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From: cmcmanis@sun.uucp (Chuck McManis)
Newsgroups: sci.electronics
Subject: Library Book detectors
Message-ID: <17024@sun.uucp>
Date: Mon, 20-Apr-87 18:25:29 EST
Article-I.D.: sun.17024
Posted: Mon Apr 20 18:25:29 1987
Date-Received: Tue, 21-Apr-87 05:38:58 EST
References: <959@obelix.UUCP> <1104@nonvon.UUCP> <5941@pur-ee.UUCP> <2835@mit-hermes.AI.MIT.EDU>
Organization: Sun Microsystems, Inc. - Mtn View, CA
Lines: 28
Summary: Maybe it 'reactivated'

In article <2835@mit-hermes.AI.MIT.EDU>, jpexg@mit-hermes.AI.MIT.EDU (John Purbrick) writes:
> My girlfriend visited a library at MIT with me a while ago, and she was
> carrying a book (legally obtained) from a different college's library.
> As we left through the stolen-book detector, the beeper went off. It 
> turned out that the "foreign" book was setting the thing off; the desk
> person ran the offending book through the deactivator, and we had no more
> trouble. So the deactivation is apparently only for certain frequencies
> or codes.

It occured to me reading this that the 'deactivation' may be nothing more
than charging up the capacitor in the LC circuit. Assuming that the system
does indeed operate by the drop in energy in the RF field when this passive
circuit comes into range. So the library runs the book through an inductive
precharger, when you leave the library since the capacitor is already charged 
there and presumably at a steady state, there would not be any sudden 
disruption in the field, however after a few hours (or maybe a few minutes)
the capacitor has discharged through leakagfe and thus re-'armed' itself.
It would certainly be damn convienient if so since you (the librarian) would
only need to restock books rather than reactivate them. This would
also explain why the 'foreign' book was causing the beeper to go off.

comments?


-- 
--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.