CharlieHack

[aroraborealis]

I ride the T every work day, and most weekend days, and for all that I complain about how it could be better, I love being able to do most of my getting around on trains and buses. I don't, however, love having to keep track of my Charliecard, not specifically because of it but just because I have so many little bits and pieces of life I have to carry around every day: Charliecard, phone, wallet, keys, etc. Sometimes, I'd like to be able to leave the house with, say, JUST my phone and my keys. But I always want my Charliecard with me, because, hey, I never know when I'm going to want to hop on a bus and go somewhere interesting.

Wouldn't it be nice if my phone could be my T pass? Well, I realized, there's no reason it can't be. So I tried sticking the card under the skin I had on my phone, but it turns out the card is too big and bulky and is too stiff to conform to the shape of the phone. Rats. Except! I don't need the whole card, right? Just the RFID tag and antenna. So I set out to do that.

I needed a Charliecard, a non-plastic container, and some acetone:


ETA: It's a good idea to make a note of your Charliecard number before you destroy it. I just put it in as a memo in my phone. I've never needed the number of my card before, but it would annoy me to find that suddenly I did and couldn't retrieve it!

I used way more acetone than I really needed; just enough to cover it would be plenty:


Then, I waited. I don't know why I expected this step to take a couple of hours, but within a couple of minutes, the card looked like this:


And after about 20 minutes, it looked like this:


So I swished it around and plucked out the important part:


At this point, I wanted to be sure it would still work, so I tucked it into my book and brought it along on my commute the next day. I tapped it on the fare machine as if to load it up with money, and lo and behold! It worked! So I put my July pass on it, and brought it back home to stick it under the new skin for my phone:


ETA: Thanks to spike, I now have a photo of the finished product:

Because I just have a protective skin, rather than a case, you can tell there's something underneath, but not in a problematic way. At the point that you just have the RFID hardware, though, you could do any number of things with it, whether put it inside a skin or case for your phone, or apparently kids in London make bracelets out of it. For me, the cell phone/T pass combination is ideal. I'm so excited!

Feel free to share this around with anyone you think might find it useful for inspiring. Also, I have a lot of leftover acetone if anyone needs any ...

Comments

[aroraborealis.livejournal.com]

For me, mainly because it means I don't have to muck around with any wiring! Would it be easy to have multiple RFID tags on the same antenna?

[xthread.livejournal.com]

It should be - I haven't read the relevant rfid radio specs in a long time, and I am not a radio hacker (we keep hams and EEs around for that), but I expect it should be fairly trivial to attach multiple chips to the same antenna. The two questions that come to mind are what are the relevant wavelengths (which would control minimum antenna size and possibly spacing of the rfid chip attachments from each other) and if the signal could be attenuated by having parallel circuit paths to the different radios, but I think that last isn't actually a concern (and actually comes from a slightly broken understanding of the relevant physics). But it's early yet, and I haven't had my coffee, and we should find a radio geek and ask them.

And then try it.
Muahahaha...

[deguspice.livejournal.com]

They're heading in that direction. With the Near Field Communication (http://en.wikipedia.org/wiki/Near_Field_Communication) standard. They want to embed a two-way RF device in cell phones (and other devices) that can emulate an RFID tag such as a Charlie Card, or be used to exchange money between two NFC devices.

With NFC technology, you could have various cards embedded in your cellphone (Nokia came out with an NFC cellphone in 2007).

[nathanjw]

Power would be an issue - recall that the chips draw their power from the signal the antenna picks up, and the system as a whole probably doesn't have a lot of margin. So powering up the T's Mifare Classic chip is OK, but powering up two or three of them might not work very well.

Getting past that, it would also be a problem if more than one tried to transmit at a time. Low-power RF systems like these often use backscattering for chip-to-reader communication - they don't actually transmit a signal of their own, they wait for their turn to speak, the reader keeps transmitting CW, and the chip tweaks the antenna's circuit so it's a bit more or less reflective of the incoming signal. Both having extra loads in the form of other chips, or having multiple chips manipulating the antenna's impedance, could make that work very badly.