How are photoelectric sensors used in burglar alarms and garage-door openers?

Photoelectric sensors used in burglar alarms and garage-door openers rely on the photoelectric effect, a phenomenon where light striking a material causes the emission of electrons.1

These devices utilize a photoelectric cell—or more modern semiconductor-based photodiodes—to maintain a constant beam of light directed at a sensor. As long as the light beam remains unbroken, the sensor detects the illumination and maintains a specific electrical state.12

When an object, such as a person or a vehicle, moves across the path of the beam, it interrupts the light. This sudden change in light intensity is immediately detected by the sensor, which triggers an electrical response to activate an alarm or stop a garage door from closing. This application is highly effective due to the fast response time of photoelectric devices, which allows for near-instantaneous detection of obstructions.12