LDRs

LDR stands for light-dependent resistor, also known as a photoresistor. It is a passive electronic component whose resistance varies with ambient light level. Most LDRs are made from a cadmium sulfide (CdS) or cadmium selenide (CdSe) semiconductor material deposited as a thin film on a glass or ceramic substrate with interdigitated metal electrodes. In darkness, the resistance is high; when illuminated, light generates charge carriers and lowers the resistance.

Electrical characteristics of LDRs vary by device, but they typically range from several megohms in bright light to a few kilohms or less in darkness. They are relatively slow compared with photodiodes or phototransistors, with response times commonly in the tens of milliseconds range. LDRs have a broad, non-linear response and are temperature dependent, so output can drift with temperature and device aging. The spectral sensitivity is strongest in the green region around 520–550 nm; they are less sensitive to red or blue and are largely insensitive to infrared.

Common applications include light-sensing circuits such as automatic lighting controls, night/day sensors, camera exposure meters, and burglar alarms. They are also used in educational experiments and in simple light meters. In circuit design, an LDR is typically placed in a voltage divider with a fixed resistor, and the resulting voltage is read by a microcontroller or analog-to-digital converter to infer light level. While inexpensive and easy to use, LDRs trade speed, linearity, and precision for simplicity and cost.