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Electronic article surveillance

Click here to learn more about RFID

New for July 2020: Electronic article surveillance is the poor cousin of RFID. EAS does not identify the article it is attached to; it merely tells a store owner that someone may be trying to move merchandise out of the store without paying. According to Wikipedia, there are three broad categories of EAS:

  • Electro-magnetic (useful for when the protected item is metallic)
  • Acousto-magnetic (trade name Emtag, you might have seen one on the cord of a power tool)
  • Radio frequency

Of course, here at Microwaves101 we are mostly interested in radio frequency stuff.

RF EAS

There are three parts to the system: the tag itself, the deactivation system, and the exit detection system.

This technology dates back to the 1950s, and there are many ways to implement it. In some cases the ID tag must be removed from the article during purchase (many clothing stores use this, as it is attached using a spike through the clothing's fabric), in other cases the cashier passes the ID card over a strong EM field and the tag is blown open (essentially all big-box stores use this).

The tag provides a resonant circuit that can be excited by a large transmit/receive coil arrangement at the store exit.  A pulsed signal is transmitted and the then the system looks for a lingering signal in receive. To use a well-worn analogy, an audio article surveillance could be set up with a tuning fork. The exit detection system would generate a loud tone at the tuning fork frequency, then go silent.  If the ID tag is present it will continue to hum out the resonant frequency. Gotcha!

Reusable RF EAS

Reusable tags must be robust enough to stop thieves from pulling them off. Typically, there is some type of magnetic latch to release the spike that holds the unit to a garment.

Below, we took apart a re-usable tag, using a hammer, of course.  It contains a simple parallel LC circuit, a so-called "tank circuit" that can store energy over a narrow band near its resonant frequency. You can calculate the inductance using any on-line solenoid inductor calculator, and cut the wire to the capacitor and measure its capacitance. We used our resonant frequency calculator  to find the center frequency and can up with 5 MHz as a rough estimate. According to Hoyle, the real frequency should be around 8.2 MHz.

Disposable RF EAS

The budget for disposable electronic article surveillance is near-zero.  The tabs are made in high-volume, using aluminum traces on cardboard substrates.  One metal layer provides a fairly accurate spiral inductor, with a second metal layer needed to fabricate a parallel-plate capacitor and inductor crossover. That capacitor has to have a low breakdown voltage as it acts like a fuse when the item is checked out.  Let's watch a video of produced by "Diodes Gone Wild" where an intact RF EAS tag reveals its secrets.  If you don't smile when he makes the "beep beep beep" noise, you have no humanity.

Diodes Gone Wild explains EAS

 

Author : Unknown Editor

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