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Light dependent resistors (LDRs) use a semiconductor material (i.e., a material that is neither a conductor nor an insulator) whose electrical characteristics vary according to the amount of incident light. The two semiconductor materials used for the manufacture of LDRs are cadmium sulphide (CdS) and cadmium selenide (CdSe). These materials are most sensitive to light in the visible spectrum, peaking at about 0.6μm for CdS and 0.75μm for CdSe.
A typical Cadmium Sulphide LDR exhibits a resistance of around 1MΩ in complete darkness and less than 1kΩ when placed under a bright light source. A typical application for such a component is in automatic lighting control. A small current is applied to the LDR and this produces a voltage drop which changes according to the level of incident light. The voltage from the LDR is passed to a transistor or operational amplifier stage which, in turn, operates a relay or solid-state switching device (such as a thyristor or triac).
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