What is Light Dependent Resistor (LDR)? Explain with diagram

The Light Dependent Resistor means an LDR. It is a passive transducer. It is also known as photoconductive cell because its conductivity changes due to change in light intensity.

Construction

The light dependent resistor (LDR) is made up of photosensitive material like Cadmium Sulphide (CdS), Selenium (Se), Cadmium Selenide (CdSe) or Lead Sulphide (PbS). It is deposited on insulating surface like ceramic substrate in the form of zigzag wire as shown in following diagram.

It is enclosed in round metallic or plastic case and two terminals are taken out for external connections. The structure is covered with glass sheet to protect it from moisture and dust and allows only light to fall on it.

When light falls on it, its resistance decreases and when it is dark, its resistance becomes maximum. The change in its resistance is inversely proportional to intensity of light falling on it.

Automatic Street Lights Control

In following circuit the light dependent resistor is used to control the street lights automatically, to switch it on during evening and switch off in the morning.

Working: When there is no light on the LDR i.e. in evening, its resistance increases. So transistor T₁ is ON and T₂ is also ON. The relay is activated and the street lights are switched on.

When it is morning, the ambient light falls on the LDR and its resistance decreases. So voltage across it is insufficient to keep T₁ in ON condition. So both T₁ & T₂ are OFF, relay is deactivated and street lights are switched off.

More Applications

1. Used in burglar alarm systems.
2. Products counter system used in industries.
3. Used in photometer to measure the brightness of light.