Hi.
I Assume you use a standart CARD READER.
there are four wires:
one for the clock
one for the data
one is GND
last I dont know, probably writing.
when you pass your card, you get clock, and data.
the clock appears proportional to how fast you pass the
card.
and the data is transfered together with the clock.
There are some standart protocols for cards, one of them
is: a long data pulse = '1'
a short data pulse = '0'
or something.
I hope its OK
Goodluck
OK