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Newsgroups: sci.electronics
Subject: Re: Anti-shoplifting devices
Message-ID: <1991May10.003238.24277@comp.vuw.ac.nz>
From: cyborg@kauri.vuw.ac.nz (Alex Ivopol)
Date: Fri, 10 May 1991 00:32:38 GMT
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While on this topic, I saw an interesting device as well. It is disguised as
a price tag sticker. Once stuck on something it is very difficult to tell it
is more than that.

Inside there is a spiral made of a narrow strip of thin aluminium foil. The
spiral is made of straight segments that bend at 90 deg angle and has about 20
turns, the whole thing is square in shape. The outer arm of the spiral ends in
a much thicker (rectangular) strip. A thin insulating foil is placed on top of
this with a small hole just on top of the start of the inner arm of the spiral.

Another narrow strip of aluminium foil is placed on top of the insulating foil,
this strip contacs the inner arm of the spiral, runs across the turns and ends
in a thick rectangular shape, just on top of the other rectangle beneath the
insulating foil.

This whole arrangement looks very much like a capacitor in parallel with a coil
Don't ask me how it works tho...  I realize that this would most probably
resonate in an electromagnetic field of the right frequency, but how does one
disable it once you bought the product and go out the shop through the gates ?
The sticker is diposable and stays on the merchandise once you bought it. The
salespersons do not do anything funny with the products (like placing them
briefly under the counter) before handing them back to you...
--
Alex T. IVOPOL
cyborg@kauri.vuw.ac.nz