1. Show-me board revision quiz 1.Draw the symbol for an LED 2.What happens to the resistance of a thermistor as it is heated up? 3.What happens to the resistance of an LDR if light intensity decreases? 4.How is the supply voltage, V s, related to the voltage across two components in series, V 1 and V 2 ?

2. What we will do today: State that a transistor can be used as a switch. Name the two types of transistor. Draw symbols for the two types of transistor. Investigate how voltage dividers work when using LDRs and thermocouples.

3. Transistor A transistor can be used as a switch. A transistor can be either ON (conducts electricity) or OFF (does not conduct) There are two types of transistor that we must know about: 1.MOSFET 2.npn – transistor We must be able to identify these transistors from diagrams.

4. 1. MOSFET A MOSFET has three main parts: Gate; Drain; and Source. They switch on at approximately 2 V. It has the following symbol:

5. 2. npn transistor Where b is the base e is the emitter c is the collector The npn transistor switches on at approximately 0.7 V.

6. 2009 Qu: 13

7. 2011 Qu: 10

8. 2007 Qu: 11

9. Voltage Dividers When dealing with thermistors and LDRs in voltage dividers we only need concern ourselves with the voltage across the lower component, we will call this V o, the output voltage.

10. LDRs – show me board Draw a voltage divider circuit with an LDR and a variable resistor to show how we could investigate the voltage across the variable resistor. Hint: Make variable resistor V o, V output

11. LDR at top With a circuit set up as shown, we can investigate Vo in both light and dark conditions: ConditionV o (V) Dark Light

12. LDR at top As L ↑, R LDR ↓ So V LDR ↓ making V VR ↑ Therefore V o ↑

13. LDR at bottom ConditionV o (V) Dark Light

14. LDR at bottom As L ↑, R LDR ↓ So V LDR ↓ Therefore V o ↓

15. Thermistor at top With a circuit set up as shown, we can investigate Vo with in both cold and warm conditions: ConditionV o (V) Cold Warm

16. Thermistor at top As T ↑, R therm ↓ So V therm ↓ making V VR ↑ Therefore V o ↑

17. Thermistor at bottom ConditionV o (V) Cold Warm

18. Thermistor at bottom As T ↑, R therm ↓ So V therm ↓ Therefore V o ↓

19. 2007 Qu: 11 D

20. 2009 Qu: 13 A