Welcome to brain Stuff from how stuff works dot com where smart Happens. Hi'm Marshall Brain with today's question, how do motion sensing lights and burglar alarms work? There are many different ways to create a motion sensor. For example, it's common for stores to have a beam of light crossing the room near the door and a photo sensor on the other side of the room. When a customer breaks the beam, the photo sensor detects the change in
the amount of light and rings a bell. Many grocery stores have automatic door openers that use a very simple form of radar to detect when someone passes near the door, and the same thing can be done with ultrasonic sound waves, bouncing them off a person and waiting for the echo. All of these are active sensors. They inject energy, light, microwaves, or sound into the environment in order to detect a
change of some sort. The motion sensing feature on most lights and security systems is a passive system that detects infrared energy. These sensors are therefore known as p i R or passive infrared detectors or pyroelectric sensors. In order to make a sensor that can detect the human being. You need to make the sensor sensitive to the temperature of a human body. Humans have a skin temperature of about ninety three degrees fahrenheit, and they radiate infrared energy
at a wavelength between nine and ten micrometers. Therefore, the sensors are typically sensitive in the range of eight to twelve micrometers. The devices themselves are simple electronic components, not unlike a photo sensor. The infrared light bumps electrons office substrate, and these electrons can be detected and amplified into a signal. You've probably noticed that your lights are sensitive to motion,
but not to a person who's standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy. When a person walks by, the amount of infrared energy in the field of view changes rapidly, and that's easily detected. You don't want the sensor detecting slower changes, like the sidewalk cooling off at night. Be sure to check out
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