At night a burglar alarm bell box may not be noticed and a would-be intruder might attempt a break-in if a house appears to be unoccupied. However, a light burning in an upstairs room is a good deterrent, and the deterrent effect is even greater if the light is switched on and off every so often. This project details the construction of a light-sensitive switch to turn a light on automatically when it gets dark, and to automatically switch the light off and on every now and again.