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Q & A: Motion detectors

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Most recent answer: 03/11/2008
Q:
I was facinated to read an old article about how microwaves work. It explains about how water molecules are heated up, but here's my question - I am currently using dual technology motion detectors on an intruder alarm system and would like to know how the microwave parts of these units are used to detect motion. Do they look at reflection or absorbtion? can they see sideways motion? Can they cause heating in objects? Most importantly for me, do the microwaves only see objects that have water in them? If you passed an object made of wood or metal in front of it, would it see it and why? could you walk past a detector and not be seen if you wore reflective materials? I very much hope you can help as I've scoured the net and have found nothing that explains about these types of microwave. Hope this is an original question
- Sean Wallace
Halifax, West Yorkshire, England
A:
There is a wide variety of motion detectors utilizing different physical principles.  The power necessary to nuke a ham sandwich is far greater than that needed to detect motion so there would be little or no heating of a passing object. .Also, it doesn't seem that the frequencies of these detectors are chosen to be in a range strongly absorbed by water, unlike microave ovens.     Some of the detectors use reflected signals, others detect IR heat radiated by warm bodies etc., etc.  Some can detect sideways motion, others can't.  There is really no end of ways to detect motion.   Here is a web site you can take a look at that describes them in more detail: 

LeeH +MW

(published on 03/11/2008)

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