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View Full Version : OT: grocery store theft detectors/alarms



lbhsbz
12-07-2012, 11:02 PM
you know, those panels you walk through when entering/exiting the store....that if you've stolen something important...make the alarm go off. I frequent the local CVS drugs...mostly for a pack of smokes on the way home from work, and when I'm out, a bottle of rum...but that's not important. The last 3 times I've walked in the door (over the course of the last 6 days)...the theft alarm sounds.....while I'm walking in....and while I'm walking out. I didn't think much of it until tonight, for some reason it prompted thought. Not sure why. Anyway, I asked the checker lady why I keep setting the alarm off...and she threw out some stupid ideas. So I went back to my truck, unloaded my rum, smokes, cell phone, keys, wallet, emptied the pockets...etc... The only thing on me was clothes...I'd emptied my breast pocket of chips from the night before in the shop earlier in the day. I walked back in the door and the alarm went off again. The only thing made of metal on me was the zipper and button on my trousers. Then I started thinking...this is the first time in a long time I've been doing a considerable and consistant amount of a machine work....and the alarm started going off about the time my latest project started....I've kept busy picking chips out of my socks/clothes/fingers/forehead, but as far as I know I've got all of 'em. What sets off those stupid alarms? Do I have a piece of swarf of a piece of busted carbide endmill stuck in me somewhere?

duckman
12-07-2012, 11:08 PM
No its just your rfid personality .

Black_Moons
12-07-2012, 11:55 PM
Some of those security tags are just little resonant antennas... Basiclly, a precision chunk of metal.

You may very well have a very precisely machined chunk of swaff somewhere in your hair :) or boot. Etc.

rdfeil
12-08-2012, 12:08 AM
I am no expert on the alarm systems, but I am fairly sure they are activated by a tuned circuit on a chip (small holder). The first thing I would think of is... are you wearing any new clothing, say six days old? Another thing you could do is take everything you were wearing when the alarm activated and bag it up. Then with a different change of clothes on (remember the shoes), go back and confirm that you don't set off the alarm. Then try it carrying the bag of clothes you were wearing. If the alarm goes off then you know it is in the clothes. With the help of a store employee you will be able to figure it out quickly and the store will be grateful.

danlb
12-08-2012, 12:40 AM
It's quite possible that you have one of those little paper tags stuck to your shoe or something.

In our area the blasted things go off all the time, and the store personnel invariably wave people through without checking... well, as long as the person looks reputable.

The paper tag holds a tuned antenna that resonates when it passes between the plates of a detector. While it is possible that you have accidentally created such an antenna from swarf, it's not likely. From: http://www.best-microcontroller-projects.com/anti-theft-tag-circuit.html



Systems have been developed wherein the circuits to be attached to protected articles comprise only a small piece of metal of a special size and shape to form a resonant circuit. These systems transmit continuously in the vicinity of the checkpoint at the resonant frequency of the circuits attached to the merchandise. When an article of merchandise bearing such circuit or piece of metal passes through the checkpoint, the circuit begins to resonate as a result of the transmitted energy. This in turn causes absorption of a portion of the energy in the vicinity of the checkpoint. Means such as a grid dip meter are provided to monitor such changes in ambient energy level and to sound an alarm when the level drops as a result of its absorption by a resonant circuit passing through the checkpoint.

Grind Hard
12-08-2012, 12:45 AM
Odds are it's a tag either sewn into your clothes or inside your shoes. Stores have had to get more creative at hiding tags.... Take the pads out of your shoes, there might be one hidden under there. For your clothing, all it takes is a bit of metallic thread in a seam and you have a "tag".

1-800miner
12-08-2012, 01:18 AM
It's a government conspiracy thing. They planted a microchip in you the last time you drank too much rum.
They know your every move.

flylo
12-08-2012, 01:25 AM
I had a customer once who truly believed the goverment was watching us thru tv sets. After thinking about it I realized that my job wasn't that bad, LOL, can you imagine having a job watching folks scatch their selves while watching hours of tv a day?:p

The Artful Bodger
12-08-2012, 01:26 AM
Walk through naked, dont forget to take off your watch, spectacles and all your body piercings!;)

armedandsafe
12-08-2012, 01:48 AM
The company which calibrates two of the stores in my area comes through once every 6 months. I can always tell when the alarms have been freshly adjusted because I set them off going in (and again going out.) I finally caught the tech when he was sorking on one of them and had him check out his device on me. I set it off and it puzzled him.

