Wiring Schematic Diagram

This is a LDR switch or light activated relay circuit. This circuit will let you make a switch that will be activated by light falling on a sensor. It is a very useful device and can be used in automatism’s, security systems, counters, remote controls etc. It is very sensitive, fast acting and reliable. The circuit uses a Light Dependent Resistor (LDR) as a sensor and IC LM311 to amplify the signals from the LDR and transistor 2N2222 to drive the relay which does the switching.

Normally the resistance of an LDR is very high, sometimes as high as 1000 Kohms, but when they are illuminated with light resistance drops dramatically. The relay on when LDR uncoverd and relay off when LDR covered. Adjust VR1 for light sensitive.LED will turn on at the same time with relay