Library anti-theft walk-thru device...????
William D. Burns
Quick question:
Our university library installed one of those anti-theft walk-thru
devices and I was wondering how the little bugger works.
Observations:
1) When walking thru with Walkman (tm sony) on TAPE-playback, I heard
4 or 5 very short, high frequence "clicks" thru my headphones. (Of
course I had to go back thru again to hear 'em again! :) They swear
that this device is safe for magnetic media (i.e. disks).
2) In the binding of the library's books was a strip of paper (dimensions
of 5 X .25 inches) that had evenly-spaced metal strips in it.
3) To allow a book to pass thru without setting off all the alarms, the
library person runs the book binding over a bulk-eraser type thing.
So, how does this little thing work? Is it sending a low energy magnetic
pulses out and picking up and strange resonances from the little strip? Or
is the wave RF in nature?
Just an enquiring mind wanting to know. :)
Bill
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Francis Vaughan
(William D. Burns) writes:
|> Greetings netlanders!
|>
|> Quick question:
|> Our university library installed one of those anti-theft walk-thru
|> devices and I was wondering how the little bugger works.
|>
|> Observations:
|> 1) When walking thru with Walkman (tm sony) on TAPE-playback, I heard
|> 4 or 5 very short, high frequence "clicks" thru my headphones. (Of
|> course I had to go back thru again to hear 'em again! :) They swear
|> that this device is safe for magnetic media (i.e. disks).
|>
|> 2) In the binding of the library's books was a strip of paper (dimensions
|> of 5 X .25 inches) that had evenly-spaced metal strips in it.
|>
|> 3) To allow a book to pass thru without setting off all the alarms, the
|> library person runs the book binding over a bulk-eraser type thing.
|>
|> So, how does this little thing work? Is it sending a low energy magnetic
|> pulses out and picking up and strange resonances from the little strip? Or
|> is the wave RF in nature?
The former. As I understand it the little chunks of metal are a very
interesting magnetic material. It has a different hysterysis curve depending
upon its state of magnetization.
In one state of magnetisation the curve has very pointy edges, so if you drive
it around the end of the curve with an alternating field, as it is driven over
the point it emits a bit of RF. The metal strip that the little chunks are
bonded to may act as an antenna helping to propogate the signal.
So if you are trying to steal a book, you walk through the alternating field,
and a detector is looking for little pulses of RF that are synchronous with the
field. If they are there it squarks.
When you borrow the book the librarian degausses (or maybe magnetises,
intuitivly
I suspect degausing) the chunks in the book so changing the shape of the curve
to have more rounded corners and hence no RF pulsing out. Returned books get
the reverse treatment.
The field strength of the detector can be quite low, it only needs to drive
around one corner of the hysterysis curve. Magnetic media should be safe.
Pacemakers however may be a different story.
Maybe someone else can provide some more specific info, and/or corrections.
Francis Vaughan.
Rop Gonggrijp
In the book is a strip of metal (any metal) covered by a piece of ferrite.
The ferrite is magnetised, thus changing the frequency at which this gizmo
resonates (it is covered in plastic, so it does resonate).
If they demagnetize the ferrite the thing goes to a lower resonating freq.
One end of the gate emits the first (high) frequency and the other end detects
it. A sudden drop in power means something is 'stealing' energy / resonating
and thus the alarm is activated.
These systems usually work at 915 MHz or so....... Have fun beating it
Oh, by the way, if you transmit this freq. and then play with the transmitted
power you should be able to make the bell ring at quite considderable distance.
--
Rop Gonggrijp (ro...@ooc.uva.nl) is also editor of Hack-Tic (hack/phreak mag.)
quote: "We don't care about freedom of the mind, | Postbus 22953 (in DUTCH)
freedom of signature will do just fine" | 1100 DL AMSTERDAM
Any opinions in this posting are wasted on you | tel: +31 20 6001480
Basshead
> They mostly work as follows:
>
> In the book is a strip of metal (any metal) covered by a piece of ferrite.
> The ferrite is magnetised, thus changing the frequency at which this gizmo
> resonates (it is covered in plastic, so it does resonate).
>
> If they demagnetize the ferrite the thing goes to a lower resonating freq.
>
> One end of the gate emits the first (high) frequency and the other end detects
> it. A sudden drop in power means something is 'stealing' energy / resonating
> and thus the alarm is activated.
>
> These systems usually work at 915 MHz or so....... Have fun beating it
>
> Oh, by the way, if you transmit this freq. and then play with the transmitted
> power you should be able to make the bell ring at quite considderable distance.
This is a reasonable assumption, but if there is any scientific
argument against what I say, please correct me.
In clothing stores at the local mall, I've heard that thieves are
able to often defeat the security systems a lot of stores use --
the ones with the detectors across the entrance and the big plastic
tags on the unpurchased clothing which is removed at the counter --
anyways they are able to shoplift merchandise by placing clothes
into a bag completely lined with foil...
Now, I assume if you did the same to a book you could theoretically
shield the book (completely wrapped) from the RF energy emitted
by the gates.? I'm not an EE major but this seems reasonable --
??????
