photoresistor

A photoresistor is a two-terminal semiconductor device made from cadmium sulfide (CdS) or cadmium selenide (CdSe) whose electrical resistance drops sharply when exposed to light. In darkness, a typical CdS photoresistor might measure 10 megohms or higher; in bright sunlight, the same component can drop to 100 ohms or less. This inverse relationship between illumination and resistance makes the device useful for detecting presence, measuring intensity, and triggering switching action in response to ambient light changes.

The physics behind this behavior is straightforward: photons strike the semiconductor material and knock electrons into the conduction band, creating charge carriers that allow current to flow more easily. The effect is roughly proportional to light intensity over a useful range, though the response is not perfectly linear. CdS types are most sensitive to visible light around 500-600 nanometers (yellow-green), while CdSe variants shift sensitivity slightly toward red wavelengths. Response time typically ranges from tens to hundreds of milliseconds, depending on material thickness and doping.

In utility and industrial settings, photoresistors appear in street lighting control circuits, where they disable outdoor lamps during daylight and activate them at dusk. They are found in light-triggered alarm systems, exposure meters, and automated dimming circuits for industrial lighting. Because of their simple construction, low cost, and two-terminal design, they integrate easily into analog circuits without requiring active amplification, though modern designs often pair them with operational amplifiers to sharpen switching thresholds or linearize output.

Limitations and alternatives

The main drawback of photoresistors is slow response time and spectral sensitivity that does not match human vision perfectly. They also exhibit hysteresis: the resistance curve differs slightly depending on whether light is increasing or decreasing. In applications demanding fast switching (faster than 10 milliseconds) or precise spectral matching, photodiodes or phototransistors are preferred. Photoresistors also degrade over time, especially under intense UV exposure, and their absolute resistance values drift significantly with temperature.

The term photoresistor is largely interchangeable with light-dependent resistor (LDR) and photocell, though older texts sometimes reserve photocell for vacuum-tube photoelectric devices. Datasheets specify dark resistance, illuminated resistance at a standard light level (often 10 lux), response time, and temperature coefficient. For any application, especially in utilities where reliability and accuracy matter, it is essential to verify spectral response, aging characteristics, and temperature stability against the actual working environment.