Motion sensor lights work by sending out a signal of some kind. When that signal is broken, the light turns on automatically. Different motion sensors operate with different types of signals.
Passive sensors use infrared energy to detect body heat emitted by people or animals. After the sensor is turned on, it works to detect any increase in heat in a specific gridded area and turns on when it finds one. Microwave sensors send out pulses that reflect off nearby objects and detect when an object is moving. Whenever a sensor determines that an object has moved too close, it turns on the light.
Some motion sensors employ more than one type of technology and are combined with alarm systems that trigger alarms when they are tripped. Using more than one type of sensor helps prevent false alarms because they both have to agree before the alarm is sounded. Some sensors have special settings to prevent domestic pets from setting off the sensor.
Many people install motion sensor lights around their home for safety and security. They help light up dark corners and turn on automatically when owners pull their cars into the driveway. When attached to an alarm system, the sensors provide even more security.