Speaker 1: Welcome to brain Stuff from how stuff works dot com
Speaker 1: where smart Happens. Hi'm Marshall Brain with today's question, how
Speaker 1: do motion sensing lights and burglar alarms work? There are
Speaker 1: many different ways to create a motion sensor. For example,
Speaker 1: it's common for stores to have a beam of light
Speaker 1: crossing the room near the door and a photo sensor
Speaker 1: on the other side of the room. When a customer
Speaker 1: breaks the beam, the photo sensor detects the change in
Speaker 1: the amount of light and rings a bell. Many grocery
Speaker 1: stores have automatic door openers that use a very simple
Speaker 1: form of radar to detect when someone passes near the door,
Speaker 1: and the same thing can be done with ultrasonic sound waves,
Speaker 1: bouncing them off a person and waiting for the echo.
Speaker 1: All of these are active sensors. They inject energy, light, microwaves,
Speaker 1: or sound into the environment in order to detect a
Speaker 1: change of some sort. The motion sensing feature on most
Speaker 1: lights and security systems is a passive system that detects
Speaker 1: infrared energy. These sensors are therefore known as p i
Speaker 1: R or passive infrared detectors or pyroelectric sensors. In order
Speaker 1: to make a sensor that can detect the human being.
Speaker 1: You need to make the sensor sensitive to the temperature
Speaker 1: of a human body. Humans have a skin temperature of
Speaker 1: about ninety three degrees fahrenheit, and they radiate infrared energy
Speaker 1: at a wavelength between nine and ten micrometers. Therefore, the
Speaker 1: sensors are typically sensitive in the range of eight to
Speaker 1: twelve micrometers. The devices themselves are simple electronic components, not
Speaker 1: unlike a photo sensor. The infrared light bumps electrons office substrate,
Speaker 1: and these electrons can be detected and amplified into a signal.
Speaker 1: You've probably noticed that your lights are sensitive to motion,
Speaker 1: but not to a person who's standing still. That's because
Speaker 1: the electronics package attached to the sensor is looking for
Speaker 1: a fairly rapid change in the amount of infrared energy.
Speaker 1: When a person walks by, the amount of infrared energy
Speaker 1: in the field of view changes rapidly, and that's easily detected.
Speaker 1: You don't want the sensor detecting slower changes, like the
Speaker 1: sidewalk cooling off at night. Be sure to check out
Speaker 1: our new video podcast, Stuff from the Future. Join how
Speaker 1: Stuff Work staff as we explore the most promising and
Speaker 1: perplexing possibilities of tomorrow. The house stuff works, I find
Speaker 1: app has arrived down at it today on iTunes