they are connected in series. This leads to an important law, Kirchoff’s 2nd Law The sum of voltages dropped across each component (V1, V2 ) is equal to the total voltage supply in the circuit. VT = V1 + V2 + V3 + …

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**Measuring Voltage Drops**

To measure d.c. voltage: Connect the black lead to the ‘COM’ socket Connect the red lead to the ‘V’ socket Make sure that ‘d.c.’ is selected Place the lead probes on the points where the voltage is to be measured Digital Multimeter

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**Measuring Voltage Drops**

Note how voltage is measured over the components Make sure you take a note of the symbol for VOLTMETER

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**Pupil Activity (Voltage Drops)**

Task: Measure the voltage drop over the 2 bulbs. Enter your findings into a table. Bulb No. Voltage (v) 1 2 9V

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**Pupil Activity (Voltage Drops)**

Task: Measure the voltage drop over the 2 bulbs and resistor. Enter your findings into a table.

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Prototype Board Prototype Board is used to test circuits prior to manufacturing the circuit in large numbers. Build a series circuit using 2 resistors of different values as shown by your teacher. Using the multimeter, check the voltage drop over each resistor. Do the results confirm Kirchoff’s law?

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Circuit Simulation As in Pneumatics, it is possible to simulate electrical circuits. In this case we will use a program called Crocodile Technology. Your teacher will demonstrate the use of Croc Clips to simulate the circuit shown below..

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Measuring Current Current is measured through components or parts of circuits, as shown in the circuit diagram opposite. Note that it is necessary to ‘break’ the circuit and connect the meter in series with the components. Take a note of the symbol for an Ammeter

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Current measurement 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.

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Measuring Resistance Connect two resistors in series on a prototype circuit board and measure the overall resistance. You should find that Rtotal = R1 + R2 And the general rule for finding the sum of any amount of resistors in series is Rtotal = R1 + R2 + R3 + Rn

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OHMS LAW Ohms law can be used to calculate theoretical Voltage drops, Current and Resistance in circuits. Using the triangle shown opposite, we can rearrange the formula to obtain V or I.

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Ohms Law in Practice The task is to calculate the resistance of the lamp.

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**Worked Example For the series circuit shown, calculate:**

The total resistance (RT) The circuit current (IC) The potential difference across both resistors (V1 and V2)

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Worked Example a) b) c)

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**Pupil Problems For the circuit shown below calculate:**

The total resistance of the circuit The circuit current

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**Pupil Problems For the circuit shown below calculate:**

The total resistance The circuit current The voltage drop across each resistor. Use Kirchoff’s second law to verify your answers to (c).

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**Pupil Problems For the circuit shown below calculate:**

The total resistance of the circuit The circuit current.

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Pupil Problems A circuit has three resistors in series. Their values are 15 R, 24 R and 60 R. Calculate the total resistance of the circuit. 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

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Series Circuits One of the problems with series circuits is if a component fails, then the whole circuit fails. Consider a set of bulbs connected in series. If one of these bulbs fail, then current cannot flow through the circuit, hence the remaining bulbs will fail to light also.

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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 Christmas-tree lights where all the bulbs require a 230 volt supply.

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**Parallel Circuits Activity**

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 car-alarm circuit is shown below with the switches wired up in parallel. Simulate the circuit shown below, then describe its operation in your note book.

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Resistors in Parallel Connect two resistors in parallel on a prototype circuit board and measure the overall resistance The formula to calculate the theoretical value of resistors in parallel is shown below.

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Worked Example Calculate the resistance of the parallel branch and the total circuit resistance. The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R.

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**Pupil Activity (Parallel Circuits)**

Task: Build the circuit, Measure the voltage over each of the bulbs. Enter your findings into a table.

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**Current in Parallel Circuits**

There are two important points to remember about resistors in parallel. The voltage drop across each resistor is the same. The sum of the currents through each resistor is equal to the current flowing from the voltage source.

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Worked Example The resistance values are R1 = 270 R, R2 = 100 R and for the buzzer 240 R. Your teacher will work through this problem on the white board.

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**Pupil Problems For the circuit shown below calculate:**

(a) The total resistance of the circuit (b) The circuit current.

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**Pupil Problems For the circuit shown below calculate:**

(a) the total resistance of the circuit (b) the circuit current (c) the current flowing though R1 (10 R) (d) the current flowing through R2 (24 R).

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**Pupil Problems For the circuit shown below calculate:**

(a) the total resistance of the circuit (b) the circuit current (c) the current flowing through R1 (660 R). (d) the current flowing through R2 (470 R).

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Pupil Problems A 6 R resistor and a 75 R resistor are connected in parallel across a voltage supply of 12 V. Calculate the circuit current. 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

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**Combined Series & Parallel**

Consider the combined series and parallel circuit shown in the figure below. You can see that R2 and R3 are connected in parallel and that R1 is connected in series with the parallel combination.

