ID this RFID

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ID this RFID

Post by jimandy » Mon May 15, 2006 8:44 pm

Finally I got one of those little anti-theft RFID tags that is still active.

Went through self-service at Wal Mart with a cheap GMRS set that had the tag. (yes I paid for it) but the sensors at the door went off with gusto. The nice lady waved me on through so I have this little jewel and would like to play with building my own sensor. For starters, I wonder if anyone can direct me to a good web source for how it might work - you know, magnetic or RF? frequency? power level? etc. Yes I know there's a lot out there. If you're familiar with the particular system that detects little white stick-on tags, about 7/16" wide by 2" long with the letters "DR would appreciate help narrowing the search.
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Post by Chris Smith » Mon May 15, 2006 9:22 pm

From memory [?] they bombard it with a low power RF in the 210 mhz?? range, and watch for a reflected digital signal?

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Post by JPKNHTP » Mon May 15, 2006 9:55 pm

-God Bless

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Post by Sambuchi » Tue May 16, 2006 3:40 am

This may help .. may give some ideas

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Post by jwax » Tue May 16, 2006 3:57 am

"Make" magazine, Vol 06 (newest) has a less-than-$100 reader project based on the Parallax RFID reader module. Tags typically use 130 KHz, 13 MHz, 900 MHz, or 2-6 GHz.
Another interesting control-access project using RFID was in April 2005 N & V mag called "You Can't Touch That: Non-Contact Access Control".

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Post by dr_when » Tue May 16, 2006 6:25 am

Actually, I don't think they did anything to your RF tag. I believe they use the cheapest read only tags. When they scan your stuff at the register, they record the tags number so the door sensors won't trip. They simply did not read your tag. You will have to have a reader for the right frequency tag and in your case you will only be able to read the serial number. I doubt if you have a more expensive writable tag.
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Post by Newz2000 » Tue May 16, 2006 10:30 am

Can the sensors at the door read the RFID tags from several feet away? I thought they could only read a few inches.

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Post by JohnDay » Tue May 16, 2006 10:51 am

If they could only read a few inches then the store is in trouble! TH eopening is generally nearer three feet and they have to be able to virtually gaurantee reading anything that goes within about 3 feet of the door sensor. For 125kHz tags with a good sized antenna - 3 feet is common.

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Post by haklesup » Tue May 16, 2006 2:16 pm

These basic security tags contain no information other than active and inactive. RFID tags with digital info will always have some sort of IC associated with it. Wall mart wants to use the digital ones but the cost per tag exceeds the price of some of the items they want to put them on for now. I do think they are testing some advanced tags on some higher priced products from forward thinking vendors but only for inventory control not for security or checkout AFAIK.

I wonder if the recording industry has thought about implementing these tags embedded directly in the CD media. That would be a cool extra for recordable CDs too. I could find the CD I want in a pile or case without flipping through them by using a hand held reader. (theres a patent waiting to happen). Great for managing digital photo collections or MP3 backups.

For RFID range is always very short. The initial radio pulse must travel from the transmitter to the tag (loosing power as a squared function) then back to the reciever loosing energy to the 4th power along the round trip. Sometimes the checker forgets to deactivate a tag, gets lazy and runs it by the deactivator too fast or the tag is shielded by another item you purchased when deactivated.

I too have wondered about exactly how this system works but conscise information is rare due to the security oriented use of these tags. A hand held device could be constructed to deactivate the tags but bringing such a device into a store would get you arrested for having burglary tools.

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Post by Robert Reed » Tue May 16, 2006 2:32 pm

Youi guys probably already know this, but from my limited knowledge on these devices, I under stand that the receiver also receives it operating power from the transmitted RF power.

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Post by ecerfoglio » Tue May 16, 2006 3:32 pm

Theese "cheap" tags may be only a tuned circuit: a printed coil and a capacitor.

The sensors are BIG coils that emit RF. When you pass between them, the tag absorbs power in that especific frecuency and trips the alarm.

When you buy the item that has the tag, they "kill" the tag in a special "deactivating station". I don´t know what does it do to the tag, but I´ve heard two versions of it (perhaps there are 2 diferent systems):

Version 1: The "deactivating station" couples a STRONG RF signal to the tag and blows a fuse inside it.

Version 2: The "deactivating station" has a STRONG MAGNET that changes some parameter inside the tag (bends something or magnetizes something) and "de-tunes" the tag so that it no longer absorbs RF in that frecuency band.
E. Cerfoglio
Buenos Aires

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