skravis@csis.dit.csiro.au (Simon Kravis) (10/12/90)
Does anyone know how the detection system for the metallic tags attached to CDs and other small and valuable items in shops works? It looks as though it's some kind of magnetic detection system, judging by the large sensing coils near the door and the fact the the tags are deactivated by running what looks like a magnetic roller over them.
cgordon@vpnet.chi.il.us (Gordon Hlavenka) (10/13/90)
I don't know how all of them work, but... Some reflect/absorb RF with a unique signature. A detector then radiates some spread of RF, and looks for the signature. This tag is deactivated by removing it from the goods. The "magnetic" ones, I think, are probably decoys. The whole system is to make you _think_ there's something going on, but even the clerks wouldn't know the truth. This may be wrong in many cases, but it _is_ true in some. There might be a second layer which is fused by _pressure_, thereby deactivating an RF-signature device by shorting it out. This would be dragged across a roller, but wouldn't involve magnetism. One system I know of is used in books. A strip about 1/8" wide by 2 or 3 inches long is inserted into the binding of the book. The strips themselves are pretty expensive, so stores that use them only protect some books. I don't know how these are turned off. I knew a guy a few years back who collected these tags. He'd secrete the active portions in his wallet, pockets, etc. And he'd set off alarms all the time. This was his idea of fun -- personally I stayed away from him in malls. He'd make up stories about having a plate in his head or somesuch, and explain this to security people. A couple of stores tried to ban him on principle, but I don't know if anything ever came of it. ----------------------------------------------------- Gordon S. Hlavenka cgordon@vpnet.chi.il.us Disclaimer: Yeah, I said it. So what?
jad@dayton.UUCP (J. Deters) (10/16/90)
> Article <271618ef-4a7.1sci.electronics-1@vpnet.chi.il.us> From: cgordon@vpnet.chi.il.us (Gordon Hlavenka) >Some reflect/absorb RF with a unique signature. A detector then radiates >some spread of RF, and looks for the signature. This tag is deactivated by >removing it from the goods. The typical tags are passive resonators. They are basically two antennas coupled to each other, the receiver being tuned to something like 133 KHz and the transmitter portion being tuned to 66.5 KHz, coupled through a diode. (The good ones are, anyway. The cheap ones retransmit on the same frequency as the transmitter, and are subject to more false positives, or so I have been told.) The antennas located at the doors transmit (directionally) across the opening and attempt to receive the re-tuned signal. (I believe that they may perform some phase-shifting to eliminate false reflections.) I have seen two types of these tags: reusable and disposable. The reusable tags have to be removed from the merchandise with a special set of pliers that your average high school kid won't have in his pocket. The disposable type I have examined are aluminum foil shapes cut to the proper dimensions, and the two halves of the circuit are coupled with a fusible link. The foil shapes are stuck on the back of a label (very similar in size to an ordinary mailing label) that has two holes in it. The 'deactivator' is a two-pronged device that simply applies a small voltage across the fusible link, burning it out. I suspect a 9 volt battery would accomplish much the same thing. >The "magnetic" ones, I think, are probably decoys. The whole system is to >make you _think_ there's something going on, but even the clerks wouldn't >know the truth. This may be wrong in many cases, but it _is_ true in some. The magnetic ones are very real. I am not sure of the physics involved, but there is a magnetic strip running between two parallel sets of wires. I think the magnetic strip couples the two antennas together. >One system I know of is used in books. A strip about 1/8" wide by 2 or 3 >inches long is inserted into the binding of the book. The strips themselves >are pretty expensive, so stores that use them only protect some books. I >don't know how these are turned off. Again, magnetic. I've seen these in libraries. They are reusable. When you czech the book out, they put the spine near a degaussing coil. When you czech the book back in, they simply polarize the strip inside the binding again. > >I knew a guy a few years back who collected these tags. He'd secrete the >active portions in his wallet, pockets, etc. And he'd set off alarms all >the time. This was his idea of fun -- personally I stayed away from him in >malls. He'd make up stories about having a plate in his head or somesuch, >and explain this to security people. A couple of stores tried to ban him on >principle, but I don't know if anything ever came of it. Sounds masochistic to me. I personally wouldn't get much "excitement" out of constantly getting busted... -j, but an occasional bust now and then never helped anyone... -- J. Deters Ask me about my PS/2. // INTERNET: jad@dayton.DHDSC.MN.ORG Then, // UUCP: ...!bungia!dayton!jad ask me about my Amiga! \\ // ICBM: 44^58'36"N by 93^16'12"W \X/
jeffj@synsys.UUCP (Jeff Jonas) (10/16/90)
I've dissected a few "inventory" tags myself. A shoe store has a square tag that has the price on it. Pull it off and you see a square spiral of foil forming an antenna. It's detuned by the foil backed "PAID" sticker directly over the center. The video store uses strips with two holes exposing a foil. I just peeled one open and I'll be darned: there's a surface mount something in there (perhaps it's a diode?). A "gun" apparently blows the "thing" with a jolt of electricity, thus detuning the circuit (a self destruct mechanism?). The clothing stores have the most effective device: the big plastic buttons/tags that come off only with the special tool. Once I bought something with TWO of those tags, and the cashier took off only one. The alarm sounded, but I was let through with NO HASSLE since I was just at the cashier. At home, I had a heck of a hard time getting it off! Under normal circumstances, they're infinitely reusable, they're built like the proverbial brick outhouse. Even without the alarm, it's quite a deterrent. Jeffrey Jonas jeffj@synsys.uucp synsys!jeffj@uunet.uu.net EOF echo "DONE !"