cmcmanis@sun.uucp (Chuck McManis) (04/21/87)
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.
ee251aaz@sdcc3.UUCP (04/22/87)
In article <17024@sun.uucp> cmcmanis@sun.uucp (Chuck McManis) writes: >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 occured to me reading this that the 'deactivation' may be nothing more >than charging up the capacitor in the LC circuit... > >comments? This is a nice idea, but I could swear that I have repeatedly entered and exited similarly equipped libraries here carrying books I checked out weeks prior. The capacitive circuit would need to possess an incredibly long time constant for this to be possible. It would seem somewhat impractical to design a security system that did not allow you to re-enter (or exit thereafter) the library with a book you checked out some time before. I hope there is someone out there who knows the answer.... Gregory Breit Department of Applied Mechanics and Engineering Science University of California, San Diego
marcus@illusion.UUCP (Marcus Hall) (04/23/87)
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
..!{ihnp4,mcdchg}!iing
h thh thhgnome@oliveb.UUCP (04/24/87)
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
larry@tapa.UUCP (04/24/87)
The one flat "tag" I took apart had a small stripline type antenna in it and a very small encapsulated device. I suspect it is a diode and the antenna/tuned circuit resonates at a harmonic of some UHF or microwave frequency. Probably a low level microwave signal is sent out and the diode produces enough harmonics to be detected by a receiver. Mixing could also be used. Sounds cheap enough to me. The deacivation device probably just burns out the diode with a little current.
elg@killer.UUCP (Eric Green) (04/25/87)
in article <17024@sun.uucp>, cmcmanis@sun.uucp (Chuck McManis) says: > 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 I don't think so. I regularly take checked-out books in and out of the library when I'm studying, and it never sets off the alarm. My personal theory is magnetic material, and the "gate" acting sort of like a giant "recording head"... except perhaps using the effect of the magnet passing thru a radio field, rather than direct inductance. I'm no electronics person (software all the way, except for a little dabbling), how about an expert correcting what I said wrong?! -- Eric Green elg%usl.CSNET Hacker-in-training, University of SW Louisiana {cbosgd,ihnp4}!killer!elg BBS phone #: 318-984-3854 1200 baud max Snail Mail P.O. Box 92191 Bayou ~~~ Lafayette, LA 70509 ~~~ Telecommunications
larry@kitty.UUCP (Larry Lippman) (04/27/87)
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 <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
kunert@nicmad.UUCP (04/27/87)
In article <1726@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes: > 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. > I worked in a library that used a similar system, but there was no way to deactivate the sensor. The "tag" inserted in the book was just a printed loop connected to a printed capacitor forming a resonant circuit- the system was designed so that the library employee would pass the books around the sensor at the checkout. Anyway, in the systems that *can* be deactivated, I believe the mechanism used at the checkout is a simple magnetizer/demagnetizer. My best guess is that a reed switch is being used to short out the tuned circuit. -- ihnp4-\ "I'm looking for a lifestyle Dick Kunert seismo!uwvax!nicmad!kunert that doesn't require my decvax-/ presence..."
