What are Light Dependent Resistors and How do they work

LDR (Light dependent resistors) are often used in circuits to detect the presence or the level of light. They can be described by a variety of names:

1.      Photoresistor

2.      Photocell

3.      Photoconductor

Please note that they are not to be confused with photodiodes or photo-transistor, which are P-N Junction based devices; LDR are purely resistive with resistance falling as the level of light increases. These changes in   resistance for a particular light level can be quite large.

LDRs or photoresistors are a particularly convenient electronics component to use such as in photographic light meters or even to control when streetlights turn on. 

So what are they made of? And how do they work?

LDR - It's all about Electrons trapped in a Crystal Lattice

Most LDR are made of semiconductor materials that have light sensitive properties. Many materials can be used, but the most popular material for these photoresistors is cadmium sulphide, CdS.

The appearance of a typical Photoresistor is as shown below:

There are two types of LDR or Photoresistors:

1.      Intrinsic photoresistors - Intrinsic photoresistors use un-doped semiconductor materials including silicon or germanium. Photons fall on the LDR excite electrons moving them from the valence band to the conduction band. As a result, these electrons are free to conduct electricity.

2.      Extrinsic photoresistors -  Extrinsic photoresistors are manufactured from semiconductor of materials doped with impurities. These impurities or dopants create a new energy band above the existing valence band. As a result, electrons need less energy to transfer to the conduction band because of the smaller energy gap.

Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light.

To understand how they work, it's first necessary to understand that an electrical current is the movement of electrons within a material based on an applied p.d (potential Difference) or Voltage. This allows us to divide material into three classes:

1.      Good conductors - have a large number of free electrons that can drift in a given direction under the action of a potential difference.

2.      Insulators - high resistance have very few free electrons, and therefore it is hard to make the them move.

3.      Semiconductors - These are materials that are in-between the properties of Good conductors and Insulators. They have charge carriers called holes (positive) and electrons (negative) that only move when a specific voltage is applied in a certain direction.

3.

As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice. Some of their energy is transferred to the electrons.  This gives some of them sufficient energy to break free from the crystal lattice causing electrons to flow. This results in a lowering of the resistance of the LDR resistance.

The relation between Resistance and light intensity is linear, once you plot the values logarithmically; as more light shines on the LDR semiconductor, the resistance falls further. This results in more electrons being released from the crystal lattice of the semiconductor material making up the LRDR, causing more electricity to flow.