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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <1726@kitty.UUCP>
Date: Sun, 26-Apr-87 17:59:23 EDT
Article-I.D.: kitty.1726
Posted: Sun Apr 26 17:59:23 1987
Date-Received: Sun, 26-Apr-87 23:49:54 EDT
References: <146@illusion.UUCP> <868@oliveb.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 35
Summary: Library book detector "secret"

In article <868@oliveb.UUCP>, gnome@oliveb.UUCP (Gary) writes:
>  > 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
>  > buy the item, it is passed over a device that overloads 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.
> 
> I took a microscope to one of those foil ditties and it seems that the
> little chunk in it is a diode.  Maybe the foil/diode pattern acts like a
> down converter (of sorts) that, when pulsed with one frequency, emits
> a very small signal at another frequency?
> 
> I've also seen similar systems where either side of the diode have
> exposed bits of foil - so that a little probe-thing can touch the
> foil pads, blow the diode, and give a little "ok beep".

	Concerning the library book detectors, don't forget these sensors
have to be (1) inexpensive, and (2) capable of controlled deactivation and
reactivation with essentially infinite life.

	The one library book detector system which I am familiar with uses
an LC resonant circuit with a stripline antenna about 10 cm long.  It can
be activated or deactivated in a second or so without physical contact
with the sensor (the sensor is usually glued inside the spine of the book).

	Now, THINK, people!  How does this system work???  It's really
rather clever!  If no one guesses the correct answer, I'll post it in a
week.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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