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    This dark activated switch can trigger a relay to operate an AC lamp at Sunset. The lamp remains on till morning and then turns off. This eliminates the need of switching the Porch lamp or Backyard lamp daily and also helps to light the premises of the house when the occupants are out of station.

    The circuit utilizes the light sensing property of LDR to activate the circuit. The LDR (Light Dependent Resistor) has very high resistance as high as 10 Meg ohms in dark which reduces to a few Ohms in bright light.

    Schematic of Dark Activated Switch Circuit

    dark activated relay circuit

    Variable resistor VR and the LDR forms a potential divider that gives a variable voltage to the inverting input (pin2) of IC1. The non inverting input (pin3) of IC1 gets a fixed voltage (half supply voltage) from the junction of the potential divider R1 and R2. Feedback Resistor R3 gives some hysteresis so that the relay turns on when the light level falls to a particular value and does not turns off again until the light level rises above this value. This prevents spurious switching by sensing mild changes in the light level. This also prevents relay clicking when the light level gradually falls at sunset.

    During day time LDR gets sunlight and it conducts. This reduces the voltage at pin2 of IC1. Since this voltage level is lesser than the voltage at pin3, output of IC1 goes high to make the PNP transistor T1 off. Thus relay remains off during day time.

    When the light level decreases at sunset, LDR cease to conduct and the voltage level at pin 2 increases above the voltage level at pin3.Output of IC1 then turns low and T1 conducts. This activates the relay and the AC load connected to the NO (Normally Open) contacts of the relay turns on. This condition remains until LDR illuminates in the morning and then the lamp turns off. Diode D1 protects T1 from back e.m.f when T1 switches off.

    VR can be used to adjust the sensitivity of LDR at the particular light level, say at 6 pm to turns on the lamp.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    32 Responses to "Dark Activated Relay Circuit"

    1. The circuit should have more hystirises, otherwise the relay with chatter near the trip points..reduce the value of 1Meg to 470K ohms

    2. It can be a 1M preset to adjust the hysterises correctly

      • Rajan Sainju says: on August 11, 2012 at 4:00 pm

        dear sir

        i have seen at internet i have devlope circuit as my requirement could you help me

        thank you sir

    3. Hi there,
      I would like to create a circuit of automatic grease/oil dispenser for motorcycle chain.Can u guys help me on that.im thinking to consider a counter before the grease/oil dispense by it self.The dispenser only can operate during running time means while the motorbike was moving to make sure its more effective.Thanks


    4. nouman nayyer says: on July 15, 2010 at 1:28 pm

      this circuit is also not working
      i m using correct components

    5. The comparator trippling point is adjusted by the sensitivity of LDR using 10k preset. In LDR and Phototransistor based circuits, the major problem is adjustment of sensitivity at the particular light level in which the device is placed.Keeping the LDR in a black tube to prevent ambient light entering will reduce the problem to a certain extent. Careful setting and patience is needed in this matter.Size of the LDR is also important. Use a 5mm or 10mm LDR. To set the correct hysteresis, replace R3 with a 500K preset and adjust

      • The comparator tripping point is adjusted by the 10K trimpot, not by sensitivity of the LDR. Sensitivity of the LDR doesn’t adjust anything. LDR changes its resistance according to the ambient light, hence the tripping point.

        It’s not a major problem to adjust the the particular light or dark level.

        Just keep away the black tube from LDR. The LDR must “see” the ambiant light.

        It can be adjusted in no more than 5 seconds if you do it properly. You don’t need patience for this operation.

    6. nouman nayyer says: on July 17, 2010 at 4:13 pm

      dear sir
      thanks alot for your very precious time
      sir it is working right now
      sir i m really thank full to u.
      sir are u in india???

    7. Happy news. It is a tested circuit.I am residing in Kerala,India

    8. hi..sir…
      my circuit is totaly work……..thank u very much…sir

    9. how this circuit can be modified to work with 12v relay?

    10. i’am a beginner. i wanna know the terminals where ac loads should be connected in this circuit. can any1 help me??

    11. [URGENT]

      Hye, want to ask.
      I have a complete dark-on relay circuit. But i need to modify it into dark-OFF relay circuit.
      So, which connection/component should i change ?

    12. Lalitha priya Radha krishnan says: on January 3, 2014 at 9:31 am

      hi sir…
      This circuit really works well…heartly thanks to you sir…
      I need more electric circuits projects would you help me sir…

    13. i really dont get the connection of relay….any same project with much better schematic design?

      • Omar Mostafa says: on March 27, 2014 at 4:27 pm

        The relay depends on your application but the main thing is to connect on part of the coil terminals to the emitter terminal of the transistor and the other coil terminal to ground.

        Then you decide if you want to use it as Normally Open(NO) or Normally Closed(NC).

        If you need more information just ask.

    14. Dear D.Mohankumar,

      I’m using this circuit for a breadboard project of mine but it seems that I can’t make it to work whatsoever!

      The transistor never provides enough voltage to switch the relay, I don’t know if it’s the LM741 that’s causing it or what but I’ve checked the LDR to be working as well as the 10K VR and by shorting the BE terminals of the transistor I got the relay to switch but never by using the LDR did the relay switch!

      Please help me it’s really urgent for me.

      If you need any more information just ask.

      • Omar Mostafa says: on March 28, 2014 at 5:54 pm

        Disregard my last comment, i figured it out on my own, it was a problem with one of my components’ wiring, that’s all.

        Can confirm it’s a working circuit although it requires too much light to operate not your avergae indoor lighting.

    15. Why the relay is rated for 6Volts while the supply is 12Volts?

    16. Sir can I use 12V relay instead of 6V?

    17. Thank you Mihai

    18. Very good light sensitive circuit. Holds NC contacts open with very small current during day and turns off at night so the NC terminal drives the lights at night.

    19. See if I can get asc file on

    20. Hi,
      Your ‘Dark Activated’ circuit is very interesting. I am seeking such to switch on some solar PV panels when the sun-light is dim (cloudy) & switch them off when there is too much light (bright sunlight).
      Can your circuit me modified to accomplish this with the voltage of 200v DC & current of 8 amps?

      PS, The reason for requiring this, is that I have added additional solar panels to an existing inverter. Thus the poor inverter gets too much power (overloaded) in summer around high noon. The extra panels then need to be automatically switched off for around 4-hours most days to prevent the inverter blowing-up (expensive).

    21. HI I made this ckt it operate but lamp is remain same in sunlight as well as in darkness
      What is reason behind this?
      Please help me.

      • Check the wiring. Disconnect the LDR(only one pin), then short the LDR and see what is happening. The relay is activated when no LDR is connected and deactivated when LDR is shorted.

        What kind of IC(UA741) do you use?

      • Yes I use ic (ua741)

    22. Good day!can any body know how to put a simple delay for this circuit? It will only turn on after may be a minute or more after presence of light was remove..thankyou!

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