How an LDR or Photoresistor Works ? LDR Basics

In this post, you will learn about the basics of LDR, its making, and LDR Working OR 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)?

An LDR is a variable resistor that changes its resistance according to the intensity of the light falling on its surface. As light intensity increases the resistance of an LDR decreases. Hence LDR or light-dependent resistor is called a light sensor. In simple words, LDR is a type of sensor that senses the intensity of the surrounding light. 

LDR Symbol
LDR Symbol

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

LDR Resistance vs Light intensity

Resistance of an LDR is inversely proportional to the intensity of light that falls on the LDR’s surface.

In other words, with an increase in light intensity, the resistance of the photoresistor or LDR decreases. That’s why the graph between the Resistance of LDR and the intensity of light is hyperbolic in nature.

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


What is a Photoresistor?

A photoresistor is another name for an LDR. It consists of “photo” meaning photons and “resistor” meaning a resistor. Thus photoresistor is a component whose resistance depends upon the number of photons that it receives.

How is an LDR made?

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 sulfide or CdS.

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

How does an LDR work?

  • Since LDR is made up of a semiconductor, it exhibits all the properties of semiconductors. One such property is photoconductivity i.e. the material becomes more conducting due to the absorption of electromagnetic radiations 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. On the basis of the number of electrons that can flow through metals, they are categorized as insulators, conductors, and semiconductors.
Conductor, Insulator and Semiconductor based on the Energy band gap
Conductor, Insulator, and Semiconductor based on the Energy bandgap
  • This categorization depends on the energy gap between the conduction band and the valence band i.e. the amount of energy required for the electrons to jump from the valence band to the conduction band.
  • For a semiconductor(assume LDR) this energy gap can be overcome by using a suitable intensity of light i.e, photons or photon-energy.
Energy band gap of the semiconductor material in an LDR
The energy bandgap of the semiconductor material in an LDR

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  • 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 the conduction band. This happens because electrons in LDR absorb the energy of photons.
  • Now since some extra electrons are in the 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 the LDR or photoresistor, more and more electrons jump from the valence band to the conduction band causing an increase in conductivity or decrease in resistance of the LDR.

How many volts does an LDR need?

An LDR can handle voltages up to 5V, but it is advisable to operate it at 3.3V to prevent potential damage.

Why are resistors connected in series with LDR while interfacing LDR and microcontrollers?

Resistors are used in series with LDRs when interfacing with microcontrollers to create a voltage divider circuit, facilitating precise measurement of varying light levels.

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