Edit: to clarify. I’m talking about these vertical things near the entrance, which set an alarm off if you bring something unpaid for close
In: Technology
The simple answer is that those things by the door are huge antennas.
They constantly transmit a signal, and if that signal gets a response, they fire up an alarm.
The response they expect comes from a nifty type of passive radio chip that gets powered by the transmitted signal, and when they fire up they send out a signal of their own. A very elementary signal. Short. With barely any data content in it. But that’s alright. All the bars at the door have to do is to send a signal and listen for something in return.
The passive radio chips are so small that they can be put *on the backside of stickers*. It’s common that the actual price tag is also the alarm tag for the merchandise.
And then they come in the form of plastic tags that you attach with a needle to a piece of clothing (those tags also often contain a anti-tamper-feature that instantly soils the sweater with ink that can’t be washed off) and also in the form of a small plastic pin about the size of a paper-clip that is commonly used to protect small items such as DVD boxes.
And that is pretty much what there is to it. If you get too close to the alarm arches by the door, and they manage to activate one of the small radio chips so that it starts to transmit, it’ll sound an alarm.
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