Design and Technology
E.M.F , Voltage and P.D E.M F This stands for Electromotive Force (e.m.f) A battery provides Electromotive Force An e.m.f can make an electric current flow around a circuit E.m.f is measured in volts (v). Potential Difference (p.d) Electric current will only flow between two points in a circuit if there is a Potential Difference between them. P.d is the difference in voltage between two points of a circuit. Potential difference is measured in volts. A potential difference (p.d) exists in this circuit because the battery is providing an e.m.f. The E.m.f creates a potential difference across the bulb Voltage We call the bottom rail (B) zero volts. If we do this we can find the voltage at A by measuring the p.d between A and the zero V point. In this case the top rail is at a voltage of 9v. Voltage is measured with a voltmeter. Design and Technology
Voltages around a Simple Circuit 1. Draw these circuits in your exercise books and write down what the voltage is at point C. 2. Draw these circuits and write down what the voltmeter would read in each case. 3. Draw this circuit and indicate which of the bulbs would light. Design and Technology
Resistance, Voltage and Current Ohms law Ohm’s law states that when you plot voltage (V), against current (I) you get a straight line. The slope of this line is the resistance R. This can be written as V = I x R A conductor with a resistance of 1 ohm needs 1 volt to drive a current of 1 amp through it. Units of resistance Resistance is measured in Ohms (or Ω). 1 ohm is a rather small resistance. Most resistances we shall meet are much larger. 1 kilohm (1kΩ) = 1,000 Ω 1 megohm (1MΩ) = 1,000,000 Ω or 1,000 kΩ Calculations using Ohms law Use this to calculate : Current flowing resistance R1 voltage v
Design and Technology
Electrical Current and Voltage Electrical Current The current flowing through part of a circuit is measured in Amps (short for amperes). The symbol is A. One amp is a large current and more often in electronic circuits we are measuring one thousandth of an ampere – one milliamps or (mA). 1000 mA = 1A or 1mA = 1/1000A or 0.001A Measure the electrical current flowing through a resistor using the following circuit. Vary the e.m.f voltage V1 from 0v to 12v plot this against the current flowing through the multimeter (I). I
and What do you notice about the relationship between the current and the voltage? Put a line through the points and measure the slope of the line. What is the value of the slope of the line? Design and Technology
Combining Resistors in series and parallel Draw the following circuits and calculate the equivalent resistance Series Combinations 1.
2
3. Parallel combinations 4. 5. 6. Series/parallel combinations 7. 8. 9. Design and Technology
Choosing the correct Resistor Standard Values Resistors are only made in certain values. They are based on the ‘E12 series of preferred values’. These are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. This means that you can buy a standard 68R resistor, 680R, 6.8K, 68K and so on, (but not a 70R or a 67R resistor). Design and Technology
Calculating Resistance Resistors in series When you have resistors in series, the current has more resistance to flow through and the overall resistance is calculated by adding together the resistors. R = R1 + R2 =
When you put resistors together in series the value of the two together is always higher. Resistors in Parallel When you put resistors in parallel you make an extra path for the current to flow down. The total current flowing is the current flowing through R1 and through R2 added together. R1 X R2 R = R1 R2
=
When you put resistors together in parallel the value of the two together is always lower. Design and Technology
Choosing the correct Resistor Standard Values Resistors are only made in certain values. They are based on the ‘E12 series of preferred values’. These are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. This means that you can buy a standard 68R resistor, 680R, 6.8K, 68K and so on, (but not a 70R or a 67R resistor). Choosing the correct resistor when using an LED For every LED there is a specified LED current (sometimes written IF – the FORWARD current). If a higher current is passed through the LED it will overheat and possibly fail. The LED is a type of DIODE. When current passes through the diode it causes a voltage drop. For every LED there is a specified LED voltage (sometimes written VF – the forward voltage drop). Circuits using LEDs need a resistor to limit the current flowing through the LED to the correct specified value (IF). This is calculated using Ohm’s Law (V = I * R ). To calculate the current flowing through the LED the equation is I = V / R, but the value for the voltage across the resistor is V SUPPLY – V LED R = VS ‐ VLED IF Question 1. What resistor would be needed to limit the current through a red LED with a forward voltage drop of 2v to 10mA when using a 12v power supply ? Design and Technology
Voltage divider circuits We can use resistors to give us a voltage at any point somewhere between the voltage of the top rail and bottom rail of the circuit. 1.
