light activated switch circuit schematic

Light Activated Switch

The light activated switch circuit can be used for switching OFF a particular lamp or group of lamps in response to the varying ambient light levels.
The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.

The power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions.

Schematic of automatic light activated switch circuit

light activated switch circuit schematic

The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed. The circuit can be used as an automatic street light controller system or a simple light activated switch.


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  • Mihai

    It depends on the relay. Find a relay for 10Amps. Take this for example:

    If it doesn’t suits you use a contactor.

  • Ajay

    Good suggestion.. But the realy which is operated from 12 volt won’t be able handle large current.

  • Che

    thanks jay for the info. or could i modify in anyway the relay contacts to handle a bigger load?

    Thanks again

    • Kuberkoos

      If you have a relay with more than one set of contacts, you may parallel the contacts to double the load controlled. This works fine on AC, however if your load is DC, the contacts should be perfectly synchronized to use this method. Rather then use this relay to drive a larger relay to drive the load.

  • Ajay

    Operate a contactor of rating 16 amp or more from the realy.. Then you can add any type of load…
    Change the contactor as per load.

  • Che

    Good day Sir,

    Can i use the circuit to control an appliance? say to turn off a fridge at night and turn it on during the day. Do you have a circuit that can manage this load?

    Thanks and more power.

  • Ajay

    use a relay to operate the motor.

  • anuar

    hi sir..i would like to ask fr ur opinion regarding the ldr curcuit..i build a curcuit consist of ldr,dry cell,relay n etc for my project where we need to cut the curcuit once the ldr no detect any light source..however,the curcuit is not happen as v expected n i dunno why because the motor keep moving although i placed the ldr in a dark place..fyi,im preparing this as a stopping mechanism fr the chemecar competition(moving car using chemical reaction)..so v got prob in stopping the curcuit..any idea??

  • Ajay

    Hi Mihai,
    Please do let me know if you have any other option for the same.
    It would be a very good help to me.
    My Email id is : ajaykumargaur@gmail.com

  • Ajay

    Small Question – will it flicker during the transistion of Light.??

    • Mihai

      YES, it will flicker(relay chatter) for sure during the light to dark or dark to light transition. This circuit doesn’t have hysteresis implemented, so don’t waste your time with it. Try something else. If you don’t find something viable, I might help you with this.


  • sanket

    @Sebastian Kushero
    Can you please give the working of this circuit?

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