You may have seen automatic doors in hotels, supermarkets etc and automatic lighting system in big hotels. The door opens and closes at proper time and light switches on/off depending on the occupancy of the hall. All these use some kind of sensors to detect the presence of people. Such sensors could in general be called motion detectors or motion sensors.
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A tiny motion detector at the door uses a simple form of radar to know when to activate the door. The radar sends out ample amount of microwave energy. These waves bounce back off the floor back to the sensor. In case a person enters, the waves bounce back a little earlier (right off the body). The detector registers a change in the time it takes for the waves to return. Whenever change in time gets registered, it opens the doors and lets the person pass through.
For activating lights when the hall is occupied and switching it off when empty, passive infrared detector (PIR) or pyroelectric sensor is used. This sensor can detect infrared energy. Our human body radiates infrared waves with a wavelength between 9 and 10 micrometers. The sensors are so designed to be sensitive in the range of 8 – 12 micrometers. On sensing heat, the electronic chip turns the light ON.