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From: charlie@oakhill.UUCP (Charlie Thompson)
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <875@oakhill.UUCP>
Date: Mon, 27-Apr-87 18:05:07 EDT
Article-I.D.: oakhill.875
Posted: Mon Apr 27 18:05:07 1987
Date-Received: Wed, 29-Apr-87 03:12:16 EDT
References: <146@illusion.UUCP> <868@oliveb.UUCP> <1726@kitty.UUCP>
Organization: Motorola Inc. Austin, Tx
Lines: 22
Summary: Library Book Detectors


OK Larry, I'll make a guess at how the LC resonant circuit with
stripline antenna works......

The circuit is activated by gaussing and degaussing a magnetically
biased inductor.  When the inductor is magnetized (gaussed) the material
that the inductor is formed from becomes *saturated* (i.e. its
small signal inductance is reduced) if the inductance is reduced
then the tuning for the tank circuit goes up (like out of resonance
for the giant grid dip meter that forms the detector).  When a person
carries the book through the RF field he does not *resonate*...provided
the book has been legally deactivated.  To re-arm the book, when it
is checked in, the little inductor is de-gaussed using a decaying
AC sinewave just like your color TV uses.  The inductor will then
be un-saturated and will allow LC resonance at the detection frequency.
So how'd I do Larry?  Gee if nobody thought of this maybe I should
patent it (fat chance now that's it's in netland!).


Charlie Thompson WB4HVD
Motorola DSP Operations
Austin, TX