The Light Dependent Resistor(LDR) is, as presto wrote, a resistor where the resistance is dependent on how strong the light that hits the light sensitive area. Typically a LDR have a resistance of several mega ohms in the dark, and less than 100 ohms in direct sunlight. The most common light-sensitive materials used in LDRs are Cadmium Sulphide(CdS) and Cadmium selenide(CdSe)

CdS based LDRs are sensitive to about the same part of the electromagnetic spectrum as the human eyes. CdS LDRs are most sensitive to light with a wavelength of about 520 nanometers(cyan).
CdSe LDRs are sensitive to infrared and the lower frequenciesof the visual spectrum. CdSe has a maximum sensitivity to light with a wavelength of 730 nm.
By mixing these two we can make LDRs with other maximum sensitivities.

LDRs may, if left in a constant light level, develop a temporary memory effect. This mean that their resistance may temporary be shifted LDRs are also very sensitive to temperature variations. Their resiatance can change by up to about 2% per Kelvin.

Electrical symbol: arrows pointing towards a normal resistor symbol
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