Transitors

Light sensitive switch

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

Time delay switch

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

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

Temperature switch