Photoresistor - Photocell - Light-Dependent Resistor

A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), photocell or photoconductor.

A photoresistor is made of a high-resistance semiconductor. If light falling on the device is of high enough, photons absorbed by the semiconductor give bound elctrons enough energy to jump into the conduction band. The resulting free electron conduct electricity, thereby lowering resistance.

Cadmium sulphide cells - CdS

CdS cells rely on the material's ability to vary its resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance. Although not accurate, even a simple CdS cell can have a wide range of resistance from less than 100 Ω in bright light to in excess of 10 MΩ in darkness. The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches.

Applications

Photoresistors (LDR) come in many different types. Inexpensive cadmium sulphide cells can be found in many consumer items such as clock radios, cameras, light meter, security alarm systems, street lights and outdoor clocks.