He took his wand to me and had me empty my pockets, remove my carry piece, belt and my boots. Still set it off. We finally found that it was the shrapnel in my right leg. He made some notes while moving his wand about on my leg. Whatever he discovered and calibrated for, I haven't set them off since.

Pops

Jaakko Fagerlund
12-08-2012, 04:06 AM
Those "product alarms" are utter crap and give much more false positives than real alarms. They peep even from a Cat5e cable in my bag or some similar.

And in here, if the alarm sounds...so what? Just continue walking. No law enforces you to stop or gives anyone the right to search you based on the peeping machine. You have to be caught in action stealing something or running from a crime scene in order to detain you until the police arrives. Only te police would be able to search you and if there is just a hunch or suspicion that you might have stolen something, they probably won't even come to check it out, as personal privacy is aknowledged quite high in here.

John Stevenson
12-08-2012, 06:01 AM
I love to 'own' this one but it happened to a guy I don't even know.

This guy pays for his groceries, puts them into two bags and is walking out the store when the alarm goes off.

Never missed a heartbeat, just dropped both bags, stuck his hands up in the air and shouts out. "DON'T SHOOT "

Quite funny over here as we don't have guns.

HSS
12-08-2012, 08:41 AM
I do service work in supermarkets and never set those things off unless I am carrying a rolled up extension cord. Sets them beeping every time. I just keep walking. The cop on duty never says a word. Yes, we have police located at the entrance of the stores to discourage shoplifting.

vpt
12-08-2012, 08:56 AM
I have also had them go off a few different times while walking in. Didn't have anything different with me at the time (at east that I knew of).

browne92
12-08-2012, 09:46 AM
I used to set them off all the time. Turns out the little antenna thingy was under the padding in my shoe.

I've been tempted many times to do Sir Johns "Dont shoot!!!" shtick. One of these days I will. If nothing else, it'll give the employees something to talk about for awhile.

ogre
12-08-2012, 03:14 PM
Just outta curiousity , do pacemakers or pain pumps set them off or do you have one in you?

Evan
12-08-2012, 03:24 PM
Pacemakers could set them off but much more important is that the alarm can interfere with a pacemaker. Some of those alarms have a pretty strong RF field. The pacemaker is designed to ignore it but only for about five seconds. After that the operation is unpredictable as it cannot sense myoelectricity in the heart properly. If you have a pacemaker don't stand around chatting to buddy near the alarms.

John Stevenson
12-08-2012, 03:41 PM
Pacemakers LOL

Now this DID happen to me.

Shortly after my mother died my brother and myself had to sell her bungalow, we agreed to get three estate agents in and see who was offering the best deal.

Booked them in at 3/4 hour intervals and waited.

First one comes, a lady, very professional and gets one of these ultrasonic tape measures out. First time I had seen one. Aims it at the far wall and beep.

I groaned and doubled up, she asked what the matter was and I said it was affecting my pacemaker. She was very concerned and then went out to her car and got a normal tape measure and measured the house up by hand.

When she had gone my brother said "You bastard "

Black_Moons
12-08-2012, 03:57 PM
Pacemakers LOL

Now this DID happen to me.

Shortly after my mother died my brother and myself had to sell her bungalow, we agreed to get three estate agents in and see who was offering the best deal.

Booked them in at 3/4 hour intervals and waited.

First one comes, a lady, very professional and gets one of these ultrasonic tape measures out. First time I had seen one. Aims it at the far wall and beep.

I groaned and doubled up, she asked what the matter was and I said it was affecting my pacemaker. She was very concerned and then went out to her car and got a normal tape measure and measured the house up by hand.

When she had gone my brother said "You bastard "

You need to adjust the story slightly. let me fix it...


I groaned, doubled up, triped and fell over, she asked what the matter was and I said it was affecting my pacemaker. She was very concerned and then went out to her car and got a normal tape measure and measured the house up by hand.

