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
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.
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