light operated relay schematic

Light Operated Relay Circuit

This light sensitive circuit can operate a relay to switch on lamps or any AC loads when it senses darkness. It is ideal to use as switch less night lamps driver.

LDR is used as the light sensor. Its resistance is low about 100 ohms in bright light but increases to 10 meg or more in dark. Preset VR1 set the sensitivity of the LDR. During day time LDR conducts so that the gate of T1 will not get base bias. So that relay remains de energized. When the intensity of light reduces, LDR offers more resistance and more current passes to the base of T1 and it conducts. Relay then switch on the load. Adjust VR1 to trigger relay at the particular light level. LED indicates the activation of relay.

Light Operated Relay Circuit Schematic

light operated relay schematic



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  • koenati

    We need the operation of the light activated alarm using 555 so that we can continue with our projects so please post it as soon as possible

  • hafeez

    thanks i will try this remove relay chattering problem

  • Rajeev

    The chattering of relay is very common problem. Try circuit using LM 741 or IC 555 as Schmitt trigger.

  • hafeez

    i designed this circuit but relay coil produce hum until proper light/dark falls on ldr whats its solution.i change sensitivity but doesnot work.

  • Ajay Kumar

    I am also facing the same problem of flickering during the transitition of light to dark.
    Please suggest.


  • Brian Gardner

    Sorry for previous comment I just came across your other circuits, please disregard it.

  • Brian Gardner

    I have lashed this circuit up on a B/board to drive a SSR for security lights but have found the lights going through a period of flickering during the transitition of light to dark.
    This needs to be addressed by some method of snapping on and off. At present I am thinking of trying a 555 IC to eliminate the flickering problem.


    sir, i am in need of a LDR based circuit that can help me to switch on /off the display boards of my shop with the movement of sunlight.
    can i buy the circuit from you.

  • D.Mohankumar

    Dear Pentaconto

    As you said, the relay coil passes around 60 mA current. Most of the new versions LEDs can handle 50 mA current. There will be some drop across the coil, transistor etc. So LED will be safe. I have used this design in some circuits If a series resistor is added, it may reduce the current through the relay.




    • dilipgaur

      you are right remove led and emitter direct ground relay working

    • victor

      m also i’ve found this prblm, but now its ok. change the vr1 10k by 22k, and use the small led, it will be ok,and sorry abt my english

  • Pentaconto

    Dear sir:

    Are you tried this circuit or is only theoretical?

    With a 100 ohm 6V coil relay, the coil current is about 60 milliamps, and therefore the collector and emitter currents of T1 is of this order, so do not think a ordinary LED can suport this current.

    Please check the circuit.