When she had gone my brother said "You clumsy bastard"

Grind Hard
12-08-2012, 05:37 PM
Those "product alarms" are utter crap and give much more false positives than real alarms. They peep even from a Cat5e cable in my bag or some similar.

And in here, if the alarm sounds...so what? Just continue walking. No law enforces you to stop or gives anyone the right to search you based on the peeping machine. You have to be caught in action stealing something or running from a crime scene in order to detain you until the police arrives. Only te police would be able to search you and if there is just a hunch or suspicion that you might have stolen something, they probably won't even come to check it out, as personal privacy is aknowledged quite high in here.

Ah see here in New York depending on a variety of profiling factors you either get a free pass to steal as much as you want (due to the fear of riots, negative publicity or fear of retaliation)... or you get thrown to the ground, a gun at the back of your head and every cop within 20 miles converging on the scene all screaming profanity and threats at you.

...all that because the cashier didn't deactivate the tag in my purchase.

Mind you that was the reaction of the store-cop (flying tackle, call in the swarm, threaten me with death every time I so much as twitched). The store manager couldn't understand why they basically tried and convicted me at gunpoint over a malfunction while "other people" were permitted to walk out the store with items in plain view without so much as a "can I see your receipt." Long story short, I ended up downtown for 38 hours while my lawyer and the store manager argued with the police chief. In the end (obviously) I was released after talking to the judge... who was very confused by the whole situation.

To this day I refuse to shop at K-mart or by extension Sears, 25 years running now.

jdunmyer
12-08-2012, 06:55 PM
A while back, I entered an AutoZone store and set off the alarm. The guy asked if I had an artificial knee or somesuch, I figured it might be my carry piece and said so. First time it ever happened...but:
later that evening, we went to Best Buy and had the alarm sound off when exiting the store, but no one bothered us. Went to Wal-Mart and had the same experience, the alarm sounded going 'in' and going 'out'. Have returned to that Wal-Mart since and had the same experience. Again, no one bothered us, so I don't know what they do when someone is actually trying to steal something.

I though those alarms were activated by the RFID tags, not simply a hunk of metal.

The local Post Office has some sort of detector at the door. As it's illegal to CCW in a US Post Office, I always disarm before entering, but tested the alarm one day by wearing a 12' tape measure on my belt. That did NOT set it off, so go figure.

Evan
12-09-2012, 01:31 AM
Most of the alarms are activated by a special designed antenna that picks up a radio signal and retransmits it back to the alarm system. It is a kind of active metal detector tuned to a specific frequency which is determined by the effective RF length of the antenna.

One very common design looks like this under the lable:

http://ixian.ca/pics10/rftag.jpg

To deactivate it, it is placed over a transmitter wire loop antenna on the till which sends a momentary high power pulse with very short range. The pulse is picked up by the antenna which blows out a tiny weak link in the wire pattern, like a fuse. That detunes the antenna so it will not work.

Jaakko Fagerlund
12-09-2012, 02:29 AM
I though those alarms were activated by the RFID tags, not simply a hunk of metal.
They are just RF tags, not ID. If it would be RFID tags the cashier would immediately know what product triggered the annoying beeping machine.

These gates are just a fear factor and not efficient and prove nothing if they alarm, so I don't bother stopping when they scream.

Mike Burdick
12-09-2012, 03:11 AM
It is common to have the electronic article surveillance (EAS) tags sewn into clothing by the manufacturer but washing will usually destroy them if the store's scanner did not deactivate it. You may have one inside your shoe lining or in something that you are wearing that is not commonly washed.

darryl
12-09-2012, 03:50 AM
What? You mean they have them in underwear too? :)

ed_h
12-09-2012, 11:41 AM
I believe these tags are different from RFID. In an RFID tag, information--minimally just the number of the tag--is returned to the reader. For the theft tags, I think the reader just senses their presence. I guess you could say it does this by sending back energy, but it is really more that it disturbs the field of the transmitter, so that it sees a small change in impedance.

Jaakko Fagerlund
12-09-2012, 12:38 PM
I believe these tags are different from RFID. In an RFID tag, information--minimally just the number of the tag--is returned to the reader. For the theft tags, I think the reader just senses their presence. I guess you could say it does this by sending back energy, but it is really more that it disturbs the field of the transmitter, so that it sees a small change in impedance.
Yep, and that is the reason they don't prove anything, as they are set of by just about anything.

