Interface LDR Sensor With 8051

A Light Dependent Resistor (LDR) or a photo resistor is a non linear device whose resistivity is a function of the incident electromagnetic radiation.

  • A light dependent resistor works on the principle of photo conductivity.
  • Photo conductivity is an optical phenomenon in which the materials conductivity (Hence resistivity) reduces when light is absorbed by the material.
  • When light falls then the resistance of the device has decreased.
  • When a light dependent resistor is kept in dark, its resistance is very high. This resistance is called as dark resistance.
  • It has high as 1012 ohm when LDR kept in dark.
  • When light is incident on a photocell it usually takes about 8 to 12ms for the change in resistance to take place.
  • While it takes seconds for the resistance to rise back again to its initial value after removal of light.


  • Automatic street light system.

Hardware Required

  • LDR.
  • ADC 0804.
  • 8051 MicroController.