I bought a dress from a mainstream shop. The item didn’t have a tag on nor a price tag, so the cashier had to manually enter the price. When I left the store the alarm didn’t set off but a machine that the security guy was holding started beeping. He approached me, showed me a tablet that had the item I bought on and asked me if I have a receipt for it , which I did, but I had to walk in and out the store again until the machine stopped beeping.
How does this alarm system work?
In: Other
Usually they have a little clasp attached to the price tag that is removed once you make a purchase. I’m not sure how else they can track the item.
A tiny piece of electronics identifies itself to the alarm bars at the doors, and the alarm bars either “just” notice that there is a tag present and starts beeping or they actually identify the alarm tag and tell what security should look for.
The electronics can be an extra sticker on the price tag. A small plastic clip. just a piece of wire and a reeeeeeeeeally small circuit, looks almost like a small needle. Or a bulkier thing that also contains some hard-to-wash paint, so that the paint ruins the piece of clothing if you to remove the alarm.
There are several versions. There is a clip on version for clothing. There are also stickers with a transponder inside that can be deactivated at the register. These are used a bunch at Target and WalMart inside the packaging. Another type is the clip on with dye. The clip that holds the transponder has a dye pack that releases if you try to remove it without a special tool.
The one you mentioned would be RFID. There’s a sticker somewhere with an antenna and a tiny chip in it. The reader sends a burst of radio waves, the antenna in the sticker picks that up and uses it to power the chip, then the chip sends its id code back. Ideally your inventory system would track which item is purchased so the anti-shoplifting scanner will just ignore it. https://www.youtube.com/watch?v=0QKrHi-G9WQ
The older type is magnetostrictive. these don’t have any unique identification, there’s just a signal, and the presence of one of the tags disrupts it. If demagnetized it no longer disrupts the signal it in the same way, letting the tag be deactivated. https://www.youtube.com/watch?v=KAm7qAKAXwI
This was probably an RFID sticker that the cashier missed deactivating when you bought the item. When security asked if you had a receipt, I would have said “yes”, and just kept walking. You’re under no obligation to stop/engage.
The most common tiny bar shaped stickers actually work differently than anything described in here.
They just contain two tiny metal strips with a weak permanent magnet.
The thing the cashier runs them over cause an electromagnetic pulse that demagnetises the magnet rendering the sticker inert.
If this doesn’t happen the detector devices at the exist detect this by projecting a weak magnetic field that causes the thin metal strips to vibrate, which in turn alters the magnetic field created by the detectors by a minuscule amount: this is then detected.
Actual rfid tags work by a magnetic field pushing enough power to make the rfid tag actually turn on. They contain microchips and antennas and a coil to receive energy. The rfid tag will send a radio signal when powered on.
However the strength of the magnetic field would need to be massive to turn on the rfid tag from such a long distance as the regular doorway theft prevention posts.
Like your credit card works that way by tapping it: it has to be within inches to be read.
Or similarly wireless charging for a phone.
This can be done, but the magnetic field wouldn’t exactly be safe anymore for all sorts of implanted medical devices. And if you got a tag very close to the emitter of such a field, it would burn out.
Hence the much more simple construction: place a slightly magnetic piece with thin metal strips into a weak pulsing magnetic field, this causes the strips to want to bend, and thus vibrate. Which takes away power from the magnetic field which is measurable.
There’s often a little strip or other device (the ones at my store are a little chunk of plastic with thin sheets of metal inside) on special items that, when they go past the sensors, cause it to beep (as someone who works at an auto parts store, they’re on our headlight bulbs most often, don’t try anything funny with ’em)
when I scan an item, I have to put it over another scanner which basically deactivates the strip or device. I’ve forgotten to do it many times and have had to tell customers to not worry about it and that my dumb butt forgot to deactivate the security device.
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