This light activated relay circuit presented here uses the 555 timer
IC and a light dependent resistor or LDR to form a light sensitive relay
in an intruder alarm system or for switching on a lamp at Sun set and
off at Sun rise.
Potentiometer R1 value must be chosen and then adjusted that under
normal conditions when the light is falling on the LDR the voltage
across the LDR is less than 1/3 of Vcc. The output of the 555 IC is high
now. The actual value of R1 will depend on the resistance of the LDR.
When the light fades or is interrupted by an intruder, the voltage across it rises above 2/3 of Vcc, tripping the IC flip-flop. The output goes low activating the relay. When the light is restored, voltage falls below 1/3 of Vcc, again tripping the flip-flop causing the output to go high and the relay drops.
The difference of 1/3 of Vcc between turning on and turning off voltages prevents relay chatter. This differential can be reduced by connecting a resistor R2 shown dotted in the schematic. Its value is about one and a half times of the LDR resistance in its illuminated condition. Use a 6V or 12V relay with a current of 200mA max.
When the light fades or is interrupted by an intruder, the voltage across it rises above 2/3 of Vcc, tripping the IC flip-flop. The output goes low activating the relay. When the light is restored, voltage falls below 1/3 of Vcc, again tripping the flip-flop causing the output to go high and the relay drops.
The difference of 1/3 of Vcc between turning on and turning off voltages prevents relay chatter. This differential can be reduced by connecting a resistor R2 shown dotted in the schematic. Its value is about one and a half times of the LDR resistance in its illuminated condition. Use a 6V or 12V relay with a current of 200mA max.
Light Activated Relay Circuit Schematic