Evan
12-09-2012, 01:17 PM
I guess you could say it does this by sending back energy,

It does return a modified signal as there is a diode in the centre of the tag. This rectifies the received signal which produces a high voltage that then rings the resonant circuit to transmit a signal back to the detector.

And yes, they are not the same as RFID at all. An RFID tag has an actual solid state logic chip that is able to be powered by the received energy and then encodes the signal sent back with information. Some RFID tags have ranges of hundreds of feet.

ed_h
12-09-2012, 02:26 PM
It does return a modified signal as there is a diode in the centre of the tag. This rectifies the received signal which produces a high voltage that then rings the resonant circuit to transmit a signal back to the detector.

Yes, I believe the impedance disturbance and the backscatter interpretations turn out to be equivlent mathematically.

Evan
12-10-2012, 10:12 AM
Yes, I believe the impedance disturbance and the backscatter interpretations turn out to be equivlent mathematically.

No. It retransmits the signal using the power provided by reception of the original pulse. That is rectified and produces a voltage of from 20 to 30 volts which then causes ringing in the resonant circuit. "Backscatter" has nothing to do with it. The transmitter loading is not being detected. The retransmitted signal is and can be detected by any device tuned to the same frequency with no reference to the original signal. That is in no way equivalent.

danlb
12-10-2012, 03:24 PM
Since there are several different designs available, it would be difficult to say with certainty whether it is using a retransmission of the signal VS disturbing the impedance.

I would think that it would be odd for a coiled cat5 cable to trigger it by transmitting anything. One of the posters said that happens to him often.

Dan

Evan
12-10-2012, 03:45 PM
There are about four different operating principles. The one I showed is the most common type but there are variations within that type too. Those types all work by retransmitting a signal produced by the power supplied by the detector system. Basically the system shoots a load of birdshot at the nearby area. If the birdshot hits anything that resembles a bell, the bell rings. If the ringing bell makes the right tone it assumes it has found a tag that is still working and sounds the alarm.

Other types do act as a load on the system which is really a fancy metal detector. Another type causes a piece of special steel to ring at audio frequencies which modulates the magnetic field. More expensive ones use microwaves and act as a 1 bit RFID system. These are too expensive to throw away so are removable. Many systems also contain an ordinary metal detector along with the tag detector. They are set up to detect large but not too large pieces of metal so they don't alarm on a shopping cart. That is the most likely cause of false alarms, not the tag detector.

michigan doug
12-10-2012, 03:55 PM
RFID technology has "advanced" rapidly. More and more capability in a smaller and smaller object. A major retailer has done significant research into putting an rfid tag on every product in the store. There would be few or no cashiers per se. As you walk in, you are surveyed for what smart credit card you have on your person. You put your purchases in the cart and you walk out. Your purchases are queried and automatically added to your smart credit card.

There is also discussion of putting rfid devices in every piece of paper money. Now, the government, or your favorite retailer, can gather all sorts of information about you, since it can literally follow the money.

The first proposal was put on hold because of problems with the shoppers not trusting or being comfortable with the process.

The second proposal I am uncertain what happened with that.

Finest regards,

doug

Evan
12-10-2012, 04:09 PM
RFID in money has been discussed for a long time. The ability to follow the money would be a boon to the state as it would make it very easy to discover tax evasion. It would also make it very easy to build a personal dossier on every person. If you really want to implement "1984" that is the way to do it.

The idea of RFID in money has fallen out of favour because it turns out it isn't really necessary. People are very willing to provide the information in other ways by using electronic transaction systems such as credit and debit cards etc.

ed_h
12-10-2012, 04:39 PM
No. "Backscatter" has nothing to do with it... The transmitter loading is not being detected... That is in no way equivalent.

Evan--

With the understanding that we are talking about one particular kind of retail tag detector, a tag like the one you posted the picture of is a passive resonant circuit. It is a parasitic element that absorbs energy from an "exciting" field of the proper frequency, and reradiates it. It is much like passive tuned "director" or "reflector" elements in directional antennas. This retransmission or backscattering (sort of a loose use of either term, but both are used in the literature) has a detuning effect on the transmit antenna, which the transmitter sees as a fluctuation of radiation impedance. It is true that detuning is how some metal dectors work, but the resonant nature of the tag makes it more sensitive.

