Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!ooc.uva.nl!ropg
From: ropg@ooc.uva.nl (Rop Gonggrijp)
Newsgroups: sci.electronics
Subject: Re: Library anti-theft walk-thru device...????
Message-ID: <19934@slice.ooc.uva.nl>
Date: 7 May 91 20:48:10 GMT
References: <050691.224256WDBURNS%MTUS5.BITNET>
Organization: Hack-Tic
Lines: 22

They mostly work as follows:

In the book is a strip of metal (any metal) covered by a piece of ferrite.
The ferrite is magnetised, thus changing the frequency at which this gizmo
resonates (it is covered in plastic, so it does resonate).

If they demagnetize the ferrite the thing goes to a lower resonating freq.

One end of the gate emits the first (high) frequency and the other end detects
it. A sudden drop in power means something is 'stealing' energy / resonating
and thus the alarm is activated.

These systems usually work at 915 MHz or so....... Have fun beating it

Oh, by the way, if you transmit this freq. and then play with the transmitted
power you should be able to make the bell ring at quite considderable distance.

-- 
Rop Gonggrijp (ropg@ooc.uva.nl) is also editor of  Hack-Tic (hack/phreak mag.)
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