Photo relay circuit

A photo relay or light activated relay is a circuit which opens and closes the relay contacts according to the light.Here a photo diode is used to sense the light.The photo diode offers a high resistance when there is no light falling on it.Here the photo diode is connected in reverse biased condition.The only current flowing through it will be due to the minority carriers.When light falls on it, the current due to the minority carriers increase and the diode offers a low resistance.As a result the voltage across the diode will not be sufficient to make the transistor Q1 forward biased and the relay will OFF.When there is darkness the photo diode resistance increases and the voltage across it will become enough to forward bias the transistor Q1 making the relay ON.The diode D2 is used as a freewheeling diode to protect the transistor from transients produced to the switching of relay. By this way the load connected through the relay contacts can be switched ON and OFF according to the light falling on the photo diode.

Circuit diagram with Parts list.



  • Assemble the circuit on a general purpose PCB.
  • The circuit can be powered from 9V DC.
  • The preset R1 can be used to adjust the sensitivity of the circuit.
  • The D1 can be any general purpose photo diode.
Show Comments

7 thoughts on “Photo relay circuit

  1. Mike Taylor

    I want to close a NO relay whenever the photodiode sees IR light. This circuit doesn’t seem to do that. I rigged up 2 transistors (darlington) but that doesn’t seem to work either.

    With R1 set anywhere between 5K and 10K, I get 8.13 volts across the relay coils and they close when I apply the 9V, with or without IR or visible light. I’m using an IR Photodiode. I have the short lead on the photodiode connected to R1 and to base on the transistor.

    When I shine an IR LED into the Photodiode, the voltage across the photodiode jumps to 0.6V (out of circuit) so I know it is working.

    What can I try?

    Thanks, Mike

  2. Matt

    I have an old Lafayette Photo Electronic Relay model# KT-133A. Does any one know what it is. There are some things that I know a lot about-electronics and women are not on that list.


  3. Dinesh

    Dear sir,
    I tried this circuit but it is not working.the relay is picked permenently even the presents of light or not.

    • ggggggg

      D1 and R1 are swapped in the circuit and the relay will stay on. make the circuit with them swapped and it will work.


Leave a Reply

Your email address will not be published. Required fields are marked *