light-dependent resistor

Component of electronic circuits whose resistance varies with the level of illumination on its surface. Usually resistance decreases as illumination increases, as the nature of the material is altered by the presence of light. LDRs are used in light-measuring or light-sensing instruments where light intensity is converted to a digital signal (for example, in the exposure-meter circuit of an automatic camera), and in switches (such as those that switch on street lights at dusk).

LDRs are made from semiconductors, such as cadmium sulphide.