Photo relay circuit

Posted on Feb 6, 2014

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 du

Photo relay circuit
Click here to download the full size of the above Circuit.

e 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.

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    1MHZ Digital Frequency Meter
    Exterior LampCircuit Of 1998 Chevrolet Blazer
    Build a Beeper visual indicator
    HiFI Headphone Amplifier
    universal serial PC interface
    Ni-MH battery charger using LT4060
    STI7167 Advanced STB Decoder With Integrated DVB-T/DVB-C Demodulator
    designing a phantom antenna power supply system
    High power siren circuit using CD40106
    Low Power Audio Amplifier
    Only one-way operation of the automatic control circuit of the motor