We found this circuit in the November 1992 Radio Electronics magazine. The circuit uses an LDR as a sensor and the transistor can be any general-purpose transistor.
In this circuit, obtained from a 1979 publication, IC1 is a 555 and IC2 is a 4020. The transistor is the...
The presented circuit uses a window comparator to keep a relay activated only when the sensor is...
Similar circuits can be found on this site. This is from an 80's American documentation. The...