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photocell

A device that detects light. Used for photographic light meters, automatic on-at-dusk street lights and other light-sensitive applications, a photocell varies its resistance between its two terminals based on the amount of photons (light) it receives. Also called a "photodetector," "photoresistor" and "light dependent resistor" (LDR).

The photocell's semiconductor material is typically cadmium sulfide (CdS), but other elements are also used. Photocells and photodiodes are used for similar applications; however, the photocell passes current bi-directionally, whereas the photodiode is unidirectional. See photodiode.




Light Dependent Resistors
Photocells come in a variety of packages such as this assortment from PerkinElmer. As the photocell receives more photons, the resistance is lowered between the two terminals. (Image courtesy of PerkinElmer, Inc., www.perkinelmer.com)






photodiode

A light sensor (photodetector) that allows current to flow in one direction from one side to the other when it absorbs photons (light). The more light, the more current. Used to detect light in camera sensors, optical fibers and other light-sensitive applications, a photodiode is the opposite of a light emitting diode (see LED). Photodiodes detect light and let electricity flow; LEDs receive electricity and emit light.

Solar Cells Are Photodiodes
Solar cells are photodiodes that are chemically treated (doped) differently than the photodiode used as a switch or relay. When solar cells are struck by light, their silicon material is excited to a state where a small electrical current is generated. Many arrays of solar cell photodiodes are required to power a house. See photocell and phototransistor.



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