Light Sensitive Switch

This circuit makes use of the wide change of resistance of the LDR. Between positive and negative supply there is a voltage divider. The bottom section is a variable resistor RV1. The top half is formed by the LDR and a 4.7K ohm resistor in series. In low light conditions when the resistance of the LDR is very high, the bias to the Darlington pair formed by TR1 and TR2 is very low, and they do not conduct. When the light level rises, the resistance of the LDR falls. This turns the transistors 'on' and pulls in the relay.

The LDR should be an ORP12 or similar. The relay should have a pull in voltage of 9V or lower and a coil resistance of 280 ohms or higher. Photo Electric