There are certainly other technologies used, probably including pulse/listen types. One I find very intriguing is the one that uses what appears to be just a plain strip of metal foil. I don't know a lot about it, but I understand that the foil is actually two kinds of metal with different magnetic properties. This forms a non-linear magnetic element that is excited from the transmitter by two frequencies. The non-linear mixing produces sum and difference frequencies that are detected by a receiver.

TRX
12-10-2012, 09:01 PM
RFID in money has been discussed for a long time. The ability to follow the money would be a boon to the state as it would make it very easy to discover tax evasion.

To a mugger, a handheld reader would help select victims with enough cash to make it worth their while...

Evan
12-10-2012, 09:47 PM
This retransmission or backscattering (sort of a loose use of either term, but both are used in the literature) has a detuning effect on the transmit antenna, which the transmitter sees as a fluctuation of radiation impedance.

Nope. The transmitter send a signal pulse and then turns off while it listens for the ringing signal. That is why it is like a shotgun.

added: The tag is much too small and too far away to produce a detectable effect on the transmitted signal. That also would not be very selective. Anything that was a harmonic size would have a similar effect and the loading effect would be extremely variable. Detecting the ring frequency is much more selective and it doesn't need to be the same frequency either.

ed_h
12-11-2012, 01:29 AM
Nope. The transmitter send a signal pulse and then turns off while it listens for the ringing signal... The tag is much too small and too far away to produce a detectable effect on the transmitted signal.

Well, there is at least one manufacturer of these systems that needs to be notified right away because he is misinformed about how his systems work.

As I've said, I don't doubt that there are or were detectors that work on a radar-like pulse and listen basis--it's an obvious approach, but there are others. A Google search might suggest that continuous swept frequency systems may be more common these days.

As for the "field disturbance" approach, here are two patents I'm aware of: US5353011 and US4686517. Each of these describes at least one embodiment that uses a single antenna and continuous excitation (swept, I assume). One of them actually shows an explicit "receiver" component, but you can see that it is coupled across the transmitter so that it can monitor the transmitter's response to "...disturbances within the electromagnetic field...".

Evan
12-11-2012, 04:11 AM
The first listed patent is 18 years old and the second is 25 years old. I doubt very much that such systems are still in use. As I said there are sensors that act like a metal detector but they aren't the type of tag I showed.

Barrington
12-11-2012, 07:12 AM
Both pulse 'ring down' and swept types are in common use. Indeed many systems can be configured on-site to either to suit tag cost/performance requirements.


It does return a modified signal as there is a diode in the centre of the tag. This rectifies the received signal which produces a high voltage that then rings the resonant circuit to transmit a signal back to the detector.I haven't come across any system which requires more than the inductor/capacitor tank circuit. I do recall some talk of a system which used a diode as a non-linear capacitor to generate a second harmonic easier to filter out for detection, but I don't know if anything came of it.

Many stages of diode/capacitor voltage multiplication are used in RFID tags to get usable voltages for both power and increased discrimination levels.


It retransmits the signal using the power provided by reception of the original pulse. That is rectified and produces a voltage of from 20 to 30 volts which then causes ringing in the resonant circuit.20-30 volts ???

Cheers

.

Evan
12-11-2012, 01:34 PM
I have taken the tags apart and examined them closely. When I had my computer store many of the products came with tags in the packaging which is common. I removed them when I sold the merchandise. Putting the tag on the outside of the package is foolish as they are ridiculously easy to disable if you have direct access to the tag. All you need to do is punch a little hole in the antenna wire and it is dead. I did trials by using a new tag that I had altered and putting it in my wallet, then visit a store.

The transmitters are quite powerful which is why they can interfere with a pacemaker. Looking up patents isn't very informative as there are many patents that have never seen the light of day as a product. Many tags use the diode as the capacitor and as the weak link for disabling. The deactivator simply shorts the diode by exceeding its voltage rating. If you look at the picture I supplied the diode is the little square part in the middle.