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The two circuits shown here make use of light sensitive voltage dividers to trip a relay whenever the light intensity changes. In the light-activated circuit, when light comes in contact with the photo resistor, the photo resistor’s resistance decreases, causing an increase in the transistor’s base current and voltage. If the base current and voltage are large enough, the transistor will allow enough current to pass from collector to emitter, triggering the relay.

The dark-activated relay works in a similar but opposite manner. The value of R1 in the light-activated  circuit should be around 1 kΩ but may need some adjusting. R1 in the dark-activated circuit (100 kΩ) also may need adjusting. A 6- to 9-V relay with a 500-Ω coil can be used in either circuit.