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A photo-resistor (or light-dependent resistor or a photocell) is a light-controlled variable resistor. The resistance of a photo-resistor decreases with increasing incident light intensity in other words, it exhibits photo-conductivity. A photo-resistor can be applied in light-sensitive circuits, and light and dark activated switching circuits.
A photo-resistor is made of a high resistance semiconductor. In the dark, a photo-resistor can have a resistance as high as several megohms (MΩ), while in the light, a photo-resistor can have a resistance as low as a few hundred ohms. If incident light on a photo-resistor exceeds a certain frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band.
The resulting free electrons (and their hole partners) conduct electricity, thereby lowering resistance. The resistance range and sensitivity of a photo-resistor can substantially differ among dissimilar devices. Moreover, unique photo-resistors may react substantially differently to photons within certain wavelength bands.
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