Automatic Light Circuit with LDR and Triac

Warning!, This circuit is not isolated from mains! Touching any parts of the circuit while connected to mains may cause high voltage through your body and can cause injury or death!

First published at ziddique.wordpress.com This circuit will turn on an AC Lamp when the sun sets, and can turn off it when the sun rises.

The working of the circuit is simple and the heart of this circuit is an LDR (Light Dependent Resistor) that is wired as a light sensor, so when the ambient light falls on the LDR, the circuit will not work and when no light falls on the (when the sun sets) LDR the circuit will turn on a 230V ac bulb or LED Circuit diagram of automatic lamp

How automatic light circuit works?

A TRIAC is used here instead of a relay, a triac can switch high voltage AC. The voltage to trigger the triac gets from the emitter of T1 so since resistance of the LDR is low as it is in day light the T1 will not get sufficient positive voltage to trigger the triac, but in darkness the resistance of LDR is very low - this causes to turn the T1 on so the triac will get triggered and the bulb will glow.

D1 and R1 are used to rectify AC Mains and lower the voltage, C1,R1 used to pass 6V DC for T1. the R3 can be set to a desired level (sensitivity) to turn the light on as the level of sunlight.