How do card scanners on buses work?

My uncle asked me this question the other day. He has a Senior Citizens’ free travel pass. It’s a card with, IIRC, a magnetic strip. He can now scan his pass without actually removing it from his wallet. He just leaves his wallet on the scanner. He was wondering how this works. This is in Northern Ireland/UK.

Is it a little RFID device?

Must be. That’s the only sort of thing which can work the way the OP describes. For the benefit of the OP, who may not know what RFID is, it’s an embedded microchip in the card which is powered by an externally-applied RF (radio frequency) signal and transmits whatever data it’s programmed with back to the reader. The power level is very low and the range is very short, so it can typically only work within a few inches from the reader.

The New York City subway is experimenting with RFID passes to augment the MetroCard system. The magstripes on MetroCards tend to get all cruddy and wear out pretty quickly, plus the readers themselves don’t always work. If you’re lucky enough to be a regular commuter on the Lexington line, you can get a single, permanent card with an RFID chip (as opposed to the flimsy MetroCards that barely last a month.)

Many office buildings here also have RFID key-cards which are used to open doors in the areas you’re allowed to go. My building has recently replaced these plain white cards with ones that have a photo-ID on the front. (They aren’t very good at it; when they took my photo they got nearly my entire upper body, leaving my head about the size of a dime on the picture.)

Yeah. For a few years they had the same thing in Burlington, ontario - now they apparently went back to paper tickets, (not sure about the monthly passes) for a while before the GTA-wide Presto system kicks in.

The burlington system was also deposit-based, you paid to have credit added to your chip, and then it was deducted when you rode. Monthly passes were done by approving the card for free after it had already spent X amount of credit in the past 31 days. Not a bad system.

Chicago rolled these out around 2003 or 2004, I believe. You can read more about the nuts and bolts of RFID on the Wikipedia page; bus cards are the “passive” type.

The weird thing (to me, as I know nothing about how these things work) about the Chicago cards is that storing them near your cellphone will, according to several bus drivers, render it inactive. This has happened to me a couple of times, when I forget and keep my wallet and phone in the same pocket. Each time, I panic as my fare won’t read, but the driver has (unofficially and against all rules, I’m sure) waved me through and assured me, “it’s okay, don’t keep it by your cellphone; it’ll work again tomorrow.” And, by gum, it does! How does it “work again tomorrow?” Is there some bus pass control wave sent out nightly to penetrate my wallet as I sleep, or does whatever interference is occurring wear off overnight?

Does the exchange of data work both ways? Oyster cards (as used on London Underground) are read in this way and each time you swipe it it deducts the appropriate fare from the cash balance on the card.

Is the cash balance actually stored in some kind of memory on the card and updated “on the go” via RFID, or is it stored on some central database somewhere? In other words, does the card transmit “I am card number 1234567 and I have £9.55 of available credit”, or does it just transmit “I am card number 1234567”, and the reader then looks up that card number to see how much credit it has?

My work ID card seems to work in a similar way. It can be loaded up with cash for use in the canteen etc, and although it does need to be inserted in a reader, it can be inserted in any of the four possible orientations (either end, and either way up) so I assume it isn’t storing data on a magnetic strip. (It also works “contactless” for access to the building, so clearly is RFID; I was just wondering if the cash system also works over RFID?)

More likely, the system closes out all “open” trips at 4 AM or some similar time, effectively resetting the card.

Is the chip hidden in the plastic because there’s no visible chip on these cards.

Yes, it’s embedded, as I said, in the card. Depending on how the card is designed, you might be able to see it if you hold it up to a bright light.

I don’t know the answer for any specific system, but I can tell you both methods are certainly possible with the technology. Central storage is more secure and reliable, generally, for obvious reasons.

No, we tried that. We couldn’t make anything out, hence the question. Thanks for your replies. That’s one more little mystery solved. :slight_smile:

I’d forgotten I covered these in a short course of study :smack:

Tried it with my work pass, I can see what seems to be a loop against a bright light, I think that might be it.

The loop is probably a little printed antenna. The actual RFID chip is small, about this size: .

Montreal and the other regional transit corporations have recently started using these Opus cards, with a visible chip on the front of the card.

The monthly pass simply causes a display on the bus/metro reader to say (in French) “Pass accepted” with a green light on, while the student and seniors pass display “Pass accepted - photo required” with an orange light on (I have yet to see a bus driver ask to see the photo!). If you simply load the card with single ride passes, it will say something like “Pass accepted - X tickets remaining” and, IIRC, a green light comes on. The bus boxes are pretty complex, because they can now read the Opus card, use the new magnetic strip single-ride tickets (which also serve as your 2-hour transfer voucher), read the old paper tickets (you’re given a magnetic strip transfer voucher) and take coins.