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How do security systems preventing stealing from shops work?

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Member level 2
Jul 15, 2001
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Security control.

Does anybody know how work the security systems that prevent stealing from big shop centers? They use passive stickers with LC components, detected at the exits of the shop. It is something similar to transponders , but is absolutely passive. I need to develop something similar , but I don't know where to start from.
Any help will be appreciated.

many patents

There are many patents in this area. The cheapes method uses a ferrous metal dipole whose resonant frequency changes with magnetic condition. Other methods use a tuned circuit. Still others use a real transponder like that used for keys for opening door locks.

What I know there are two different types of detectors.
1) a magnetic wire long about 10 cm polarized along the lenght with a code like NNNSSNSNSNNNSSSNSSNSSN and so on.
If you pass in front of the cash without paying, a coil detects the code associated withe the magnetization in the wire and the alarm is ON.
= How to steal : pass a very strong magnet on the wire is the solution but also the audio or video tape will be demagnetized so no fun.

2) a coil tuned around 900 MHz or 130/140 KHz ( depends on the type) and a diode across the tuned circuit; If you pass through the coils installed at the exit of the shop the tuned circuit receive the signal transmitted from the fixed coils , the diode rectifies the signal and creates a lot of 1800 MHz ( or 260/280 KHz )signal; the signal is detected from an antenna and the alarm is ON
= How to steal : wrap around the antenna and diode a foil of domopak aluminum tape; take care the receiver is usually very sensible and this way don't work well.
Also usually the antenna and diode are hidden under the bar code label.
Hope not to have helped any thief ( anyway these methods are well known )


Thanks for your information.
I suspected something close to method 2) but was not shure how they do that.
Finaly... I don't want to steal nothing at all :) :)
The goal is to protect stuff from stealing.
It will be very helpful for me, If there are links to practical designs.
Thanks again for your help.

You might also find this document interesting:

**broken link removed**

Good luck,

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