Smart Cards, Smart Phones and a Cashless Society

Dissecting the SmarTrip

Originally Published on December 14, 2005

DCist.com - ... The Washington Metropolitan Area Transit Authority's (WMATA) SmarTrip card. Ever wonder how it works — or better yet, how to disassemble it and turn it into a keychain-ready size? If so, read on...

The SmartTrip card is composed of three sheets of laminated cardboard (or plastic; it's tough to tell). The middle one is where the important guts are located. The outer two serve a decorative and protective function. The first order of business is to remove those outer layers. But before you do, resign yourself to destroying the card. You might be able to transplant its brain into a new body, but DCist doesn't want any angry emails demanding money for replacement cards. You've been warned.

In order to plunge heedlessly onward you'll need to find some acetone — aka nail polish remover. Another word of caution: despite its over-the-counter status, acetone is not particularly good for you. It's very flammable, a skin irritant, and pretty bad for your kidneys. In addition to being an inhalation hazard it can be absorbed through the skin. So wear gloves, work in a well-ventilated space, and use common sense.

It's also worth noting that acetone can dissolve many types of plastic — that's why we're using it. But that also means that you should avoid using tupperware for this. So find a glass or ceramic bowl and fill it with about half an inch of acetone. If you have an x-acto knife handy, you might want to score the back of the card (not diagonally) in order to help the acetone penetrate, but it's not strictly necessary to do so. Plop the card in the bowl, cover with aluminum foil (to cut down on fumes) and wait for about 15 minutes. When that time is up, remove and dry it (use tweezers or gloves, please). You should find the outer laminate stretchy and pliable. Scrape off the layer of laminate from the back of the card — a butterknife should suffice, although this DCist irresponsibly used his fingernails. You'll end up with something like this:

Place the card back in the acetone and go to bed. It needs to soak for a bit. In the morning you should be able to remove it, dry it, and peel off the back layer of paper. If you're careful, you might be able to destroy less of the antenna than we did. Here's what the separated layers will look like:

At last, SmarTrip's mysteries revealed! Let's get down to it: the card is an RFID chip. You might have heard of RFID — Walmart is using the technology to revolutionize its inventory practices; others are concerned about its privacy implications. The chips work thanks to the photoelectric effect induction: when radio waves hit metal they produce an electrical current. Most radio devices amplify, read and respond to this signal using their own power sources. With RFID chips, the incoming signal is the power source. The chip uses it to transmit another, much weaker signal back to the RFID reader. The result is a sort of barcode that can be read from any direction within a range measured in inches or feet (depending on the specific RFID technology being used).

But the SmarTrip isn't just an RFID tag. It's also a Smart Card: an electronic identification card that performs calculations on its own. In SmarTrip's case, this means keeping track of the balance on your card — that's how the card can work on MetroBus. Because the balance is stored on the chip, there's no need for a network connection to a central database.

All of the action takes place on that little chip in the upper left (or upper right, when the card is viewed from the front). The chip should lift out easily. Here's a closer view, both from the front and back:

As mentioned, the rest of the card acts as an antenna — the card needs a pretty big one in order to gather enough charge for its return transmission. We estimated the antenna's length at around 40 inches. Say, forty inches of wire sounds like something we might have lying around the house...

A little soldering later, we ended up with this (slightly blurry) monstrosity. To be honest, it wasn't much smaller than the original card. But it still worked like a champ — its range might have even been better than the original card's.

Our subsequent experiments have been slightly less successful. We mounted the chip on a 40" loop of magnet wire, applied some heat-shrink tubing, and stuck the conveniently small loop that resulted on our keychain. It looked great. The only problem: it didn't work on about half of the SmarTrip machines we tested. More loops and a bigger gauge of magnet wire are on our to-do list. We'll let you know when we've got a winning recipe.

But for now, we're hoping that some like-minded geeks take up the gauntlet (let us know how it goes!). As for the rest of you: how about some inspiration? We know that the original card isn't that big, but it never hurts to remove an unnecessary card from your wallet. And we've seen too many ladies digging through their purses outside the Metro turnstiles for us to think that SmarTrip technology has reached its ideal form.