Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mcnc!gatech!udel!wuarchive!uunet!isis!whester
From: whester@isis.cs.du.edu (William R. Hester)
Newsgroups: sci.electronics
Subject: Re: Stock Control Tags and Theft
Message-ID: <1990Oct16.221817.14620@isis.cs.du.edu>
Date: 16 Oct 90 22:18:17 GMT
References: <1118@cameron.egr.duke.edu>
Reply-To: whester@isis.UUCP (William R. Hester)
Distribution: usa
Organization: Math/CS, University of Denver
Lines: 58

In article <1118@cameron.egr.duke.edu> wdp@dukee.egr.duke.edu (William D.
Palmer) writes:
>Friday, 12 OCT 90
>
>Simon Kravis writes:
>
>> From: skravis@csis.dit.csiro.au (Simon Kravis)
>> Subject: Stock control tags
>> Date: 12 Oct 90 04:48:01 GMT
>> 
>> Does anyone know how the detection system for the metallic tags attach
d to
>> CDs and other small and valuable items in shops works? ...
>
>The tags of this type that I have seen are LC tank circuits.  A flat coi
 is 
>made of copper (or some other non-magnetic material) with a capacitor (s
mply
>two layers of foil separated by an insulator) made of some magnetic meta
 in
>the center.  The coil and capacitor are connected in parallel.  When the

>magnetic material in the center is demagnetized, the flat coil will have
a 
>certain inductance, producing one resonant frequency for the tank.  Magn
tize 


Looks like there are different types of detectors...the tags that I took
apart do have the L-C resonant tank circuit but also have a small diode
which I assume acts as a frequency doubler or tripler and re-radiates
a signal on a harmonic which is then picked up as an alarm.

 



















-- 
Bill Hester, Ham Radio N0LAJ, Denver CO., USA
Please route replies to: whester@isis.cs.du.edu or uunet!isis!whester   
Public Access Unix @ University of Denver, Denver Colorado USA
(no official affiliation with the above university)