per-el@obelix.UUCP (04/08/87)
In article <2634@phri.UUCP> roy@phri.UUCP (Roy Smith) writes: > This is also, BTW, one of the ways those stolen-item detectors in >stores work. The big plastic clips contain some sort of passive tuned >circuit, with a diode detector. The gizmos on either side of the door emit >RF at some frequency (presumably up in the several hundred Mhz range) and >listen for harmonics comming back..... A girlfriend of a friend of mine found one detector in a box she had got from a shop. I opened it and found a coil with about 10 turns and a capacitator in it. No diode! I was puzzled, because I did also think it used harmonics for detection. I measured the resonant frequency with a grid-dip meter, and found it to be about 5.5 MHz. Later I went into a radio-shop and listened on a shortwave radio and found a strong signal around 5.5 MHz. I started to think about what principle it worked it used. 1. Send a signal and measure the loss when the resonant circuit enters the e-m field. (Would that work? Would it not be very sensitive for other things entering the field.) 2. I talked to a service man at a shop. He was adjusting the boxes at the door. I looked into them but could not figure out how they worked. The service man did not seem to know either, only how to adjust it. He talked about *two* frequencies, one at longwave and one at microwave, but I did not believe that. 3. (My favourite theory!) The boxes sends out pulses of 5.5 MHz radio energy. They deliver energy to the resonant circuit. When they stops transmitting, they start to listen. If a resonant circuit is near, it will send out energy a short period of time after the excitation. The boxes hears that and starts the alarm. Does anybody have other theories, or doues anybody know how this works? per-el@obelix.uucp
sow@luthcad.UUCP (04/11/87)
In article <959@obelix.UUCP> per-el@obelix.UUCP (Per Elmdahl) writes: >In article <2634@phri.UUCP> roy@phri.UUCP (Roy Smith) writes: >> This is also, BTW, one of the ways those stolen-item detectors in >>stores work. The big plastic clips contain some sort of passive tuned >>circuit, with a diode detector. > > A girlfriend of a friend of mine found one detector in a box she had >got from a shop. I opened it and found a coil with about 10 turns and >a capacitator in it. You will be even more puzzled, if tell you that you probably have a lot of this coils already. They are hidden under some sort of barcode markers. Most record stores use them. When you pay, you got a marker with the text "paid" on the barcode marker, the paid marker is some sort of shield. Peel of the paper and take a look. Sven-Ove Westberg, CAD, University of Lulea, S-951 87 Lulea Tel: +46-920-91677 (work) +46-920-48390 (home) UUCP: sow@luthcad.UUCP or seismo!mcvax!enea!luthcad!sow ARPA: enea!luthcad!sow@seismo.css.gov
larry@kitty.UUCP (04/11/87)
In article <959@obelix.UUCP>, per-el@obelix.UUCP (Per Elmdahl) writes: > > This is also, BTW, one of the ways those stolen-item detectors in > >stores work. The big plastic clips contain some sort of passive tuned > >circuit, with a diode detector. The gizmos on either side of the door emit > >RF at some frequency (presumably up in the several hundred Mhz range) and > >listen for harmonics comming back..... > > A girlfriend of a friend of mine found one detector in a box she had > got from a shop. I opened it and found a coil with about 10 turns and > a capacitor in it. No diode! I was puzzled, because I did also think > it used harmonics for detection. I measured the resonant frequency > with a grid-dip meter, and found it to be about 5.5 MHz. Later I went > into a radio-shop and listened on a shortwave radio and found a strong > signal around 5.5 MHz. I started to think about what principle it worked > it used. > > 1. Send a signal and measure the loss when the resonant circuit enters > the e-m field. (Would that work? Would it not be very sensitive for > other things entering the field.) You got it! All theft-prevention systems that I am aware of use this principle. The operation is not unlike a giant fix-tuned grid-dip meter. The detector element consists of an LC circuit, which when placed in an electromagnetic field at its resonant frequency will absorb energy from the field, thereby affecting the power drawn by the oscillator circuit in the detector apparatus which generates the field (this is somewhat over-simplified, but close enough). One of the first theft-prevention systems was manufactured by a company called Sensormatic, and used a UHF excitation frequency. The detector elements were rather small and were like a flexible printed circuit sandwiched between two pieces of cardboard. Since other metallic objects (keys, jewelry, etc.) could conceivably cause UHF energy absorption within the sensitivity threshhold of a UHF device, HF systems in the range of 2.0 to 10.0 MHz have become more prevalent. While HF detector elements are larger than the UHF variety, they offer a greater detection range and fewer false alarms. Stores are _awfully_ concerned about false alarms - since they usually generate ill will among customers. An example of an HF system using detector elements about 1-1/2 by 2 inches by 1/8 inch thick is manufactured