A photoresistor, also known as a light-dependent resistor (LDR) or photocell, is a type of resistor that changes its resistance in response to light. Photoresistors are made from semiconductor materials such as cadmium sulfide (CdS) or cadmium selenide (CdSe), and have a high resistance in darkness and a low resistance when exposed to light.
Photoresistors are commonly used in a variety of applications as sensors for detecting light levels. For example, they can be used in automatic light switches to turn lights on and off based on the ambient light level, or in burglar alarm systems to detect intruders. Photoresistors can also be used in circuits to control the brightness of a display or the speed of a motor, or to measure light intensity for scientific or medical purposes.