fotoresistos

A fotoresistor, also called a photoresistor or light-dependent resistor (LDR), is a passive electronic component that changes its electrical resistance in response to light intensity. The most common type is made from cadmium sulfide (CdS), though other cadmium-based materials have been used. These devices are typically produced as small round disks or rectangular packages with a thin photosensitive semiconductor layer between interdigitated metal contacts.

Operation and characteristics: when exposed to light, photons generate charge carriers in the semiconductor, reducing its resistance. In darkness, the resistance is high; in bright light, it drops by several orders of magnitude. The spectral sensitivity generally peaks in the green portion of the spectrum (approximately 500–550 nm) and diminishes toward the red and blue ends. Compared with modern photodiodes or phototransistors, photoresistors are relatively slow, with response times ranging from tens to hundreds of milliseconds or more.

Construction and specifications: resistance values vary widely by part, commonly ranging from a few kilohms in bright illumination to several megohms in darkness. They are inexpensive, compact, and require no external power to sense light, though the surrounding circuit must supply a bias and read the resulting voltage or current.

Applications and limitations: photoresistors are used in light meters, outdoor lighting controls, camera exposure metering, alarm and novelty projects, and educational experiments. Limitations include nonlinear and temperature-dependent response, limited spectral selectivity, aging under intense illumination, and slower response compared with solid-state photodetectors. They are often supplanted in precision or high-speed applications by photodiodes or phototransistors.