charlie@oakhill.UUCP (Charlie Thompson) (04/28/87)
OK Larry, I'll make a guess at how the LC resonant circuit with stripline antenna works...... The circuit is activated by gaussing and degaussing a magnetically biased inductor. When the inductor is magnetized (gaussed) the material that the inductor is formed from becomes *saturated* (i.e. its small signal inductance is reduced) if the inductance is reduced then the tuning for the tank circuit goes up (like out of resonance for the giant grid dip meter that forms the detector). When a person carries the book through the RF field he does not *resonate*...provided the book has been legally deactivated. To re-arm the book, when it is checked in, the little inductor is de-gaussed using a decaying AC sinewave just like your color TV uses. The inductor will then be un-saturated and will allow LC resonance at the detection frequency. So how'd I do Larry? Gee if nobody thought of this maybe I should patent it (fat chance now that's it's in netland!). Charlie Thompson WB4HVD Motorola DSP Operations Austin, TX
larry@kitty.UUCP (Larry Lippman) (04/29/87)
In article <875@oakhill.UUCP>, charlie@oakhill.UUCP (Charlie Thompson) writes: > OK Larry, I'll make a guess at how the LC resonant circuit with > stripline antenna works...... > > The circuit is activated by gaussing and degaussing a magnetically > biased inductor. When the inductor is magnetized (gaussed) the material > that the inductor is formed from becomes *saturated* (i.e. its > small signal inductance is reduced) if the inductance is reduced > then the tuning for the tank circuit goes up (like out of resonance > for the giant grid dip meter that forms the detector). When a person > carries the book through the RF field he does not *resonate*...provided > the book has been legally deactivated. To re-arm the book, when it > is checked in, the little inductor is de-gaussed using a decaying > AC sinewave just like your color TV uses. The inductor will then > be un-saturated and will allow LC resonance at the detection frequency. > So how'd I do Larry? Gee if nobody thought of this maybe I should > patent it (fat chance now that's it's in netland!). You hit the nail right on the head! In one system that I have seen, the stripline antenna has four or so (the actual number is not that important) pieces of metal along its length. These pieces of metal have a fairly high magnetic remanance. When the sensor is "active" (i.e., capable of being detected), there is little or no magnetism in the above pieces of metal. As a result, the stripline antenna and capacitor present an LC resonant circuit in the UHF frequency range which can be detected by its RF power absorption. When the sensor is "inactive" (i.e., when the book is checked out), a magnetizer upon which the book is placed causes a magnetic field to remain in the pieces of metal. Two mechanisms now come into effect (not being privy to the sensor design calculations, I don't know which effect is predominant): (1) The magnetic field of the stripline antenna-inductor is saturated due to the permanent magnet, and its effective Q is severely reduced. (2) The UHF signal across the stripline antenna undergoes several phase-shifts as caused by the magnets, thereby severely reducing the amount of RF energy that can be absorbed by the effective LC network. The power absorption of the LC network is now so reduced by the above methods that it is no longer detectable. I don't have the precise design specs of the system, but I would guess the magnetic remanance to be measured in hours, or a few days at most. A book when returned is run through a demagnetizer to assure the detector element is "active". The nice aspect of this system, is that the magnetic remanance is sufficiently short in life such that ALL books return to an "active" state in a few days. This system is decently immune to fraud. The magnetizer uses a capacitive discharge into an electromagnet, resulting in a pretty high momentary magnetic field; a permanent magnet will not work to deactivate the sensor. Of course, one could rip out the sensor, but some libraries glue and imbed the sensor in such a fashion as to make it non-trivial to remove. The only assured method of defeating a sensor is to wrap the book tightly in metal foil - but that's a little obvious while strolling out the library door! Peter Wu, who sent me mail, also correctly guessed the "secret". <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
dave@sdeggo.UUCP (04/30/87)
In article <1732@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes: > This system is decently immune to fraud. The magnetizer uses a > capacitive discharge into an electromagnet, resulting in a pretty high > momentary magnetic field; a permanent magnet will not work to deactivate the > sensor. San Francisco Public Library (and a few others I've seen, but can't remember which) appears to use a permanent magnet. It's an awfully big sucker, though. (About the size of a brick). Would the field generated by this be enough to do it or are we back to square one on theories about their system? Originally they used a powered gadget, but not anymore. I think the sensitizer is powered. -- David L. Smith sdcsvax!sdamos!sdeggo!dave, ihnp4!jack!man!sdeggo!dave, hp-sdd!crash!sdeggo!dave sdeggo!dave@sdamos.ucsd.edu "Morals are one thing. Ratings are EVERYTHING!"
