dark activated relay circuit

Dark Activated Relay Circuit

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.


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  • elson

    Nevermind…it worked….the pot I used was faulty…
    Worked perfectly fine …when replaced it with a new one…

  • elson

    I made the circuit in my breadboard, but when I give the +12V,the relay just gets energised. the ldr and 10k pot has no effect on it…
    I made the circuit 3 times from the beginning(hoping that I made a connection mistake or a loose connection somewhere)…but still the same result…
    Also I checked the collector voltage and it’s only 1.2V(the relay is rated 6V, so how is it getting energised,I’m left scratching my head!!)….
    Can somebody help me….asap…

  • JCR

    can we check the output using led and how?

  • Ford

    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!

  • Mayur

    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.

    • Mayur

      Yes I use ic (ua741)

    • Mihai

      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?

  • JohnnyK

    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).

  • TyTower

    See if I can get asc file on

  • TyTower

    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.

  • Santosh

    Thank you Mihai

  • Santosh

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

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