Many years ago, when the giant reptiles walked the Earth, another gentleman and I built (at the time) what was considered a high security system using mag cards. (I'm saying my memory may be a little fuzzy.)
Look at MagTek's website. They used to have a lot of white papers discussing the technical side of things.
You will need an o'scope. Those magnetic heads vary in output to an incredible extent. Your project will not work without a scope.
Find powdered iron. (hobby shop, High school or college chem lab or (what I originally did) iron filings and a mortar and pestle. Get a bottle with an eyedropper, put some iron powder in it and fill the bottle with acetone or high concentration isopropyl alcohol. Shake it up and put a few drops on the back of the mag stripe card. Now you can see the magnetic tracks or even the magnetic transitions. Now you can see what you are doing.
We used a dual (or maybe it was a quad) op-amp and 4 (or 6?) AA batteries. Battery + was, obviously analog power +. Battery ground was analog - (negative supply) and digital ground. "center tap" of the batteries was analog ground. After the mag stripe signal was recovered and amplified, the 4th op-amp "shifted" it from negative to ground.
Keep in mind that as people swipe the card the card will move at different speeds and the magnetic transitions will not be a uniform width on the card because people accelerate the card as it moves through the reader. Most of the systems did not have "clock transitions" encoded on them because there wasn't enough space. So, we wrote some initial transitions and would count clock cycles between "start" and "end," and with a known pulse with of 1 and 0 we could calculate an initial "real width" for the particular person/card. That way, even if the card accelerated or decelerated, as long as a particular pulse was within "X" number of counts with our initial calculated time, we knew if it was a 1 or 0.
We used one of the TLxxx series op amps. I don't really remember the specifics, but a lot of heads would "saturate" if the card moved too fast and the output waveform would "collapse."
Of course, MagTek used to sell devices (ICs) that had power supply, amp, "discriminator" and such all on board a 16 or 18 pin device.
Well - it took us about a month to learn all that. You are welcome.
There are a couple books floating around in PDF format on using smart cards and mag stripe cards. There is also a really informative (old) file called "hacking mag stripe cards" or something like that that has a lot of good information in it.
Before I forget, everyone has the great idea about halfway through this project of using a couple transistors and a couple zener diodes to try to drive the microcontroller with a (maxed out) magnetic head signal, then try to square it off with the zener diodes because it's cheap. Don't bother. Doesn't work.
here's the file: http://www.hackcanada.com/ice3/card/phrack37-6.txt