LDR Working: How an LDR or Photoresistor Works ? LDR Basics

In this post you will learn about the basics of LDR, how its made and “LDR Working: How an LDR or Photoresistor Works?

Before jumping to LDR working, let’s cover some basics of LDR or photoresistor first.


What is an LDR(Light Dependent Resistor) or Photoresistor?

An LDR is a type of variable resistor which changes its resistance according to the intensity of the light falling on its surface. Hence it can be considered a sensor or light sensor. LDR-sensor which senses the intensity of the surrounding light. 

So in what fashion does the resistance of LDR changes with Light intensity?

Resistance of an LDR is inversely proportional to the intensity of light that falls on LDR’s surface. In other words, with an increase in light intensity, the resistance of photoresistor or LDR decreases. And that’s why the graph between Resistance of LDR and intensity of light is hyperbolic in nature.

LDR working basics: graph between intensity of light and the resistance of LDR
Graph between the intensity of light and resistance of LDR

Source: Kitronik.com


What is the meaning of Photoresistor?

Photoresistor consists of photo meaning photons and resistor meaning a resistor. Thus photoresistor means a component whose resistance depends upon the number of photons that it receives.

So how does an LDR works? Before jumping to LDR working we must know how an LDR is made. An LDR or photoresistor is made up of a high resistance semiconductor having no P-N junction. Hence an LDR or photoresistor is a passive component. One such semiconductor material is cadmium sulphide, CdS.

An LDR can either be made using intrinsic Semiconductors or extrinsic semiconductors. LDR made up of former is called an intrinsic photoresistor and that of latter is called an extrinsic photoresistor.


How an LDR(Light Dependent Resistor) or Photoresistor Works?

Since LDR is made up of a semiconductor, it exhibits all properties of semiconductors. One such property is photoconductivity i.e, the material becomes more conducting due to absorption of electromagnetic radiation such as normal visible light and ultraviolet light etc.


Now we all know that the flow of current in any metal is actually caused by the flow of electrons. And On the basis of the number of electrons that can flow through metals, they are categorized as an insulator, conductor, and semiconductor. This depends on the energy gap between the conduction band and the valence band i.e, the amount of energy required for electrons to jump from the valence band to the conduction band.


For a semiconductor(assume LDR) this energy gap can be overcome by using suitable intensity of light i.e, photons or photon-energy. The semiconductor material of an LDR or photoresistor is of high resistance. LDR’s semiconductor is of high resistance because there are only a few electrons available for conduction. So when the light of a suitable intensity falls on an LDR, some electrons jump from the valence band to conduction band. This happens because electrons in LDR absorbs the energy of photons. Now since some extra electrons are in conduction band, more current flows through the LDR or the resistance of LDR decreases.


Hence on increasing the intensity of light that falls on the surface of LDR or photoresistor, more and more electrons jump from the valence band to conduction band causing an increase in conductivity or decrease in resistance of the LDR.


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