Photocell based night light

Many automatic night light circuits had been published here. This one uses a photocell for detecting the light intensity. At full light the resistance of the photocell will be few ten ohms and at darkness it will rise to several hundred ohms. The IC1 uA741 is wired as a comparator here. At darkness the resistance of photocell increases and so the voltage at the inverting input of the IC1 will be less than the reference voltage at the non inverting input. The output of the IC1 goes to positive saturation and it switches ON the transistor to activate the relay. By this way the lamp connected through the relay contact glows. The diode D1 works as a freewheeling diode.

Circuit diagram.



  • The circuit can be assembled on a Vero board.
  • Use 9V DC for powering the circuit.
  • POT R7 can be used to adjust the sensitivity of the circuit.
  • The relay K1 can be a 9V, 200 Ohm SPDT type.
  • L1 can be a 230V,60W lamp.
  • R8 can be a ORP 12 photocell.