Photocells are circuit elements that have many applications. A major application of photocells is in automatic lighting devices such as "dusk-to-dawn" lights. Photocells react to ambient light via a changing electrical resistance.
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Photocells are simply resistors that have a light-dependent resistance. Photo-sensitive plates are mounted on the front of the resistor, and as more or less light hits these plates, the resistance value of the photocell changes.
Resistance and Current
As the resistance changes, the current in the circuit also changes, since resistors affect current in electric circuits. Current determines the brightness of a light.
Application to Lights
Photocells are commonly used in dusk-to-dawn lights. These lights turn on when the surrounding environment is dark and turn off when it becomes bright again. The photocells in the lights tell the circuit when to provide more or less current, thus turning the light on or off as needed.