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**Combined Series & Parallel**

Some points to remember when you are dealing with combined series and parallel circuits are: The voltage drop across R2 is the same as the voltage drop across R3 The current through R2 added to the current through R3 is the same as the current through R1 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.

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Worked Example 2 For the combined series and parallel circuit shown, calculate: The total circuit resistance (RT) The circuit current (IC) The voltage drop across resistor R1 (VR1) The current through resistor R2 (I2).

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**Pupil Problems For the circuit shown calculate:**

The resistance of the parallel combination The total circuit resistance. The branch currents

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**Pupil Problems For the circuit shown calculate: The total resistance**

The circuit current The branch current The voltage drop across each resistor.

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**Pupil Problems For the circuit shown calculate:**

The total resistance of the circuit The circuit current The current through each resistor The voltage drop across each resistor.

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**Power = Voltage x Current (watts)**

Power in 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)

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Worked Example An electric household lamp consumes 60 watts from a 240 volt supply. Calculate the current drawn by the lamp and the resistance of the lamp.

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Pupil Problems In the following simplified circuit for a vacuum cleaner motor, calculate: The power consumption of the motor The voltage of the lamp The total power drawn from the power supply.

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Pupil Problems The torch circuit below is supplied with two 4.5 volt batteries connected in series, with the current being 20 mA. Determine: The resistance of the bulb The voltage across the bulb The total power drawn from the supply The power drawn by the bulb.

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Pupil Problems 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 half- heat setting? A kettle and a toaster use the same double socket. If the kettle draws a current of 10 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. An electric drill draws a current of 1.5 amps from a 110 volt supply. Calculate the power rating of the drill.

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Pupil Problems An emergency power generator has to drive 80 lamps. Each lamp takes 60 W at 230 V. Calculate the current through each bulb if: They are connected in series. They are connected in parallel. How many 150 W lamps can be connected in parallel to a 250 V supply through a 5 A fuse?

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Pupil Problems The rear screen heater in a car is connected to the 12 V system and draws a current of 2 A. Find the resistance of the circuit. In reality the 12 V, 0.5 A interior light is on the same circuit. State whether this is a parallel or series circuit and calculate the power and current when both lamp and heater are on.

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Voltage Dividers 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.

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**Voltage Dividers Activity**

Build a voltage divider circuit using any 2 values of resistor. Using the multimeter measure the voltage drop over R2. This voltage is known as Vo or the output voltage from the divider.

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**Voltage Dividers Activity**

Measure the resistance of the 2 resistors from the last activity. Enter the values into the formula below and calculate Vo. Simulate the circuit using croc clips and measure Vo. Hopefully! The value of Vo should be the same in all three cases, (within reason).

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Worked Example

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Pupil Problems Calculate Vo in the following exercises

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Pupil Problems Calculate Vo in the following exercises

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**Switches Switches are useful input devices (transducers).**

There are several ways in which the contacts in mechanical switches can be operated. Such as push button, key, slide, toggle, magnetic (reed) and tilt. These switches are digital input devices as they can only be on or off. The contacts on a switch can be NO or NC (normally open, normally closed)

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**Switch Contacts Types of switch contacts:**

SPST (Single Pole Single Throw) SPDT (Single Pole Double Throw)

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Pupil Activity Copy the circuit into your note book. Simulate the circuit using Croc Clips, then describe in your own words how the circuit operates.

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Analogue Transducers A thermistor is a device whose resistance varies with temperature. It is a temperature-dependent resistor. There are two main types. Negative temperature coefficient (t or NTC) – where resistance decreases as temperature increases. 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 on the next slide.

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Thermistor NTC is the most common thermistor

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Data Charts In your Data book you should find a graph which describes how the resistance of a thermistor changes with temperature. Your teacher will work through the use of the chart.

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Strain Gauges Strain gauges are really load sensors. They consist of a length of resistance wire and when stretched their resistance changes. Strain gauges are attached to structural members (beams, etc.) and as they are loaded, a reading on a voltmeter can be obtained.

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**Light Dependent Resistor**

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).

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Pupil Activity Use your Data Book to find the resistance of an ORP12 LDR for the following light conditions: 10 Lux 40 Lux 100 Lux

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**Pupil Activity Copy the circuit shown below into your note book.**

Using the Electrical Modular Boards, construct the voltage divider circuit. Using a multimeter measure Vo. Warm the thermistor up with your fingers and re measure Vo. Describe the operation of the voltage divider. Reverse the position of the thermistor and resistor. Repeat 3,4 & 5.

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**Pupil Activity Copy the circuit shown below into your note book.**

2) Using the Electrical Modular Boards, construct the voltage divider circuit. 3) Using a multimeter measure Vo. Cover the LDR up with your hand and re measure Vo. 4) Describe the operation of the voltage divider. 5) Reverse the position of the LDR and resistor. Repeat 3,4 & 5. Describe what is happening.

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Pupil Activity 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. Build a voltage divider using a potentiometer. Check its operation by measuring Vo from the voltage divider.

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Potentiometers Some more examples of potentiometers.

