WEBVTT - How do motion sensors work?

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<v Speaker 1>Welcome to brain Stuff from how stuff works dot com

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

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<v Speaker 1>do motion sensing lights and burglar alarms work? There are

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<v Speaker 1>many different ways to create a motion sensor. For example,

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<v Speaker 1>it's common for stores to have a beam of light

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<v Speaker 1>crossing the room near the door and a photo sensor

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<v Speaker 1>on the other side of the room. When a customer

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<v Speaker 1>breaks the beam, the photo sensor detects the change in

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<v Speaker 1>the amount of light and rings a bell. Many grocery

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<v Speaker 1>stores have automatic door openers that use a very simple

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<v Speaker 1>form of radar to detect when someone passes near the door,

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<v Speaker 1>and the same thing can be done with ultrasonic sound waves,

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<v Speaker 1>bouncing them off a person and waiting for the echo.

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<v Speaker 1>All of these are active sensors. They inject energy, light, microwaves,

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<v Speaker 1>or sound into the environment in order to detect a

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<v Speaker 1>change of some sort. The motion sensing feature on most

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<v Speaker 1>lights and security systems is a passive system that detects

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<v Speaker 1>infrared energy. These sensors are therefore known as p i

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<v Speaker 1>R or passive infrared detectors or pyroelectric sensors. In order

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<v Speaker 1>to make a sensor that can detect the human being.

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<v Speaker 1>You need to make the sensor sensitive to the temperature

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<v Speaker 1>of a human body. Humans have a skin temperature of

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<v Speaker 1>about ninety three degrees fahrenheit, and they radiate infrared energy

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<v Speaker 1>at a wavelength between nine and ten micrometers. Therefore, the

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<v Speaker 1>sensors are typically sensitive in the range of eight to

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<v Speaker 1>twelve micrometers. The devices themselves are simple electronic components, not

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<v Speaker 1>unlike a photo sensor. The infrared light bumps electrons office substrate,

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<v Speaker 1>and these electrons can be detected and amplified into a signal.

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<v Speaker 1>You've probably noticed that your lights are sensitive to motion,

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<v Speaker 1>but not to a person who's standing still. That's because

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<v Speaker 1>the electronics package attached to the sensor is looking for

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<v Speaker 1>a fairly rapid change in the amount of infrared energy.

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<v Speaker 1>When a person walks by, the amount of infrared energy

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

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<v Speaker 1>You don't want the sensor detecting slower changes, like the

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<v Speaker 1>sidewalk cooling off at night. Be sure to check out

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