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Re: Security sensors



On Fri, 7 Jan 2000, Tony Wayne wrote:

I *faintly* remember a discussion once regarding how store security stickers
work. I'm looking at 2 types of sensors in my hand. One is a rectangle with
a spiraling wire on the back of a sticker. The other looks like 2 pieces of
metal in a plastic container. Could anyone explain to me how these work?

Vague recollection, might not be entirely accurate: One of the two pieces
in the plastic container is "metglass" foil, and is magnetized so it can
be vibrated with an AC magnetic field. The two metal pieces also act as a
UHF (microwave?) antenna and resonant circuit. The shoplifting alarm
system sends out an AC b-field to physically vibrate the metal foil, and
also sends out a UHF signal in order to detect the (oscillating) resonant
circuit. To disable the sensor, demagnetize the metglass foil.


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