CdScell

CdS cell, commonly called a cadmium sulfide cell, is a light-dependent resistor used as a photosensor in electronic circuits. It consists of a thin film of cadmium sulfide deposited on a ceramic or glass substrate with interdigitated metal contacts, forming a variable resistance element controlled by incident light.

Operation: When photons are absorbed by CdS, electron-hole pairs are generated, lowering the material’s resistance as light intensity increases. In darkness the resistance can be on the order of megaohms; in bright light it may drop to kiloohms or less. The response is nonlinear and relatively slow compared with modern photodiodes, with response times ranging from milliseconds to seconds depending on device construction and temperature.

Spectral response: CdS cells are most sensitive in the green region, with a peak around 540–560 nanometers. They respond across roughly 400–700 nm but lack strong wavelength discrimination, making them sensitive to ambient lighting conditions and color temperature.

Construction and characteristics: common CdS photoresistors are inexpensive, rugged, and easy to read with simple analog circuits. They exhibit significant device-to-device variation, drift with temperature, and a nonlinear relationship between light level and resistance.

Applications: used in light meters for photography, automatic exposure controls, street or facade lighting controllers, and other ambient-light sensing tasks where slow response and low cost are acceptable.

Safety and environmental considerations: cadmium compounds are toxic; CdS devices require proper handling, recycling, and compliance with regulations governing cadmium-containing electronics.

History: Cadmium sulfide photoresistors were developed in the early 20th century and enjoyed widespread use before silicon photodiodes, but they remain available as inexpensive light sensors for specific low-speed applications.