ken@argus.UUCP (Kenneth Ng) (04/30/87)
In article <1640@nicmad.UUCP>, kunert@nicmad.UUCP (Dick Kunert) writes: > In article <1726@kitty.UUCP>, larry@kitty.UUCP (Larry Lippman) writes: > > 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. > I worked in a library that used a similar system, but there was no way > to deactivate the sensor. The "tag" inserted in the book was just a > printed loop connected to a printed capacitor forming a resonant circuit- > the system was designed so that the library employee would pass the books > around the sensor at the checkout. At least one system works magnetically. Before the remodeling the local college library had a device it passed books through before giving them back to you. I balanced my trusty 6 inch steel ruler on a finger. When the book went through the ruler went flying and inpinged against the device. Unfortunately the new system is moved out of range, so I can't test it. Maybe my Cheshire cat grin gave them the hint that I figured out at least part of the system. > ihnp4-\ "I'm looking for a lifestyle > Dick Kunert seismo!uwvax!nicmad!kunert that doesn't require my > decvax-/ presence..." -- Kenneth Ng: Post office: NJIT - CCCC, Newark New Jersey 07102 uucp !ihnp4!allegra!bellcore!argus!ken *** WARNING: NOT ken@bellcore.uucp *** bitnet(prefered) ken@orion.bitnet Kirk: "I don't care if you hit the broadside of a barn" Spock: "Why should I aim at such an object?"
larry@kitty.UUCP (05/01/87)
In article <45@sdeggo.UUCP>, dave@sdeggo.UUCP (David L. Smith) writes: > > This system is decently immune to fraud. The magnetizer uses a > > capacitive discharge into an electromagnet, resulting in a pretty high > > momentary magnetic field; a permanent magnet will not work to deactivate the > > sensor. > > San Francisco Public Library (and a few others I've seen, but can't remember > which) appears to use a permanent magnet. It's an awfully big sucker, though. > (About the size of a brick). > > Would the field generated by this be enough to do it or are we back to square > one on theories about their system? Originally they used a powered gadget, > but not anymore. I think the sensitizer is powered. I have no doubt that a large alnico magnet could _reliably_ magnetize the sensor element, thereby deactivating it so that a book could be taken out. My comment was really directed to small novelty-type magnets which could be readily carried in one's pocket. Using a permanent magnet to magnetize something is, in general, not as reliable as using a capacitive-discharge electromagnet. Such an electromagnet will result in a much higher magnetic field intensity, and will always result in the proper magnetic orientation. The problem with a permanent magnet is that it requires some finesse to use; i.e., one false move - and you have just canceled the magnetic field that you just created. The "sensitizer" which reactivates the sensor must work by destroying any permanent magnetic field in the sensor. The only practicable way to do this is by means of an AC magnetic field; hence, it is a line-powered device. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
koko@uthub.UUCP (05/01/87)
> Of course, one could rip out the sensor, but some libraries glue > and imbed the sensor in such a fashion as to make it non-trivial to remove. > The only assured method of defeating a sensor is to wrap the book tightly > in metal foil - but that's a little obvious while strolling out the library > door! > Not if the book is concealed in, say, your briefcase! (However, if you must display the contents of your briefcase to the desk clerk as you leave, then you might be in trouble.
mitch@stride1.UUCP (Thomas P. Mitchell) (05/14/87)
In article <875@oakhill.UUCP> charlie@oakhill.UUCP (Charlie Thompson) writes: > >OK Larry, I'll make a guess at how the LC resonant circuit with >stripline antenna works...... > >The circuit is activated by gaussing and degaussing a magnetically >biased inductor. Changing the tuning. >So how'd I do Larry? Gee if nobody thought of this maybe I should >patent it (fat chance now that's it's in netland!). Fat chance -- I wonder -- If Charlie's posting would qualify as a notebook entry. With all the date time stamps and copies the date of the 'invention' might be well documented. If so all he needs is a working model to go the the patent office. Thomas P. Mitchell (mitch@stride1.Stride.COM) Phone: (702) 322-6868 TWX: 910-395-6073 MicroSage Computer Systems Inc. a Division of Stride Micro. Opinions expressed are probably mine.
egb@mitisft.Convergent.COM (E. G. Bradford ) (05/14/87)
I have looked in various old issues of Radio Electronics and asked a few friends and no one seems to know the answer to this question. What is the technology that systems like Quotrek use for broadcasting real time stock market quotations over the FM radio frequency? I assume it a subcarrier of a local station, but can guess no further. Does anyone know how this is done? Ed Bradford
jpexg@mit-hermes.AI.MIT.EDU (John Purbrick) (05/15/87)
In article <667@stride.Stride.COM>, mitch@stride1.UUCP (Thomas P. Mitchell) writes: > In article <875@oakhill.UUCP> charlie@oakhill.UUCP (Charlie Thompson) writes: > >OK Larry, I'll make a guess at how the LC resonant circuit with > >stripline antenna works...... > >The circuit is activated by gaussing and degaussing a magnetically > >biased inductor. > >So how'd I do Larry? Gee if nobody thought of this maybe I should > >patent it (fat chance now that's it's in netland!). After the "public use, disclosure, or sale" of an invention, you have 1 year to file your patent application, but for foreign protection you must file before disclosure. It's not too late!
