1. Transitors

2. Light sensitive switch

3. How it works LDR When the LDR is put in the dark the bulb lights up
Automatics lights at night

4. Time delay switch

5. How it works The capacitor charges up slowly
It is several seconds before the voltage across it is high enough to switch the transistor on
The switch next to the capacitor when closed causes the capacitor to discharge
More capacitance – more delay

6. Temperature sensitive switch
Resistance falls when temperature increases
Part of a potential divider circuit
At low temp the thermistor has high resistance
At high temp the thermistor has a low voltage
The transistor is switched on

7. Temperature switch