Input Devices Microphone Symbol: Energy Change:soundelectrical Solar Cell Symbol: Energy Change:lightelectrical

Thermocouple Symbol: Energy Change:heatelectrical The higher the temperature at the junction, the more heat energy converted to electrical. +-

The Thermistor A thermistor is a special type of resistor whose resistance changes with temperature. Symbol:

Experiment A thermistor is connected to an ohmmeter as shown. Ω The resistance of the thermistor is measured by the digital ohmmeter at several different temperatures. Results Resistance of thermistor at hand temperature = Ω Resistance of thermistor at room temperature = Ω Resistance of thermistor at 0 °C = Ω

Conclusion As the temperature increases the resistance. As the temperature decreases the resistance will. decreases T U R D temperature up, resistance down increase

Light Dependent Resistor (LDR) A light dependent resistor (LDR) is a special type of resistor whose resistance changes with light intensity (brightness). Symbol:

Experiment A thermistor is connected to an ohmmeter as shown. The resistance of the LDR is measured by the digital ohmmeter in bright light and in darkness (covered up). Results Resistance of LDR in bright light = Ω Resistance of LDR in darkness = Ω Ω

Conclusion As the light intensity increases the resistance. So as the light intensity decreases the resistance will. decreases L U R D light up, resistance down increase

Thermistor and LDR Problems A thermistor is connected to an ammeter and a supply voltage of 12 volts as shown. A 12 V T Temp (°C)Resistance (Ω) (a)Calculate the reading on the ammeter at 20 °C. (b)Calculate the reading on the ammeter at 100 °C. (c)The reading on the ammeter at 150 °C will be: (i) 20 mA (ii) 30 mA

Solution (a) (b) (c)TURD – temperature goes up so resistance will go down. Smaller resistance means bigger current. Current will be 30 mA (0.03 A)

A light dependent resistor (LDR) is connected to an ammeter and a supply voltage of 10 volts as shown. 10 V A Light IntensityResistance (Ω) 100 units 500 units 4, Calculate the reading on the ammeter at a light intensity of (a)100 units (b)500 units.

Solution (a) (b)

Yellow Book Thermistors and LDR’s – Page 47 Q25, Q26, Q27

The Capacitor A capacitor is an input device which introduces a time delay before something happens. Symbol: A capacitor is able to store charge. It takes a certain amount of time for an uncharged capacitor to charge up. Capacitance is measured in farads (F). A large value capacitor (e.g μF) takes longer to charge up than a smaller capacitor (e.g. 200 μF).

Converting Units 1.Convert 150 μF into farads. OR You may leave in scientific notation. 2.Convert the following into farads: (a)750 μF (b)10 μF (c)1500 μF 750 x F F 10 x F F 1500 x F F

Charging a Capacitor Experiment An uncharged capacitor is placed in a circuit as shown. S R 1000 μF 0 V 6 V V Switch S is closed. The capacitor starts to charge up. The voltage across the capacitor is measured every 5 s after switch S is closed.

Results A graph of the results was plotted: Time (s)Voltage (V)

The time it takes to charge a capacitor depends upon the size of: CAPACITORit takes longer to charge a bigger capacitor RESISTORit takes longer to charge with a bigger resistor (this is because the current is smaller) ** Need to know these factors affect time to charge capacitor **

Potential Divider This is an input device which consists of two resistors in series. The resistors divide the voltage supply into two parts. 0 V R1R1 R2R2 VSVS V1V1 To calculate V 1 : To calculate V 2 : ** NOT on data sheet **

Example 1 In a potential divider circuit, a 6 volt supply is connected to two resistors as shown. 0 V 500 Ω 250 Ω 6 V V1V1 Calculate the size of the voltage across each resistor. V2V2

The size of V 2 is found by: Points to Note: V 1 + V 2 = V S The BIGGER RESISTOR gets the BIGGER SHARE of the voltage supply. If it is twice as big it gets twice as many volts etc.

Example 2 In a potential divider circuit, a 10 volt supply is connected to two resistors as shown. 0 V 4 kΩ 1 kΩ 10 V V1V1 Calculate the size of the voltage across each resistor. V2V2

The size of V 2 is found by:

Example 3 A 2 kΩ and 7 kΩ resistor are connected in a potential divider circuit as shown. 0 V 2 kΩ 7 kΩ 5 V V1V1 Calculate the size of the voltage V 1.

The size of V 2 is found by: 0 V 2 kΩ 7 kΩ 5 V 1.11 V V2V2

Yellow Book Potential Dividers – Page 46 Q18, Q19, Q20, Q21, Q22

Comparing Calculated Values Experiment The calculated value of V 1 will be compared to the measured value using a voltmeter. Calculation 0 V 1 kΩ 10 kΩ 6 V V1V1

Experimentally The circuit shown was built and the voltage across the 1 kΩ is measured using a voltmeter. voltmeter reading = V Extension Reverse the positions of the two resistors and repeat experiment.

What Input Device? Selecting Input Devices If a TIME DELAY is involved – use a CAPACITOR. In other cases – think carefully about the energy change.

Example 1 Choose a suitable input device from the following list for each application given: microphone; thermocouple; solar cell; thermistor; LDR; capacitor (a)energy source for a satellite (b)time delay for arming a burglar alarm to allow householder out front door (c)temperature control for an aquarium (d)alarm warning parents in another room that baby is crying (e)measurement of temperature inside a blast furnace (f)circuit to reduce brightness of TV screen when room lights are switched off. solar cell capacitor thermistor thermocouple mic LDR (GENERAL)

Example 2 Give a suitable input device for each application: (a)coin detector in a drinks machine (b)fog detector (c)heartbeat monitor (d)circuit switching hand drier on for 10 seconds (e)flame sensor for a gas fire LDR microphone capacitor thermocouple (CREDIT)

Questions Q1.Name an appropriate input device for the following applications: (a)automatic light switching on when light level becomes dim (b)heating system to switch on when temperature falls below 20°C (c)baby monitor detecting noises made (d)energy saving light system that switches lights on in hotel staircase for a time if 60 seconds (e)alternative energy source used in many calculators. LDR thermistor microphone capacitor solar cell (CREDIT)