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Speaker 1: Welcome to brain Stuff from how stuff works dot com

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Speaker 1: where smart Happens. Hi'm Marshall Brain with today's question, how

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Speaker 1: do motion sensing lights and burglar alarms work? There are

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Speaker 1: many different ways to create a motion sensor. For example,

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Speaker 1: it's common for stores to have a beam of light

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Speaker 1: crossing the room near the door and a photo sensor

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Speaker 1: on the other side of the room. When a customer

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Speaker 1: breaks the beam, the photo sensor detects the change in

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Speaker 1: the amount of light and rings a bell. Many grocery

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Speaker 1: stores have automatic door openers that use a very simple

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Speaker 1: form of radar to detect when someone passes near the door,

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Speaker 1: and the same thing can be done with ultrasonic sound waves,

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Speaker 1: bouncing them off a person and waiting for the echo.

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Speaker 1: All of these are active sensors. They inject energy, light, microwaves,

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Speaker 1: or sound into the environment in order to detect a

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Speaker 1: change of some sort. The motion sensing feature on most

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Speaker 1: lights and security systems is a passive system that detects

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Speaker 1: infrared energy. These sensors are therefore known as p i

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Speaker 1: R or passive infrared detectors or pyroelectric sensors. In order

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Speaker 1: to make a sensor that can detect the human being.

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Speaker 1: You need to make the sensor sensitive to the temperature

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Speaker 1: of a human body. Humans have a skin temperature of

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Speaker 1: about ninety three degrees fahrenheit, and they radiate infrared energy

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Speaker 1: at a wavelength between nine and ten micrometers. Therefore, the

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Speaker 1: sensors are typically sensitive in the range of eight to

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Speaker 1: twelve micrometers. The devices themselves are simple electronic components, not

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Speaker 1: unlike a photo sensor. The infrared light bumps electrons office substrate,

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Speaker 1: and these electrons can be detected and amplified into a signal.

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Speaker 1: You've probably noticed that your lights are sensitive to motion,

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Speaker 1: but not to a person who's standing still. That's because

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Speaker 1: the electronics package attached to the sensor is looking for

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Speaker 1: a fairly rapid change in the amount of infrared energy.

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Speaker 1: When a person walks by, the amount of infrared energy

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Speaker 1: in the field of view changes rapidly, and that's easily detected.

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Speaker 1: You don't want the sensor detecting slower changes, like the

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Speaker 1: sidewalk cooling off at night. Be sure to check out

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