Light sensor


This project make use of an LDR (light dependent resistor) to sense light and darkness and control the bright white LED.

Circuit diagram


Short Description

  • The circuit uses IC LM324 (quad op-amp).
  • One of the four op-amps is used as a voltage comparator.
  • When it is dark, resistance of LDR is high. Voltage across it is higher than the “preset voltage” at inverting terminal and op-amp output is high. (You can set the threshold of darkness by adjusting the inverting voltage).
  • High op-amp output makes  the bright white LED “ON“.
  • If light intensity in the room is high, the voltage across LDR is low and is less than the inverting voltage. This makes the op-amp output “low”.
  • This forces the LED not to glow.
  • You can test the circuit simply by covering and un-covering the LDR by hand.

Material list

Sr. No. Component Specifications Quantity
1 IC LM 324 standard IC 1
2 resistor 10k,0.25W 2
3 resistor 2k2,0.25W 1
4 preset 5k,top screw settable,blue 1
5 LED bright white, 5mm 1
6 LDR standard 1
7 Battery 9v, flat, with snapper 1
8 PCB general purpose, 4″X4″ 1
9 Misc solder metal, wire etc.

This project is available in the kit form. Contact Omkar to buy it. This site has no commercial connection with his activities.


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