Using a Transistor to Light Up the Night

A light can be made to come on when the light intensity falls below a certain point using a variable resistor and a fixed resistor as essentially a potential divider. The circuit is shown below.

When the light level falls the resistance of the light dependent resist rises, so the voltage across it (which is equal to V BE above) also rises. When the voltage rises above 0.6V, current will flow between emitter and base, switching the transistor on. This will switch the lamp on also. The circuit above can be used in night lights, street lights, or wherever a component is required to switch on for light below a certain level.