Introduction to LDR

I hope you all are fine; this is the fifth article in the series on the introduction of basic electronic components. In my previous article I have briefly discussed the Phototransistor in this tutorial I will discuss the Light Dependent Resistor commonly referred to as LDR.


Introduction to LDR:


After reading this post the reader will be able to learn about the working of the light dependent resistor, construction of the LDR, circuit and application of the light dependent resistor. So sit back, keep reading and enjoy learning.


What is LDR (Light Dependent Resistor):

Light Dependent resistor also known as the photoresistor is a two terminal passive electronic component that is made up of a semiconductor element or compound. As the name implies the resistance of the Photoresistor depends upon the intensity of light projected onto it. With the increase in the intensity of light the resistance of the light dependent resistor decreases. The photoresistor along with its schematic symbol is as shown in the following image:

The LDRs are used in a bridge sensor circuit on order to convert the light into the voltage and thus different actions can be performed for example by feeding the voltage signal into the microcontroller the circuit can be used to determine the darkness in the room and turn on or off the light.



Construction of the Photoresistor:

The photoresistor is made of the semiconductor material an important thing to note here is that unlike the photodiode or the phototransistor the photoresistor does not have PN junction. The photoresistor exploits the light dependent properties of the semiconductor material. Some of the semiconductor materials commonly used to manufacture the photoresistor are;

  • Cadmium Sulfide.
  • Lead Sulfide.
  • Cadmium Selenide.
  • Lead Selenide.
  • Indium Antimonide.

In a photoresistor it is desirable to increase the dark resistance and also to reduce the dark current. In order to achieve this, the resistive path on the photoresistor is designed in a zigzag manner.


Working of the Photoresistor:


Photoresistor works on the principle of photoconductivity. The Photoresistor is covered with the transparent object which allows the light to pass through it. When the light falls on photoresistor an electron from the valence shell gets excited and enters into the conduction band thus in this way the number of available of electrons for conduction increases and the resistance of the light dependent resistor falls.


The photoconductivity is a phenomenon in which the absorption of light makes the material more electrically conductive.


The electrons jump into the conduction band from the valence only when the photon of sufficient energy is absorbed by the photoresistor. The energy that is required by the electron to excite from the valence band to the conduction band is called the energy gap and it varies from one semiconductor material to the other.

Applications of the LDR:

Although the Light Dependent Resistor is not a commonly used electronic element in electronic circuit however it is used in the embedded systems in which light is to be manipulated. Some of the embedded system applications of the Light Dependent Resistor are:

  • LDR is used in home automation system to automatically control lighting.
  • LDR is used in greenhouse monitoring system to regulate the light inside the greenhouse.
  • LDR is used as proximity sensor that measures the distance of the light source from the target object on which the sensor is mounted.
  • LDR is used in solar tracking system.

I hope this article would be helpful for you. I have briefly discussed the LDR, it basic construction, working and applications. That is all for now in the next article I will come up with more interesting topics in engineering. Till then stay connected, keep reading and enjoy learning.