Circuit Lighting :
Light Detector by IC CD4001

Kindly submitted by Mick Devine from the UK

Notes:
Variable resistor R1 adjusts the light threshold at which the circuit triggers. R1’s value is chosen to match the photocells resistance at darkness. The circuit uses a CMOS 4001 IC. Gate U1a acts as the trigger, U1b and c form a latch. S1 resets the circuit. The output device may be a low power piezo buzzer.

Source :: http://www.zen22142.zen.co.uk/Circuits/Switching/ldet.htm