--
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=-*=-=*=-
University of California, San Diego - ICC
e-mail: gy...@ucsd.edu "DOS sucks!"
Andy Tefft
at his local video store, the alarm goes off when he leaves the
store, but not when he comes in (you go through the detector either
way), whether he has any movies (properly desensitized) or not.
Why would this be?
Dan Meyer
>Greetings netlanders!
>
>Quick question:
> Our university library installed one of those anti-theft walk-thru
> devices and I was wondering how the little bugger works.
>
> Observations:
> 1) When walking thru with Walkman (tm sony) on TAPE-playback, I heard
> 4 or 5 very short, high frequence "clicks" thru my headphones. (Of
> course I had to go back thru again to hear 'em again! :) They swear
> that this device is safe for magnetic media (i.e. disks).
>
> 2) In the binding of the library's books was a strip of paper (dimensions
> of 5 X .25 inches) that had evenly-spaced metal strips in it.
>
> 3) To allow a book to pass thru without setting off all the alarms, the
> library person runs the book binding over a bulk-eraser type thing.
>
>So, how does this little thing work? Is it sending a low energy magnetic
>pulses out and picking up and strange resonances from the little strip? Or
>is the wave RF in nature?
>
>
>Bill
As I understand the system in use here in the Twin Cities Metro area, *any*
book that passes through the detectors will sound the alarm. the reason the
alarm does not go off when you check out books is the librarian passes the
books *around* the detector. If you don't believe me, check out a book, and
then go back in and the back out with the book.
-- Dan
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Mike Bell
took to pieces a store anti-theft tag, and I'm stuck on how this
one works:
It consists of a rather bulky plastic self-adhesive package, about 40mm
x 13mm. Inside are two mettalic strips. One is a rectangle 38mm x 12.2
mm. The other is a parallelogram:
__________________
/ / ^
/ / |12.2mm
/_________________/ |
< 29mm >
The angle is 45 degrees. All dimensions are +/- 0.25mm
The two strips are separated by a plastic membrane.
Anybody able to explain the principle of operation?
What are the relevant frequencies of operation?
Jonathan Rice
>They mostly work as follows:
>In the book is a strip of metal (any metal) covered by a piece of ferrite.
>The ferrite is magnetised, thus changing the frequency at which this gizmo
>resonates (it is covered in plastic, so it does resonate).
I can give you an example of how one particular system worked. The St. Paul
public library, 4th St. branch (the main one), had the usual detector pylons
by the exit door, with the familiar warning notices. And there were gizmos,
to borrow Mr. Gonggrijp's technical term, in the books.
Apparently, though, the checker-outers of books found that it slowed them
down too much to have to quell the gizmos, so the entire system was turned
off for more than a year. The theft prevention device was nothing more than
the expectation created by the signs and detectors.
Good old psychology. Of course, when this was divulged, nobody was willing
to say how well it had worked. I've no idea if they ever turned it back on
again.
--
Jonathan C. Rice | Internet: ri...@cray.com | UUCP: uunet!cray!rice
Phil Howard KA9WGN
The layers are separated, but there is an indentation at a couple
of points to "connect" the metallic layers. They form a capacitor
and inductor (the spiraling metal lines).
I have no idea what the frequency is. I don't have a generator to
set this with. There are several frequency ranges the FCC has
authorized for motion detectors.
--
/***************************************************************************\
/ Phil Howard -- KA9WGN -- ph...@ux1.cso.uiuc.edu | Guns don't aim guns at \
\ Lietuva laisva -- Brivu Latviju -- Eesti vabaks | people; CRIMINALS do!! /
\***************************************************************************/
Pete Hartman
[ description of sticker device to foil shoplifters ]
>This whole arrangement looks very much like a capacitor in parallel with a coil
>Don't ask me how it works tho... I realize that this would most probably
>resonate in an electromagnetic field of the right frequency, but how does one
>disable it once you bought the product and go out the shop through the gates ?
>The sticker is diposable and stays on the merchandise once you bought it. The
>salespersons do not do anything funny with the products (like placing them
>briefly under the counter) before handing them back to you...
I don't remember precisely, but I took one of these apart some time ago,
and noticed that there was a puncturing of some sort that shorted the
capacitance. I don't recall quite how it was produced, but it was
clearly not there in the store.
--
-----
Pete Hartman Bradley University p...@bradley.bradley.edu
Alex Ivopol
a price tag sticker. Once stuck on something it is very difficult to tell it
is more than that.
Inside there is a spiral made of a narrow strip of thin aluminium foil. The
spiral is made of straight segments that bend at 90 deg angle and has about 20
turns, the whole thing is square in shape. The outer arm of the spiral ends in
a much thicker (rectangular) strip. A thin insulating foil is placed on top of
this with a small hole just on top of the start of the inner arm of the spiral.
Another narrow strip of aluminium foil is placed on top of the insulating foil,
this strip contacs the inner arm of the spiral, runs across the turns and ends
in a thick rectangular shape, just on top of the other rectangle beneath the
insulating foil.