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**Voltage Divider Sensitivity**

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). This allows us to fine tune the sensitivity of the voltage divider.

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Pupil Activity 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 modular circuit boards. Adjust the variable resistor so as Vo goes higher when your hand is about 100mm above the LDR

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Pupil Problems Calculate the voltages that would appear across each of the resistors marked ‘X’ in the circuits below. 6v 0v

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Pupil Problems In each of the following voltage divider circuits determine the unknown quantity.

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Pupil Problems In each of the following voltage divider circuits determine the unknown quantity.

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Pupil Problems 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: 80C OR 20C.

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**Pupil Problems What would happen to the voltage across the**

thermistor in the circuit shown previously as the temperature increased? What would happen to the voltage across the resistor in the circuit shown previously as the temperature increased?

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Pupil Problems A thermistor (type 5) is used in a voltage divider circuit as shown below. The characteristics of the thermistor are shown in your Data Book. If the voltage V2 is to be 4.5 V at 100 C, determine a suitable value for R1. State whether V2 will increase or decrease as the temperature drops. Explain your answer

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Voltage Dividers We have seen that Voltage Dividers, divide the voltage depending on the value of resistors used. In addition, if we include a variable resistor, we can alter the sensitivity of the voltage divider. If we include a thermistor, we can measure changes in temperature. If we include a LDR, we can measure changes in light levels. If we include a potentiometer, we can measure changes in position.

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Transistors The 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. Transistors come in many variations and sizes. However, they all are reliable, efficient, small and relatively cheap.

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**Transistors A transistor is an electronic switch**

Transistors amplify current which enables them to drive heavy loads such as motors A voltage of 0.7V will switch on a NPN transistor Collector Base Emitter NPN Bipolar Transistor

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Transistors Activity Simulate the circuit using Croc Clips. Fill in the table shown below. Base resistor value (K) Base/Emitter Voltage (mV) Base current (A) Lamp on/off 2200 1000 470 220 100 47 33 22 10 1

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Transistors Your teacher will work through a number of problems which will look at calculating: Base current Voltage drop over R Base – Emitter voltage M R Vin

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**Transistors Activity 5V (B) 5V (A)**

Build the following transistor circuit using modular boards. Adjust the voltage reaching the transistor base by altering the value the potentiometer. At what voltage does the transistor switch on? Measure the current flowing to the base. Now measure the current flowing in the collector leg. What is the transistor doing? 10K Buzzer 1k

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Relays Although relays are often considered to be output devices, they are really output switches from electric or electronic circuits. When an electric current flows into the relay coil, the coil becomes an electromagnet. This electromagnet attracts the armature and moves the contacts. This movement provides the switching, just as the contacts in any other switch do.

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Relays The relay is a very useful device because it is the vital link between microelectronics and high-energy systems that require substantial amounts of current. The relay is perhaps the most commonly used switch for driving devices that demand large currents.

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**Relays – Protection Diode**

As seen earlier, relays have a 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. The transistors and other sensitive components can be protected by the inclusion of a diode that provides a path for the current caused by the reverse voltage to escape.

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Solenoid A solenoid is another output transducer that has a coil inside. Circuits containing a solenoid require a protective diode as well.

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Pupil Activity - Relay Build the following circuit using modular boards Change the resistance of the thermistor by heating it up with your hand Listen carefully, you should hear the relay contacts opening and closing Now add a bulb and 5V power supply to the output side of the relay and heat the thermistor up again, (your teacher will demonstrate the correct connections). The bulb should light when the thermistor is hot. It might be necessary to replace the 10K with a potentiometer.

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DPDT Relay As electric motors normally draw larger currents, relays are ideal devices for such circuits. By using DTDP switching, relays can control the direction of rotation of motors. Simulate a sensing circuit using an LDR in a voltage divider Add a transistor driving circuit and a DPDT relay Connect the relay up so as the motor drives clockwise and anticlockwise depending on the amount of light hitting the LDR

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**Motor Reversal Circuit**

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Capacitors Capacitors are electronic components that store electricity for short periods of time within electronic circuits or networks. Electrolytic capacitors are polarity conscious. This means that they must be connected ‘the right way round’. The negative lead must be connected to zero volts with the positive terminal towards the higher voltage side of the circuit.

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Capacitors Polyester capacitors are for small-value uses and can be connected without regard to polarity. Capacitance in measured in farads, but because this is a very large measurement most capacitors are rated in F (microfarads) or in nF (nanofarads).

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**Pupil Activity 9V Construct the following circuit**

Allow the capacitor to charge up Connect the end of the LED to 0V The LED should light up for a short period of time 10K + 100uF 0V

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555 Integrated Circuit 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.

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**Pupil Activity - Monostable**

Simulate the circuit shown, press the switch and observe the circuit operation. Monostable means one stable state. The light comes on but always goes back to its original state.

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ASTABLE CIRCUIT

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**Pupil Activity - Astable**

Build the following circuit on Breadboard, describe its operation.

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