under the trade name "No-Go". The HF systems generally use loop antennas to create a confined electromagnetic field, and thereby prevent interference to adjacant detector systems and other radio services. There are still UHF systems on the market, and I believe that at least one of them uses an element having two absorption frequencies to minimize false alarms. > 2. I talked to a service man at a shop. He was adjusting the boxes at > the door. I looked into them but could not figure out how they worked. > The service man did not seem to know either, only how to > adjust it. He talked about *two* frequencies, one at longwave and one > at microwave, but I did not believe that. There could be two frequencies in some theft-detection systems, but they are probably not very far apart. > 3. (My favourite theory!) The boxes sends out pulses of 5.5 MHz radio > energy. They deliver energy to the resonant circuit. When they stops > transmitting, they start to listen. If a resonant circuit is near, > it will send out energy a short period of time after the excitation. > The boxes hears that and starts the alarm. Not a good idea. While you could "ping" the detector element with a short high-energy RF pulse and listen for received damped oscillations, the receiver would have to be rather sensitive with a rather fast T-R switch; without getting expensive and complex, such an arrangement could be unreliable and susceptible to false alarms. > Does anybody have other theories, or does anybody know how this works? The use of a varactor diode to emit a totally different frequency from that of the excitation frequency would be nice, but it is presently cost-prohibitive for retail store theft-detection system applications. The advantage of such a varactor diode scheme would be virtual elimination of false alarms since the RF excitation could be pulsed, with the expected received frequency signal being fed through a coincidence circuit. <> 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?"
apn@nonvon.UUCP (04/12/87)
in article <959@obelix.UUCP>, per-el@obelix.UUCP (Per Elmdahl) says: > > In article <2634@phri.UUCP> roy@phri.UUCP (Roy Smith) writes: >> This is also, BTW, one of the ways those stolen-item detectors in >>stores work. The big plastic clips contain some sort of passive tuned >>circuit, with a diode detector. The gizmos on either side of the door emit >>RF at some frequency (presumably up in the several hundred Mhz range) and >>listen for harmonics comming back..... > > Does anybody have other theories, or doues anybody know how this works? > I visited the local music shop (Wherehouse) and after purchasing a tape, I disected the security device that was taped to the side. It contained to foil plates mostly covered with paper with the exception of two small openings used as access to destroy the function of the device. Two wires each leaving one plate joined on opposite sides of a silicon die. Examining the silicon die under a occulars. It seemed to be only a simple diode and nothing more. I should have placed the entire device in an RF cavity and measured absorptions at RF with a network analyzer ... but it was already in a destroyed stayed upon leaving the store. Maybe next one....... Alex P Novickis -- UUCP: {ihnp4,ames,qantel,sun,seismo,amdahl,lll-crg,pyramid}!ptsfa!nonvon!apn {* Only those who attempt the absurd ... will achieve the impossible *} {* I think... I think it's in my basement... Let me go upstairs and check. *} {* -escher *}
parnass@ihuxz.UUCP (04/12/87)
x
Some of the shoplifting detection transmitters found in
retail stores purportedly employ a low power swept sig-
nal near the following frequencies: 2.0, 4.5, 8.2 MHz.
--
===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===-===
Bob Parnass, Bell Telephone Laboratories - ihnp4!ihuxz!parnass - (312)979-5414larry@kitty.UUCP (04/12/87)
In article <1104@nonvon.UUCP>, apn@nonvon.UUCP (apn) writes: > I visited the local music shop (Wherehouse) and after purchasing > a tape, I disected the security device that was taped to the side. It > contained to foil plates mostly covered with paper with the exception > of two small openings used as access to destroy the function of the > device. Two wires each leaving one plate joined on opposite sides > of a silicon die. Examining the silicon die under a occulars. It > seemed to be only a simple diode and nothing more. Is it possible that what you thought to be a diode was in fact an unencased "chip" capacitor? If it were a capacitor, it would imply that the detector element was just an LC circuit. The above device sounds to me like one of the detector elements of a UHF theft-detection system which relies upon RF power absorption. While varactor diodes have been considered for use in the detector elements of theft-detection systems as a frequency multiplier circuit, it is my understanding that the cost of varactor diodes is still prohibitive for such applications. Most retail stores are _cheap_ when it comes to spending money for items which do not directly attract business, so the cost of most disposable theft-detection elements is << $ 0.15; if the cost were much more, stores would not buy 'em. <> 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?"