This whole arrangement looks very much like a capacitor in parallel with a coil
Don't ask me how it works tho... I realize that this would most probably
resonate in an electromagnetic field of the right frequency, but how does one
disable it once you bought the product and go out the shop through the gates ?
The sticker is diposable and stays on the merchandise once you bought it. The
salespersons do not do anything funny with the products (like placing them
briefly under the counter) before handing them back to you...
--
Alex T. IVOPOL
cyb...@kauri.vuw.ac.nz
William D. Palmer
> From: ART...@psuvm.psu.edu (Andy Tefft)
> Date: 8 May 91 03:02:28 GMT
> I have a friend who uses an electric wheelchair ...
(Stuff about theft detection device deleted.)
> Why would this be?
Try getting him to go out backwards. If that gives you the same response
as coming in, there's some non-symmetry in the magnetic metal parts on the
chair that's disturbing the theft detection device.
Dev Palmer
w...@dukee.egr.duke.edu
Duke Electrical Engineering
Room 309 Old Engineering Bldg.
Durham, NC 27706
(919) 660-5282
"I'm from North Carolina and I DON'T support Jesse Helms!"
Tony Arnerich
It has two curved elements (antenna?), an hourglass detail that was
blistered at the junction (burned out fuse for disabling), two exposed
circles of circuit trace (connection to disabling wand?) and a VERY
tiny silicon chip. There were two wires going into the chip, so I figured
that it might be a diode. Under a microscope (this thing measured only
about 0.2mm on a side!), there did not appear to be any complex circuit
pattern.
The overall dimensions on the "antenna" are about 3/4" by 5".
Filip Gieszczykiewicz
Greetings. I've seen 20+ posts on people taking security devices
apart - what I'm wondering is where did they get them.... ;-)
Take care.
P.S. Thank you to all the people who replied to my post on the
couple SIP power amps that I posted a few days ago.
--
_______________________________________________________________________________
"The Force will be with you, always." It _is_ with me and has been for 11 years
Filip Gieszczykiewicz "... a Jedi does it with a mind trick... " ;-)
FMGST@PITTVMS or fm...@unix.cis.pitt.edu "My ideas. ALL MINE!!"
John Whitmore
>I dissected one of these gizmos that was on a CD box.
>
>It has two curved elements (antenna?), an hourglass detail that was
>blistered at the junction (burned out fuse for disabling), two exposed
>circles of circuit trace (connection to disabling wand?) and a VERY
>tiny silicon chip. There were two wires going into the chip, so I figured
>that it might be a diode.
With either a varactor or a PIN diode, this could easily
be a frequency doubler/tripler; send out an interrogation pulse and
look for a triple-the-frequency response, and you've located
a tag that hasn't been disabled.
At a guess, the curved elements would be resonant at the
interrogation frequency and the tripled frequency, respectively.
John Whitmore
Louis K. Scheffer
>While on this topic, I saw an interesting device as well. It is disguised as
>a price tag sticker. Once stuck on something it is very difficult to tell it
>is more than that.
>Inside there is a spiral made of a narrow strip of thin aluminium foil. The
>spiral is made of straight segments that bend at 90 deg angle and has about 20
>turns, the whole thing is square in shape. The outer arm of the spiral ends in
>a much thicker (rectangular) strip. A thin insulating foil is placed on top of
>this with a small hole just on top of the start of the inner arm of the spiral.
>Another narrow strip of aluminium foil is placed on top of the insulating foil,
>this strip contacs the inner arm of the spiral, runs across the turns and ends
>in a thick rectangular shape, just on top of the other rectangle beneath the
>insulating foil.
Assuming this is the gadget I'm familiar with:
There is a diode in between the coil and the capacitor. When you hit it with
microwaves, you get back second harmonic due to the non-linearity of the diode.
When you buy the product, the cashier zaps the diode with a small gun like
tool - then you get no second harmonic, and no alarm.
-Lou Scheffer-
Leonard J. Popyack
try going through backwards.
Luns Tee
> This whole arrangement looks very much like a capacitor in parallel with a co
> Don't ask me how it works tho... I realize that this would most probably
> resonate in an electromagnetic field of the right frequency, but how does one
> disable it once you bought the product and go out the shop through the gates
> The sticker is diposable and stays on the merchandise once you bought it. The
> salespersons do not do anything funny with the products (like placing them
> briefly under the counter) before handing them back to you...
>
>
The local library uses the same sort of stickers on their cassettes. I
haven't found anything in the books and magazines though. They get around
things by passing books around the detector.
As for disabling them, some stores have bins in which the cashiers fill
your bags with sold goods. I'd suspect these bins to also be degaussers.
Timothy Newsham
>
>When you borrow the book the librarian degausses (or maybe magnetises,
>intuitivly
>I suspect degausing) the chunks in the book so changing the shape of the curve
>to have more rounded corners and hence no RF pulsing out. Returned books get
>the reverse treatment.
>
>Maybe someone else can provide some more specific info, and/or corrections.
>
> Francis Vaughan.
So if i really wanted a book alls i gotta do is run upstairs to
the audio visual center and use their bulk tape eraser???
hmm.. interesting
Tim Newsham