Path: utzoo!mnetor!uunet!husc6!mailrus!tut.cis.ohio-state.edu!bloom-beacon!mit-eddie!ll-xn!ames!pacbell!att-ih!ihnp4!ihuxv!tedk
From: tedk@ihuxv.ATT.COM (Kekatos)
Newsgroups: sci.electronics
Subject: Re: Library book detectors.
Message-ID: <2530@ihuxv.ATT.COM>
Date: 18 Mar 88 16:30:43 GMT
References: <5398@swan.ulowell.edu| <1261@uop.edu| <2521@ihuxv.ATT.COM| <1674@uhccux.UUCP> <20719@bu-cs.BU.EDU>
Reply-To: tedk@ihuxv.UUCP (Kekatos,T.G.)
Distribution: na
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 49
Summary: re-useable L-C circuits

In article <20719@bu-cs.BU.EDU| madd@bu-it.bu.edu (Jim Frost) writes:
|In article <1674@uhccux.UUCP| taro@uhccux.UUCP (Taro Nobusawa) writes:
||In article <2521@ihuxv.ATT.COM| tedk@ihuxv.UUCP (Kekatos,T.G.) writes:
|||The wafer if manufactured to one "pre-determined" frequency. The machine
|||at the doorway is a radio transmitter-receiver. It senses the 
|||presence of the TUNED circuit. 
||
||Yes, but how do you de-tune the foil strips?  In the library, the books can
||be removed after beeing zapped by some device without setting the sensors
||off, and the strips can then be reactivated when returned.  How does this
||work?  Record stores and the like remove these tags, libraries don't.
|
|The most common way (now) is to permanently mount the tag in the
|binder and deactivate it with a special magnet.  I have no clue how
|it's reactivated, since I've never seen someone reactivate one.  I
|could imagine a couple of ways, though -- could the tag be made so
|that running it along a magnet in one direction activates it, and in
|the other (perpendicular) direction, deactivates it?
|

Well, One system that I happened apon, used a hand-held device which
was only a power supply with two sharp pins. The foil tag was layered
in paper and had really bad fake printing on it to make it look a price tag. 
There was two holes in the paper which exposed the foil. The two sharp 
pins are matched with the two holes. ZAPPP! The foil is blown away 
like a fuse. I opened up the paper and foil layers to see. The L-C 
circuit is destroyed!

I have not seen any of the library systems  which are described 
as having "re-useable" tags. But I can think of a few methods of
designing a re-useable tag. There are magnetic REED switches that
latch. The tag could be deactived with a magnet. Although this
sounds to be too simple. Maybe there is a combination of REED
switches? Then there would be a special magnetic "key" to 
deactive/active.

Does anyone know of the history of using L-C circuits. I heard
that they were used in WWII. The term "Friend or Foe" circuit
comes to mind. 






Ted G. Kekatos
backbone!ihnp4!ihuxv!tedk                     (312) 979-0804
AT&T Bell Laboratories, Indian Hill South, IX-1F-460
Naperville & Wheaton Roads - Naperville, Illinois. 60566 USA