photoresistors

A photoresistor, or light-dependent resistor (LDR), is a passive electronic device whose resistance changes in response to light. Typically, the resistance decreases as light intensity increases, enabling it to sense light levels for simple automatic control circuits.

Most photoresistors are made from a photosensitive semiconductor film such as cadmium sulfide (CdS) or cadmium sulfide selenide, deposited on a glass or ceramic substrate with interdigitated metal contacts. The resulting element behaves as a variable resistor whose conductance rises with illumination. The spectral response is determined by the semiconductor material, with CdS devices sensitive to visible light and a peak response in the green region. The electrical behavior is nonlinear, and the resistance can span several orders of magnitude from dark to bright conditions.

Typical performance characteristics include relatively slow response times compared with photodiodes and dependence on temperature and aging. LDRs can drift over time and may require conditioning or calibration. They are also sensitive to environmental factors such as humidity.

Common applications include light meters in exposure control for cameras, automatic street or garden lighting, light-sensing circuits in consumer electronics, alarms, and educational kits. In circuits they are often used in a voltage divider to convert light level into a readable electrical signal.

Cadmium-based photoresistors raise environmental and safety concerns; many regions regulate their use in consumer electronics. Alternatives include metal oxide photoresistors and semiconductor photodiodes or phototransistors, which offer faster response, better linearity, and improved stability, though at higher cost or complexity.