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From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group)
Newsgroups: sci.electronics
Subject: Anit-shoplifting device captured
Message-ID: <75638@iuvax.cs.indiana.edu>
Date: 1 Dec 90 16:34:09 GMT
Sender: daemon@iuvax.cs.indiana.edu
Lines: 20

A friend's wife bought a dress from a store which uses electronic 
anti-shoplifting tags.  Not only did the clerk fail to remove it, but 
it did not trip the alarm when she left.

It's a thick white plastic device.  Being a true electronics person, 
my friend dissected it.  He found a coil which absorbs energy from an 
external exciter, which is then rectified and used to power a one-
transistor oscillator on a different frequency.  He damaged the device 
during the inspection and could not determine frequencies, but 
believes that the output is LF, perhaps around 200 kHz.

We joked about what fun it would be to lurk near the store exit with a 
concealed oscillator, but decided that the exciter is probably 
modulated and the alarm receiver uses synchronous detection, which 
would be an easy way to reduce false alarms.  Anybody know details of 
these gadgets?

--

Frank Reid     reid@ucs.indiana.edu


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