Photoresistor/LDR

A photoresistor (LDR - Light Dependent Resistor) is a passive electronic component whose resistance changes inversely with the amount of light falling on its surface. When exposed to bright light, its resistance decreases significantly, and in darkness, its resistance increases to very high values. Commonly made from cadmium sulfide (CdS) or other photosensitive materials, photoresistors are widely used in light-sensing circuits such as automatic street lights, camera light meters, and alarm systems.