In a residential alarm system, which type of sensor is commonly used to detect motion?

In a residential alarm system, an infrared sensor is commonly used to detect motion because it operates by sensing changes in infrared radiation, which is emitted by objects based on their temperature. When a person or object moves within the sensor's range, it changes the amount of infrared radiation detected, triggering the alarm.

Infrared sensors are popular in alarm systems because they can cover a wide area and have the ability to differentiate between moving objects and stationary ones, making them effective for security applications. Their ability to ignore small animals while detecting larger heat sources contributes to their effectiveness in home security environments.

Magnetic reed switches are primarily used for detecting the opening and closing of doors or windows, rather than motion within a space. Photoelectric cells are typically used for detecting light levels, and temperature sensors measure the ambient temperature rather than detecting movement. Therefore, among the options provided, the infrared sensor is the most suitable choice for detecting motion in a residential alarm system.