Not the kind where you swipe a card with a magnetic strip into a reader.
I have two different entry cards (parking garage and office building) that you simply wave in front of a scanner, and it activates it.
I assume it’s something magnetic, and that neither one is shooting invisible rays at the other, but what is so special about this magnetism?
(BTW, these two different “scanners” are made by different people, and the cards are not interchangeable.)
Uninformed WAG:
It’s possible that the cards have active circuitry, little transponders that output a signal when activated by proximity to the scanner. So there may be invisible rays involved after all.
Let’s see. I lathered and I rinsed. But did I repeat?
pluto–right on the money. If Mjollnir were to cut apart his card, he’d discover that it has a small chip connected to a copper wire loop. The card is not magnetic, and is in fact totally inert when not near a reader. When the card comes within 1-4 inches, it reads the signal being tranmitted by the reader (I know that doesn’t make too much sense), then sends back a responding signal. Click! Door open.
Whoa! How long does the little bugger last? Is there a little battery in there, or can it somehow “live” off the magnetic signal sent out by the reader?
Livin’ on Tums, Vitamin E and Rogaine
IIRC, manhattan, there is no battery. It is activated by the signal, although I don’t think it’s magnetic. I believe it’s more like a radio signal.
I use the same kind of scanner at my workplace and we were told to keep the cards dry (they were given to us in plastic sleeves), so we had to punch holes in the sleeves to keep moisture from condensing on the cards.
I think the hint is in the copper wire loop that Guy described. Waving a conductor in a magnetic field induces a current.
My own contribution to this thread is that the signal will pass through materials such as denim or paper, or both. Of course you’re all probably thinking, “No kidding” right now.
But the thing is, I’ve noticed this because at the dorms at my college, the card sensors are located at such a height that you don’t need to actually take the card from your pocket for the sensor to read it. People just leave it in their pockets. It’s funny, watching people open the front door to the building with a pelvic thrust to the sensor 
“I’m just too much for human existence – I should be animated.”
–Wayne Knight
This got mentioned in a thread several months back - basically the electronics in the prox card (that’s what they’re called in the biz) has such low energy requirements that some sort of power(electric, IIRC) being emitted from the reader was enough to power the cards.
Anyone else remember this thread?
Daniel:
Actually, several guys I know keep them in their wallets and just hoist their butts up to the reader (especially if they are carrying a load of reports).
I never do that (short legs).
Tom~
Pluto’s WAG is right on target, at least for what I think the sort of proximity tag is being used. The reader supplies the power (via an RF signal) which the transponder (the cardkey) uses to transmit the transponder’s identity back to the reader.
The overly interested reader might check out the Transponder News site for more info on the technology behind this.
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That’s right; prox cards will work through cloth, paper, etc. Some of the more powerful prox card readers can work from several feet away(!).
There is no battery; the power source is in the reader.
BTW, Mjollnir, while there are some proprietory prox card systems, some cards can work on multiple systems. To do this, you would have to know the code associated with the card, then have it programmed into the computer that controls the access card system. Obviously, you’d need someone on the inside to help you, but it could be done.