Draw Circuit 1 and calculate what the voltage is at points A, B, C, D and E. Circuit 1 For this general circuit the voltage at point C is calculated as Vc = V x R2 R1 + R2 Draw the circuits and calculate the voltage at point C. 2. 3. 4. Design and Technology
Diode A diode is an electronic one‐way valve – it allows current flow only in one direction. to The negative end of the diode the CATHODE – this has a is The positive end is the ANODE Current will flow when the anode voltage is more than 0.7V higher than the cathode. Diodes are used to : 1. Protect circuits from having the batteries connected the wrong way around 2. To prevent components being damaged by the high voltage spike caused when a relay or solenoid coil is switched off 3. To turn ac into dc Design and Technology
Transistors A transistor has three leads. The base is used as an input. Current flowing into will control the current flowing to the output – the collector. the base How a transistor behaves If a transistor is connected between a bulb and the 0V rail we can turn the bulb on and off with tiny changes to the current flowing into the base of the transistor. Voltages round a transistor When the transistor is off, the collector voltage is high When the transistor is on, the collector voltage is low The voltage at the base is never above 0.7V. Base resistor The base resistor Rb is used to make sure that the current flowing into the base is small – without Rb the transistor may overheat and fail. Sensors – Input Devices for the Voltage Divider Circuit Use the diagrams below to show what sensors are needed to make a Light Sensor Darkness Sensor Moisture Sensor Dryness Sensor Heat Sensor Cold Sensor Draw the components needed for the Voltage Divider arrangement (both as symbols and as a sketch of the input device) Design and Technology
Switches A single throw switch is one that can be either OPEN or CLOSED. A double throw switch is one that closes one set contacts in one position and another set of contacts in the other position. of Normally open means that when the switch is in normal position the contacts are OPEN. its Single pole switches have only ONE set of contacts, double pole switches have TWO sets contacts. of Push to make means that the contacts are CLOSED when it is pushed. Push to break means that the contacts are OPEN when it is pushed. Push button switch
Microswitch
Toggle switch
Slide
Reed switch
Rocker
switch
Switch
Label each of the above – what type of switching action does each switch have ? Design and Technology
Relays A relay is a switch that is operated by a solenoid. Solenoids A solenoid uses a coil of wire to operate a moving plunger. When a current is passed through the coil it becomes magnetised and pulls the plunger in one direction – when the current is removed a spring moves the plunger back. Relays Relays use a solenoid to open and close the switch contacts. A small current through the coil can open and close switch contacts that can pass much higher currents – this is what makes relays so useful. Relays can have single or double pole contacts. They are usually double throw contacts with sets of normally open contacts and sets of normally closed contacts. When using a transistor sensor circuit the transistor is often used to switch the relay solenoid. When the solenoid coil is de‐energised there will be a voltage spike generated – remember to use a diode across the relay coil to prevent damage to the transistor. Design and Technology
Capacitors When a capacitor is connected across a potential difference (e.g. a battery) it will charge up until it reaches the same voltage as the applied voltage source. If a capacitor is connected in series with a capacitor then this process happens gradually – this can be used to give a time delay. There are two types of capacitor. Electrolytic capacitors have larger values. They must be connected the right way round – this is marked on the case. Non‐electrolytic capacitors usually have a lower capacitance. Capacitance is measured in farads. A farad is a very large capacitance – most capacitors are usually: microfarads ‐ µF (one millionth of a farad, 0.000,001F) nanofarads ‐ nF (0.000,000,001 F or 1000 nF = 1µF) picofarads – pF (0.000,000,000,001 F or 1000pF = 1 nF) Combining capacitors Capacitors in parallel Capacitors in series C1 x C2 Design and Technology
Integrated Circuits Integrated Circuits (silicon chips) are made by treating small areas of silicon so they become ‘semi‐conductors’, to form miniature transistors, diodes, resistors and interconnections – i.e. a complete circuit in miniature ! These are put into a case. The commonest cases are DIL – Dual In Line. Most of the chip is the case and wires – the circuit is contained within the tiny square of silicon The connections to each pin are called the ‘Pinout’. This is the pinout for an NE555 timer IC In the circuits we will use, Pin 4 and Pin 8 are connected to + V In the monostable timer Pin 6 and Pin 7 are connected together – the circuit is triggered by a signal voltage to Pin 2 and will stay on (Pin 3 high) until the voltage at Pin 6 reaches 6V (at which point Pin 3 will switch to low 0 V). This circuit can be used for many projects involving timing (e.g. leaving level crossing gates or lift doors open for a set period of time). In the astable flasher circuit Pin 2 and Pin 6 are connected together. Used like this the 555 chip can be used to produce flashing lights or warning beeps from a buzzer (or as an electronic clock pulse for other circuits or for motor speed control). Design and Technology
555 Monostable Timer Circuits Build the following circuits using Circuit Wizard. Add the probe and see how the capacitor charges up and discharges. Try adding a second LED (with current limiting resistor) between the 9V rail and output pin 3. Monostable Here the timer is triggered by applying a voltage for a short time to input pin 2 The ON period ends when the voltage at pin 7 (which rises as the capacitor charges up) reaches 6V These are approximate values for the ON period R C T (kΩ) (μF) (s) 100 10 1 500 10 5 500 50 25 500 100 50 Try these and control the ON period of your circuit Use the circuit on the right to add a second switch. When Pin 4 is taken down 0V the circuit is reset and the output switched off. Design and Technology
555 Astable Timer Circuits Build the following circuits using Circuit Wizard. Add the probe and see how the capacitor charges up and discharges. When Pin 3 is ON the capcitor is charging up. When it reaches 6V at Pin 6 the IC switches OFF. The capacitor discharges until it reaches 3V. When Pin 2 drops to 3V this acts as a trigger pulse to switch the IC back on again. Using two variable resistors lets us control both the ON period and the OFF period. Try adding a second LED (with current limiting resistor) between the 9V rail and output pin 3. Design and Technology
Component Symbols Producing a Printed Circuit Board 1. 2. 3. 4. 5. Design and Technology
6. 7. 8. 9. 10.