tedk@ihuxv.ATT.COM (Kekatos) (03/19/88)
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
bill@sigma.UUCP (William Swan) (03/22/88)
In article <2530@ihuxv.ATT.COM> tedk@ihuxv.UUCP (Kekatos,T.G.) writes: >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. The use of "L-C" circuits goes a ways back.. but not as IFF (Identification Friend or Foe) gear. IFF equipment was essentially a transponder device, designed to reply with a coded radio sequence upon (as I recall) radio interrogation. My father worked on this stuff for a while during WWII, and said it was a bear to service, it was all wired with white wires with thermite charges distributed throughout, and a .38? shell set up at one end of the chassis to fire through all the vacuum tubes should self-destruct be required. Once in a while, of course, a particularly difficult module would accidentally misfire, making servicing it a moot point... :-) -- William Swan {ihnp4,decvax,allegra,...}!uw-beaver!tikal!sigma!bill
tedk@ihuxv.ATT.COM (Kekatos) (03/24/88)
In article <1574@sigma.UUCP> bill@sigma.UUCP (William Swan) writes: >In article <2530@ihuxv.ATT.COM> tedk@ihuxv.UUCP (Kekatos,T.G.) writes: >>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. > >The use of "L-C" circuits goes a ways back.. but not as IFF (Identification >Friend or Foe) gear. > >IFF equipment was essentially a transponder device, designed to reply with >a coded radio sequence upon (as I recall) radio interrogation. Something like modern radio-transponder equipment in most airplanes. >My father worked on this stuff for a while during WWII, and said it was a >bear to service, it was all wired with white wires with thermite charges >distributed throughout, and a .38? shell set up at one end of the chassis >to fire through all the vacuum tubes should self-destruct be required. Once >in a while, of course, a particularly difficult module would accidentally >misfire, making servicing it a moot point... :-) > I wonder if this kind of explosive "copy-protection" is still being used in today's miltary and defense electronics. > >William Swan {ihnp4,decvax,allegra,...}!uw-beaver!tikal!sigma!bill 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
glenn@c3pe.UUCP (D. Glenn Arthur Jr.) (03/27/88)
In article <2530@ihuxv.ATT.COM>, tedk@ihuxv.ATT.COM (Kekatos) writes: > > 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. I didn't see the beginning of this discussion, but here are my clues. First, the system described above doesn't sound like systems I've seen in libraries -- it sounds like it was designed for retail, where a tag need only be deactivated. Second, just how small to reed switches get? I used to work in a library, and we had foil strips, with glue on both sides, about an eighth of an inch wide and several inches long. They were *very* flexible, and although they were easily creased, I don't *think* we ever damaged one by creasing it. They were activated and deactivated by passing them over an electrical device (I think it had a switch to select activation or deactivation), but I have no idea just how complex the device was. As far as one could tell from the outside, it could have been merely an electromagnet. Other systems I have seen have included mainly variations on the physical dimensions of the tag. Some are squares maybe an inch on a side and as thick as a fingernail. Others are strips, but narrower or wider than the ones I used to use. Somebody else in this newsgroup mentioned strips about a quarter of an inch wide. Do the warnings to pacemaker patients posted on the sensing equipment constitute a clue? And has anyone mentioned yet that cardkeys will set off the detector but that some libraries provide magnetic shields the same size as a cardkey which, if placed next to the cardkey, render it invisible to the sensor? D. Glenn Arthur Jr. -- D. Glenn Arthur Jr. "The Human Vibrator" ..!decuac!c3pe!glenn
ttims@watdcsu.waterloo.edu (Tracy Tims) (03/29/88)
Some of the libraries that I have been in (University and public) do not bother to deactivate the markers in their books. When someone wants to remove materials from the library, they give them to a staff person near the door, who makes sure they are checked out, and then passes them around the exit sensors to the person as they are leaving. I have also seen this method used in large video rental stores.
