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LDR (Light Dependent Resistor)

A light dependent resistor(LDR) is used to detect the presence or level of light. They work on the basic principle of photo-conductivity. It is also called a photoresistor, photoconductor or photocell. The circuit symbol of an LDR is shown in the figure.

ldr light dependent resistor circuit symbol
LDR Circuit Symbol

LDR consist of a resistor made up of semiconductor material. Cadmium sulphide and cadmium selenide are the most commonly used materials for constructing these resistors. They have high resistivity and their resistance value varies according to light intensity. Photons from the fallen light are absorbed by the material lattice and some electrons gain this energy to break the bond and be free. As a more light strike, more free electrons are emitted that can conduct electricity thereby decreasing the resistance.

A snake-like track is formed using cadmium sulphide on the surface as shown in the figure. This structure provides maximum surface and reduces resistance.

ldr structure
Structure of LDR
ldr construction
Internal Structure of LDR

Advantages of LDR

  • Low cost.
  • Ease of manufacture.
  • Ease of use.

Disadvantages of LDR

  • High response time.


  • Automatic Street Lighting.
  • Burglar Circuits.
  • Cameras, Alarm Clocks.
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