Photocell sensors are light-detecting devices that act as automatic switches to power electrically powered devices on or off in the presence of light. One common use is in automatic lights that turn on at dusk and off at sunrise.
Manufacturers construct photocell sensors using high-resistance semiconductors. In the dark, the resistance is high enough to prevent the flow of electricity. However, in the light, the resistance drops, allowing the power to flow. Manufacturers pair the dissimilar materials to produce the photocell with the characteristics the application requires.
In a typical American streetlight, the photocell sensor powers a heating element. When the element is powered and hot, the switch remains open. However, at night, when the photocell is not receiving sunlight, the heating element cools, the switch closes and the streetlight powers to the on position.
Older movie projectors use photocells to convert the sound recording on the film into the voices of the players. In automated restrooms, a sensor detects a person moving into a position to break the light beam to start the water flowing in the sink, flush the toilet and dispense soap or paper towels. Burglar alarms use the sensors in motion detectors to warn of intruders into a home or business.