A motion detector is a device that detects movement. It is often used as a security measure to alert people to the presence of unwanted visitors. Motion detectors can also be used to automate tasks, such as turning on lights when someone enters a room.

There are many different types of motion detectors, but they all work on the same basic principle. They use a sensor to detect changes in the environment, such as changes in temperature, light, or sound. When the sensor detects a change, it sends a signal to a control unit, which then activates an alarm or performs another action.