How LDR Works? Photoresistor Basics, Construction, Limitations

How LDR Works? Photoresistor Basics, Working, Limitations

What is meant by LDR?
LDR = Light Depended Resistor.
As the name indicates its resistance value depends upon the amount of light falling on it.
It is also known as Photoresistor, Cadcell, photoConductive Cell and Solar cell...
Photoresistors(Photo - Resistor, Photo=light) are light-controlled variable resistors. The photoresistor is used as a light detecting device.

It is defined as two terminal transducer which have a resistance determined by the amount of light falling on the cell.

Symbol of Photoresistor:

Construction of Photoresistor(LDR):

  • They are made from either Cadminum Sulfide(CdS) or Cadminum Selenide(CdSe).
  • They are sensitive to light having wavelengths between 4000°A (blue light) and 10000°A(infrared).
  • When this semiconductor is placed in the dark, electrons within its structure do not want to flow through the resistor because they are too strongly bound to the crystal’s atoms.
  • But when the light strikes it,the incoming photons of light collide with the bound electrons, and release the valance electrons.
  • This causes the current flowing through the device (the resistance goes down).

Photoresistor Working Principle:

  • When placed in the dark, the photoresistor is usually having very resistance (in the range of megaohms).
  • When the light energy strikes it, it's resistance decreases to as low as few hundred ohms (depends upon the light intensity).
  • This property is used in the real-time applications like light and dark-activated switching circuits and in light-sensitive detector circuits. 

Limitations of LDR:

  1. They require a few milliseconds or more to respond fully to the changes in light intensity.
  2. They will take few seconds to return to their normal dark resistance once light is removed.
  3. The sensitivity and resistance range of the LDRs will vary from one device to another. 

Thanks for reading about Light Dependent Resistor basics

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