Circuit Bending RFID


I hate keys. My goal in life is to remove as much junk from my pockets as possible. Here at SparkFun, we have grown big enough we got an RFID access system. Everybody gets a card:

http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-2-M.jpg




Including our friend Chris. He decided to be silly on picture day. Payback is a ...

I forget my card a lot, but I never forget my car keys. The goal was to remove the card internals, and embed them into my car key fob - so I would just have to remember my car keys. I realize the range of the new-recoiled fob would be much shorter, but I don't care if I have to rub the thing on the scanner - I want to carry less!
http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-3-M.jpg



Step one, peel back the cover. Pete Dokter (ex-HID guru) says these are 13MHz cards.
http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-4-M.jpg



The entire coil was stuck into the trench with some serious adhesive. With a heat gun and a little persuasion we were able to remove the coil, but it wasn't easy or pretty.
http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-1-M.jpg



Close-up of the bit-o-silicon.
http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-5-M.jpg



Now to fold it into my car's key fob...
http://www.sparkfun.com/tutorial/KeyFob-RFID/KeyFob-RFID-6-M.jpg



Ok - I could lie and say this worked, but it didn't. We picked Chris' card to practice on (and photograph) because his card had quit working (each card is ~$5 a pop). We were able to remove the coil, intact, from his card, but because it started out not working, it certainly didn't work after we re-coiled it into my car fob. But the idea was proven! We then grabbed a new, functioning RFID card and practiced the same method. Unfortunately during the second heating-un-gluing process, the coil broke. Pete claims that we could create our own coil but I'm not convinced. So to date, I still have to carry my keys and a SparkFun key card. Booo.

Yes, we could look into purchasing a key chain RFID fob, but that defeats the purpose - I want to (tear things apart and) carry less things in my pocket.

Time for new stuff!



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Comments 18 comments

  • lolol…… I am one of the patent holders on the 13.56mhz rfid tagging system readers and have been messin' with these readers and tags for almost 20 years. To just pull the coil and fold it is a No-no…lolol. Now, if you go back many years and look up a bit on the ‘Mifare’ system you can find out a little on how to use these. Quite a number of years ago, I designed an antenna as just a coil trace on the circuit board, (with the chip of course), which was then injection molded in a teflon button, (25mm dia. x 4.5mm). It would take up to 550 Fahrenheit….when applied to the bottom of the stool legs it made it REALLY slide in the lab too. So……is that small enough for a keyfob?

    • Cool! I hope you get a small fraction of the $5 we’re paying for our cards. I was really shooting in the dark. But it was worth a try, no? I debate little and try (and fail) a lot.

  • hey, I didn’t mean to dump on ya, pete. I had a card key system put in at my house. All the installers seem to know is HID. Like a bunch of frickin zombies “HID HID HID…”.
    I’ve been round and round this issue with tag makers and they are all pretty close mouthed when it comes to HID stuff. If there are alternate tags, I’d like to get one. If SF doesn’t have HID readers then maybe there is hope.

  • I’ve looked into making an RFID antenna for a project once. They have to have a very particular resistance and inductance in order to resonate properly during the charging cycle, and incorrect antennas can damage the chip. By folding the antenna in half like that, you’re changing its inductance; I’d bet it wouldn’t work at all. It’s not that bad to design and wind a new antenna, though, at least for someone in your line of work.. and especially if you have an inductometer handy. Here’s a nice doc with all the theory, math, and design spelled out for you: http://ww1.microchip.com/downloads/en/AppNotes/00710c.pdf

  • Very impressive! It’s amazing how RFID and RFID Readers can create such unique uses and opportunities. If you check out www.rfidreaderinfo.com you can find a ton of other RFID technologies too!

