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From: larry@tapa.UUCP
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <522@tapa.UUCP>
Date: Thu, 23-Apr-87 21:02:04 EST
Article-I.D.: tapa.522
Posted: Thu Apr 23 21:02:04 1987
Date-Received: Sat, 25-Apr-87 07:45:27 EST
References: <959@obelix.UUCP> <1104@nonvon.UUCP> <5941@pur-ee.UUCP> <2835@mit-hermes.AI.MIT.EDU> <17024@sun.uucp>
Reply-To: larry@.UUCP (Larry Pajakowski)
Organization: Tapa Systems, Deerfield, IL
Lines: 9

The one flat "tag" I took apart had a small stripline type antenna in it
and a very small encapsulated device.  I suspect it is a diode and the
antenna/tuned circuit resonates at a harmonic of some UHF or microwave
frequency.  Probably a low level microwave signal is sent out and the
diode produces enough harmonics to be detected by a receiver.  Mixing
could also be used.  Sounds cheap enough to me.  

The deacivation device probably just burns out the diode with a little
current.