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1 of 32 © Boardworks Ltd 2009 2 of 32 © Boardworks Ltd 2009 Sensing devices 3 of 32 © Boardworks Ltd 2009 A Light Dependent Resistor (LDR) is an input transducer, converting light energy to a change in electrical properties. Its resistance decreases as light intensity increases. cadmium sulfide track LDR symbol resistance (Ω) Light dependent resistors light intensity (lux) As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band. The resistance can fall from 1 MΩ in darkness to 500 Ω in light. 4 of 32 © Boardworks Ltd 2009 Thermistors resistance (Ω) Negative temperature coefficient (NTC) thermistors are input transducers that have a decreasing resistance as temperature is increased. temperature (°C) As the surrounding temperature increases, the electrons in the metal oxide of the thermistor gain energy. This increases thermistor the number of charge carriers, symbol decreasing resistance. 5 of 32 © Boardworks Ltd 2009 Sensors summary 6 of 32 © Boardworks Ltd 2009 7 of 32 © Boardworks Ltd 2009 Sharing voltage 8 of 32 © Boardworks Ltd 2009 Potential dividers Potential dividers reduce voltage. Varying the ratio of a pair of resistors changes the output voltage of a circuit. VIN R1 VOUT VOUT = VIN × R2 R1 + R2 R2 0V 0V VOUT will be a fraction of VIN. The magnitude of VOUT is dependent upon the ratio of the two resistors R1 and R2. 9 of 32 © Boardworks Ltd 2009 Using the potential divider equation 10 of 32 © Boardworks Ltd 2009 Sensors and potential dividers 11 of 32 © Boardworks Ltd 2009 Potential divider questions 12 of 32 © Boardworks Ltd 2009 Potential dividers summary 13 of 32 © Boardworks Ltd 2009 14 of 32 © Boardworks Ltd 2009 Electrical power The power, or rate of energy transfer, of a device is a product of the voltage and current passing through the component. power (W) = voltage (V) × current (A) What is the power of a bulb which uses a 230 V mains supply and has a current of 0.44 A passing through it? P = V × I = 230 × 0.44 = 101.2 W What is the voltage across a microchip if it has a normal operating power of 0.5 W and draws a current of 0.1 A? V = P ÷ I = 0.5 ÷ 0.1 = 5 V 15 of 32 © Boardworks Ltd 2009 Different forms of the power equation Electrical power can also be calculated using resistance. The equations linking power to resistance are found by substituting the equation V = I × R into the power equation: P=V×I P=V×I and… V=I×R and… I=V÷R Therefore, using substitution: Therefore, using substitution: P=I×R×I P=V×V÷R P = I2 × R P = V2 ÷ R 16 of 32 © Boardworks Ltd 2009 Energy in circuits 17 of 32 © Boardworks Ltd 2009 Efficiency Efficiency is a measure of how well a device transforms energy into useful forms. A light bulb converts electrical energy to useful light and wasted heat. light electrical heat What is the efficiency of the bulb if it useful energy out converts 50 J of efficiency = × 100 total energy in electrical energy into 45 J of heat energy? 5 = × 100 = 10 % 50 18 of 32 © Boardworks Ltd 2009 Efficiency of a motor 6V 2A pulley motor 1.5 m What is the efficiency of this system, if the motor takes 5 seconds to lift the weight? (take gravity to be 9.81 N/kg) energy into system: electrical energy = I × t × V = 2 × 5 × 6 = 60.0 J 1.4 kg energy used: gravitational potential energy = m × g × h = 1.4 × 9.81 × 1.5 = 20.6 J useful energy out efficiency = 19 of 32 total energy in × 100 = 20.6 60.0 × 100 = 34.3 % © Boardworks Ltd 2009 Electricity in the home 20 of 32 © Boardworks Ltd 2009 21 of 32 © Boardworks Ltd 2009 Current and drift velocity Current is a flow of charge. Electrical devices activate almost instantly once they are supplied with power, however the electrons actually move around a circuit quite slowly. Their velocity is called drift velocity. Current and drift velocity are linked by the following equation: I = current (amps) I = nAve n = charged particles per unit volume A = cross-sectional area (m2) v = drift velocity (m/s) e = charge on an electron (1.6 x 10-19 C) 22 of 32 © Boardworks Ltd 2009 Understanding I = nAve 23 of 32 © Boardworks Ltd 2009 Alternating current and direct current 24 of 32 © Boardworks Ltd 2009 RMS voltage The voltage of AC can be viewed using an oscilloscope. There are three common voltage measures, namely peak, peak-to-peak and RMS (root mean squared) voltage. peak voltage RMS voltage peak-to-peak voltage zero volts RMS is a measure of the average magnitude of the voltage. 25 of 32 VRMS = VPEAK √2 © Boardworks Ltd 2009 RMS current and RMS power To investigate voltage we use an oscilloscope connected across a resistor. As V I, the equation for calculating RMS current is similar to the equation for RMS voltage: IRMS = IPEAK √2 The equation for RMS power is a little different: PPEAK = IPEAK × VPEAK PRMS = IRMS × VRMS = PRMS = 26 of 32 IPEAK √2 × VPEAK √2 PPEAK 2 © Boardworks Ltd 2009 AC calculations 27 of 32 © Boardworks Ltd 2009 AC/DC summary 28 of 32 © Boardworks Ltd 2009 29 of 32 © Boardworks Ltd 2009 Glossary 30 of 32 © Boardworks Ltd 2009 What’s the keyword? 31 of 32 © Boardworks Ltd 2009 Multiple-choice quiz 32 of 32 © Boardworks Ltd 2009