light activated relay circuit schematic

Light Activated Relay with 555 IC

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.

Light Activated Relay Circuit Schematic

light activated relay circuit schematic


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  • Dio Aranilla

    What kind of flip flop that is used in this project?

  • Tharanga

    My Relay chatters at the time switching on at dusk & also at dawn before switching off.I am using a 103k Mylar Capacitor as C1.

    What is the best solution to prevent Relay chatter ?

  • Vishal Ahuja

    I have made this circuit. Working perfectly. I have used a 10K pot for R1

  • ajaykumargaur

    What can be done to remove flickering during light transition? We are operating the relay.

  • Tariq996

    Please explain me that without connection to ground how does 555 activates relay.
    I’ve made an automatic light switching controller circuit whose performance is exactly same as this circuit.
    But in my circuit I’ve given +ve to pin1 of relay & connected pin 2 with the collector of an NPN transistor whose emitter was connected to ground & the potential divider of pot & ldr biasis the transistor.

    • farhan

      I suggest you to use 6.8k r2 and 12k r1 if it not work reverse it

    • Sabbir

      I think you are using the circuit I am using but the problem is that when I get the bc547 transistor to work,that is when the LDR is working I don’t get the relay to turn on because I see when I tested it draws only around 1 mA. So, this much current can’t turn the 6volt relay on right? But then what is the solution? I am using 6volt battery, 100k pot

    • m

      hello sir cn u please mail me ur project details which u did earlier