Photoresistive Element

A photoresistive element, also known as a photoresistor or light-dependent resistor (LDR), is a passive semiconductor device whose electrical resistance decreases with increasing incident light intensity. It operates on the principle of photoconductivity, where photons with sufficient energy excite electrons from the valence band to the conduction band, increasing the number of charge carriers and thereby reducing resistance. In industrial photocell sensors, it serves as the primary sensing component for detecting presence, position, or light levels in automated systems.

The working principle is based on photoconductivity. When photons strike the semiconductor material (typically cadmium sulfide or cadmium selenide), they provide enough energy to excite electrons from the valence band to the conduction band, creating electron-hole pairs. This increases the number of free charge carriers, which lowers the electrical resistance of the material proportionally to the light intensity. In darkness, the resistance is high (typically megaohms), while under illumination, it drops significantly (to kiloohms or less). This resistance change is measured by a circuit to produce an output signal.