A light-dependent resistor (LDR) (a.k.a. photoresistor, photocell, photoconductor) is a basic electronic component which can be used to measure light levels. It contains a resistor which changes it’s resistance when exposed to light.
LDRs are made with semiconductor-based materials to allow them to be sensitive to light. A once popular semiconductor for LDRs was cadmium sulphide (CdS), however it’s use is becoming restricted due to the environmental problems that Cadmium can cause.
The semiconductor has a crystal lattice which, in the absence of light, greatly restricts the movement of electrons across it’s structure. This gives the component a large resistance in the dark. When light strikes the lattice, some of the photon energy is transferred to the electrons, which allows them to break free and conduct. This causes the resistance of component to decrease.
Even though an LDR contains semiconductor material, it is not considered an active device (such as a diode or transistor) as it does not contain a PN junction.
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