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From: marcus@illusion.UUCP (Marcus Hall)
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <146@illusion.UUCP>
Date: Thu, 23-Apr-87 12:05:04 EST
Article-I.D.: illusion.146
Posted: Thu Apr 23 12:05:04 1987
Date-Received: Sat, 25-Apr-87 10:10:45 EST
References: <959@obelix.UUCP> <1104@nonvon.UUCP> <5941@pur-ee.UUCP> <2835@mit-hermes.AI.MIT.EDU> <17024@sun.uucp> <3875@sdcc3.ucsd.EDU>
Reply-To: marcus@illusion.UUCP (Marcus Hall)
Organization: Magic Numbers Software, Bloomingdale, IL
Lines: 24

I have always been under the impression that there was a tuned circuit in the
return date cart pocket and the return date card itself de-tuned this circuit
when you checked a book out.  Since the return date card is very close to the
pocket, it could perhaps simply be a metal shield to prevent the RF from
being absorbed by the circuit in the pocket.  The libraries in my area don't
seem to do anything to the book to de-activate the sensor, they just put
a return date card into the book.  This would imply that if you were to check
out a book and take it outside the library, remove the date card from the
book, return inside the library, and put the card into another book, you
could then just walk out with the new book.  If this is the case, it could
be a good reason why this system isn't in use in many places!

I have seen a system in use in bookstores where a sensor is placed inside the
spine of a book.  When you buy the book it is slid over a box that disables
the sensor.  Similar systems in record stores that I have seen seem to use
a printed circuit on the back of an adhesive label (appears to be a foil
inductor and capacitor printed on the adhesive side of a label).  When you
but the item, it is passed over a device that overloads the circuit and burns
out a fuse in it thereby detuning the circuit.  This could be the same as
the bookstore's system.  Such systems cannot be re-enabled, so they don't
make much sense for use in a library, however.

Marcus Hall
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