  • If this does turn out to be a 125 kHz HID card, they’re pretty easy to read and emulate. I built a really simple homebrew HID card reader a while back as a garage door opener, and just for fun I built my own emulated HID card using just an AVR and a coil.
    Here’s a schematic and details on the HID card reader. It’s just a Propeller microcontroller and a few passive components:
    http://forums.parallax.com/forums/default.aspx?f=25&m=287204
    And this is a blog post on the AVR firmware that emulates a HID card:
    http://scanwidget.livejournal.com/32928.html
    Cheers, and good luck on your quest to carry fewer things (and take more of them apart)!
    –Micah

  • 125 khz is a little tougher nut to crack. The antenna coil is a VERY long length of 42ga. copper. Also, the communications is near field, manchester encoded, back feed coil shorting. It works by shorting the fields out and the reverse emf is the modulation. In actuality, it is pretty clever, but very noise sensitive. Range is rather problematic as the received signal level falls off at the cube root of the distance.

  • Just for reference, an instructable on how to make an RFID tag.
    Unfortunately a 13.56 MHz tag, not the 125 KHz version.
    http://www.instructables.com/id/RFID_Reader_Detector_and_Tilt_Sensitive_RFID_Tag/

  • My Emag isn’t that great but I’m guessing that when you fold the coil like that you create two loops, whereby the current induced in each loops is in the opposite direction causing your signal to be completely canceled out.
    Here is an idea:
    You always have your keyfob with you, but I bet you also always bring your wallet to work. Perhaps you can sew the coil and IC inside of you wallet w/o folding it. (I see a lot of people carrying the card in the wallet and scanning the badge via swiping the wallet, but that isn’t geek enough IMO.)

  • I agree that the RFI card you are showing is most likely a 125khz card. A 13.5 Mhz card has only one or two windings.
    Here’s the thing. When you fold the coil to fit into the key, you change it’s inductivity as you increase the number of coil windings (by folding it). Inside the chip is a fixed value capacitor which forms a resonant circuit with the coil. The resonant frequency of this circuit determines the operating frequency of the card. If you folded the coil you have increased it’s inductivity. thereby lowering it’s resonance frequency. You could counteract this by reducing the number of coil windings (or, better, wind a completely new coil). If you want to be completely sure of the operating frequency of it’s base station, wind 3 feet of wire into a coil and attach it to an oscilloscope (or a counter) and watch the waveform.

  • Yes this would be SparkFun to make the name badge type fit into that small space, but the RFID key fob type would already have a compact antenna that should fit into your car key fob. The SparkFun way would be use the chip from the name badge and use the antenna from a key fob.

  • http://www.hidglobal.com/prod_detail.php?prod_id=22

    • That’s exactly what I use at work. I cracked one open for a practical joke: if you remove the chip/antenna element and stick it to the bottom of the reader, nobody can authenticate.
      The antenna is wrapped around the chip itself in a small package, like a fat grain of rice. It’s quite small, I bet it would fit in your car fob.

  • Check out those miniature ferrite core RFID antennas, I guess there other manufacturers, too.
    http://www.kaschke.de/english/pdf/transponder_coils_0208.pdf

  • Yeah, very likely 125K. probably an HID system so you can’t just use any tag with it. If some one finds HID compatible tags other than cards, I’d love to know… A little keyring fob would be way awesome.

    • Alright, so sue me. I also agree that it’s a lot of turns. But at the time I was there, there was a great migration to 13.56MHz. Without knowing what the tuning capacitance is, I made an assumption, and we all know what that gets you (or me, as the case may be). The coil form factor has also changed since I’ve opened any HID cards, so…meh. I screwed up.

  • I don’t think that’s a 13.56MHz antenna – way too many turns, and most access control systems are 125kHz, anyway.
    Any reason you can’t just buy smaller tags? (e.g., http://www.identisource.net/pd_hid_1391.cfm)

  • Your crusade for “less in your pocket” is a a worthy one, sign me up!.
    Several phones now support Near Field Communication (NFC) which can emulate a swipe card.
    One NFC microcontroller chip I came across is the ST21NFCA. I would love a breakout board for that chip, c'mon Sparkfun!


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