wn9nbt@pur-ee.UUCP (Dave Chasey) (04/13/87)
I have always wondered about these alarms also. The ones mentioned here seem to involve clips that are removed, or the ones that the ones that are destroyed when you pay for the item. How about the ones that are in Library books in some libraries ? They are able to activate the sensor when the book is on the shelf and deactivate it when you check the book out. In our engineering library, I saw a student check out a few books, but they forgot to deactivate one of the books. The alarm went off as the student left, so he returned and they found the book and deactivated the sensor and he went thru with no problem. I have been told that the sensor is on the back side of the envelope that they attach inside the rear cover that holds the checkout card. Any ideas ??............Dave
sdp@omssw1.UUCP (04/15/87)
In article <5941@pur-ee.UUCP> wn9nbt@pur-ee.UUCP (Dave Chasey) writes: > >How about the >ones that are in Library books in some libraries ? They are >able to activate the sensor when the book is on the shelf and deactivate >it when you check the book out. My High School had one of those. I don't know how it works, but we had a great time peeling the metal strips out of books when we found them and slipping them into peoples backpacks, etc. When I bought The Connection Machine from B. Dalton's here in portland, one of those things fell out. It wasn't even attached to the book. -- Scott Peterson, Intel Corp., Hillsboro, OR, ...!tektronix!ogcvax!omssw1!sdp
jpexg@mit-hermes.UUCP (04/18/87)
In article <5941@pur-ee.UUCP>, wn9nbt@pur-ee.UUCP (Dave Chasey) writes: > > I have always wondered about these alarms also. The ones mentioned > here seem to involve clips that are removed, or the ones that the > ones that are destroyed when you pay for the item. How about the > ones that are in Library books in some libraries ? They are > able to activate the sensor when the book is on the shelf and deactivate > it when you check the book out. 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.
mfb@cci632.UUCP (04/23/87)
Sender: In article <5941@pur-ee.UUCP> wn9nbt@pur-ee.UUCP (Dave Chasey) writes: > > How about the >ones that are in Library books in some libraries ? > I have been told that the sensor is on the back >side of the envelope that they attach inside the rear cover that holds >the checkout card. I recently checked a book out of the R.I.T. library and I noticed that there was a piece of gray paper that was rather thick affixed to the inside of the front cover. It was easy to peel back one corner of the paper to discover that it had a foil backing (aluminum or lead, I'm not sure which). This is the only book that I've ever found with this foil affixed to the inside of the cover. Perhaps most books that are sold to libraries have this foil built-in to the cover. I recall that one of the people working at the check-out counter said something about having to "discharge" the book. On the other hand, I recently checked out some reading material that was on 24 hour reserve, and the a check-out person had to hand it to me _around_ the detector, apparently because they could NOT "discharge" the item. The items that can be discharged are just rubbed briefly on a metal plate that sits under the edge of the counter. Perhaps the metal plate is neutralizing all the positive or negative charged particles on that metal foil. Kind of like the "Zero Stat" guns sold by "Discwasher" for neutralizing the static on your vinyl audio disks? Does that sound plausible ? ...Anybody ?
tomm@voodoo.UUCP (Tom Mackey) (04/24/87)
In article <1696@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: > Since other metallic objects (keys, jewelry, etc.) could conceivably >cause UHF energy absorption within the sensitivity threshhold of a UHF device, >HF systems in the range of 2.0 to 10.0 MHz have become more prevalent. While >HF detector elements are larger than the UHF variety, they offer a greater >detection range and fewer false alarms. Stores are _awfully_ concerned >about false alarms - since they usually generate ill will among customers. I had a store up in Bellingham, WA "finger" me several years ago. The device that set the alarm off was my Hewlett Packard HP-25 that I was carrying in its belt-case. I had to let the sales clerk carry it through the doorway before they were convinced I had ripped nothing off. You are correct. I was _very_ annoyed. -- Tom Mackey (206) 342-1442 (wk) Boeing Computer Services ....uw-beaver!ssc-vax!voodoo!tomm M/S 03-73, P.O. Box 24346, Seattle, WA 98124-0346