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From: gnome@oliveb.UUCP
Newsgroups: sci.electronics
Subject: Re: Library Book detectors
Message-ID: <868@oliveb.UUCP>
Date: Thu, 23-Apr-87 20:08:21 EST
Article-I.D.: oliveb.868
Posted: Thu Apr 23 20:08:21 1987
Date-Received: Sat, 25-Apr-87 08:08:56 EST
References: <146@illusion.UUCP>
Organization: Olivetti ATC; Cupertino, Ca
Lines: 22

in article <146@illusion.UUCP>, marcus@illusion.UUCP (Marcus Hall) says:
 > 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
 > ..!{ihnp4,mcdchg}!ilusion!marcus

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".

Gary