Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!valid!caber!lou
From: lou@caber.valid.com (Louis K. Scheffer)
Newsgroups: sci.electronics
Subject: Re: Anti-shoplifting devices
Message-ID: <443@valid.valid.com>
Date: 14 May 91 19:52:28 GMT
References: <050691.224256WDBURNS%MTUS5.BITNET> <19934@slice.ooc.uva.nl> <1991May9.135021.22131@sparrms.ists.ca> <1991May10.003238.24277@comp.vuw.ac.nz>
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cyborg@kauri.vuw.ac.nz (Alex Ivopol) writes:

>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.

Assuming this is the gadget I'm familiar with:
There is a diode in between the coil and the capacitor.  When you hit it with
microwaves, you get back second harmonic due to the non-linearity of the diode.
When you buy the product, the cashier zaps the diode with a small gun like
tool - then you get no second harmonic, and no alarm.

-Lou Scheffer-