What is LDR and How it works

LDR stands for Light Dependent Resistor. It is also known as a Photoresistor and is another special type of Resistor it has no polarity so it can be connected in any direction.

A photoresistor or LDR is a light-sensitive electronic component. The resistance changes when exposed to light. The resistance of an LDR can vary over many orders of magnitude, decreasing as light intensity increases.


Working of LDR or Photoresistor

It works on the principle of photo-conductivity, when light hits its photoconductive material, the light absorbs its energy and the electrons of this photoconductive material in the valence band are excited to enter the conduction band and thus increasing the conductivity also the light intensity increased.

Furthermore, the energy of the incident light must be greater than the energy of the band gap for electrons to be excited from the valence band to the conduction band.

LDRs have a maximum resistance of about 1012 ohms in the dark, and this resistance decreases as the light increases.

Types of LDR or Photoresistor

  • Intrinsic Photoresistor
  • Extrinsic Photoresistor

Intrinsic Photoresistor

Intrinsic photo-resistors use undoped semiconductor materials such as silicon or germanium. A photon incident on the LDR excites an electron, causing it to move from the valence band to the conduction band.

Extrinsic Photoresistor

Extrinsic photoresistors are made from semiconductor material doped with impurities. These impurities or dopants create new energy bands above the existing valence bands. As a result, the energy gap is smaller, so less energy is required for electrons to enter the conduction band.

Symbol of LDR or Photoresistor


Applications of Photoresistor or LDR

  • Automatic Street Light
  • Detect Day or Night
  • Automatic Head Light Dimmer
  • Position sensor
  • Used along with LED as an obstacle detector
  • Automatic Bedroom Lights
  • Automatic Rear view mirror

Leave a Comment