krc@cs.purdue.EDU (Kenny Crudup) (03/29/88)
In article <1574@sigma.UUCP> bill@sigma.UUCP (William Swan) writes: ] My father worked on this stuff for a while during WWII, and said it was a ] bear to service, it was all wired with white wires with thermite charges ] distributed throughout, and a .38? shell set up at one end of the chassis ] to fire through all the vacuum tubes should self-destruct be required. Once ] in a while, of course, a particularly difficult module would accidentally ] misfire, making servicing it a moot point... :-) ] William Swan In article <2536@ihuxv.ATT.COM>, tedk@ihuxv.ATT.COM (Kekatos) writes: + I wonder if this kind of explosive "copy-protection" is still + being used in today's miltary and defense electronics. + Ted G. Kekatos I dunno, but if the warning label on the outside reads "Qualified Service Personnel Only", you'd better believe it!! First time a service tech should get hazard pay, with the possible exception of working on certain Zenith TV's :-) -- Kenny "_R_o_b_o_B_r_o_t_h_e_r" Crudup krc@arthur.cs.purdue.edu Purdue University CS Dept. W. Lafayette, IN 47907 inews: Error code 257: .signature +1 317 494 7842 joke too stupid: Not included
reid@decwrl.dec.com (Brian Reid) (03/30/88)
I have explored the inside of one library anti-theft system; I expect that others might work the same way. As several posters have noted, the anti-theft device itself is a collection of high-Q tuned circuits. A band-sweep ping is sent out a few times a second, and the response frequency signature is recorded. This system used devices with 4 different tuned circuits in them, and each tuned circuit could be one of 100 different center frequencies. This gives a large number of possible combinations. The frequencies are in the hundreds of megahertz; I don't recall the exact band. Something like 320 MHz comes to mind. When a book is checked out, the librarian runs it over a "deactivator". In fact, that deactivator is another sweep pinger, and it records the frequency signature in a short-term database. For the next K minutes, the alarm will not ring when the matching anti-theft tag passes through the doorway detector. A few hours later, the entry expires from the short-term memory. This scheme does not even TRY to prevent duplicates. They just take the detectors out of the box and stick them into books. Because there are a lot of different combinations, the probability of duplicates is very low, but if by some remote chance I check out a book and then you try to steal one, and your anti-theft code is the same as mine, then you can steal the book undetected. At the library whose operation I am familiar with, the expiration period was about 2 hours. This allowed people to make a trip or two back into the library with the recently-checked-out books without ringing any bells. In other words, the anti-theft device wasn't deactivated, but the detector was trained not to buzz on its number. Brian Reid
dhesi@bsu-cs.UUCP (Rahul Dhesi) (03/31/88)
In article <341@bacchus.DEC.COM> reid@decwrl.UUCP (Brian Reid) writes: >When a book is checked out, the librarian runs it over a "deactivator". >In fact, that deactivator is another sweep pinger, and it records the >frequency signature in a short-term database. However, all the libraries I've used deactivate books that are checked out so that they will not trigger the detector even days or weeks later. This lets the user bring checked-out books into the library for reference during subsequent visits. Speculation: A deactivator simply changes the magnetization in the little "target" strip that's in the book. This changes the characteristics of the L-C circuit in the strip because the inductance of a coil changes as its magnetic core gets saturated. Alternatively, it could just be casting a magic spell which, after all, is indistinguishable from any sufficiently advanced technology. -- Rahul Dhesi UUCP: <backbones>!{iuvax,pur-ee,uunet}!bsu-cs!dhesi
coconut@cup.portal.com (04/05/88)
Where I went to school in California we had a system where bye passing a book over a sensor it would make the sensing device not pick up the book. We could then pass the bok back over it with a switch held down and if the book then was passwd through the detector it would sound off.. What I am curious of though is....The plastic detectors we used were made of thin plastic and metal...If the sensors work by picking up a field produced by the tag...How about just squeezing the darn thing so hard that it would resonate on a differest freq// Oliver@cup.portal.com