I got given a piece of electronics to take apart that has a magnetic swipe card reader built into it.

Upon taking it apart the card reader itself (which appears very much like a tape deck head) seems to have 5 pins, only three of which have wires attached in the current circuit (I'll call these 1 through 3 and will ignore the disconnected pins).

Pin 2 is obviously the ground, as it's connected to the outer case (this was also obvious from the circuit board it was attached to).

Resistance from pin 1 to pin 3 is about 65 ohms. 1-2 and 2-3 are OL.

I tried putting +5V (limited by a 1k ohm resistor) across 1 with 3 hooked up to my oscilloscope, but the voltage was a steady +5V with no change when I swiped a card.

Same results for +5V through 3 with 1 hooked up to the scope.

Oscilloscope does not show any voltage through ground (pin 2) when voltage is applied to 1 or 3.

So I'm sort of stuck.

I tried googling around, but so far I haven't really found much information on how the mag stripe readers actually work that gives me a better idea of how I might provoke a reaction from this thing.

Do any of you guys have any thoughts or suggestions?


Here's a decent summary of how the readers work: http://www.eettaiwan.com/ARTICLES/2001OCT/PDF/2001OCT04_AMD_CT_AN2112.PDF

  • \$\begingroup\$ After skimming through that document I am wondering what you would see if you applied no voltage to either pin. So, connect ground to ground and then use your Oscope to probe the pins. \$\endgroup\$ – Kellenjb May 21 '10 at 13:38
  • \$\begingroup\$ Yeah, that's a good idea. I bet the magnets induce a current in some sort of loop, which you are expected to feed to an amplifier/comparator. \$\endgroup\$ – pingswept May 21 '10 at 14:37
  • \$\begingroup\$ After drawing it out I think I have a better idea of what is going on. I bet pin 1 and pin 3 are connected to the inductor. The resistant that is being read is most likely the internal resistance of the reader head. Pin 2 is most likely just a shielding ground and not connected to the reader head itself. \$\endgroup\$ – Kellenjb May 21 '10 at 15:10
  • \$\begingroup\$ I thought the same thing, but doesn't 65 ohms seem high for an inductor? I'd expect less than 1 ohm at DC. Maybe the ohmmeter is doing something funny? \$\endgroup\$ – pingswept May 21 '10 at 16:17
  • \$\begingroup\$ I have seen inductors with significantly higher resistance. you can get very high inductance inductors by using very very thin wire with a very high resistance/foot plus a lot of coils. I have used uH inductors that break 500 Ohms because size and inductance were a large factor while the resistance was not. \$\endgroup\$ – Kortuk May 21 '10 at 21:03

@kellenjb and @pingswept:

Your comments were on the mark.

My eventual setup was to hook the probe up to pin 1 and the scope's ground to pin 3.

I found that this produced peaks/valleys of around 10-20mV with about 1-2 ms between "0" peaks (1s are half this width) depending on the card and speed of swipe. It's very obviously data in the format described in @pingswept's PDF.

I found setting the scope to trigger in single shot mode at around 7 mV did an excellent job of capturing the data.

Now all I need to do is transcribe and translate it.

Thanks guys.

As an additional note, not all hotel swipe cards I tried worked. Only one out of the three I had around produced any data. My AAA and Costco membership cards both worked, though.

  • 2
    \$\begingroup\$ Sweet! Might we see a blog post with the results? \$\endgroup\$ – pingswept May 22 '10 at 22:19
  • 1
    \$\begingroup\$ I second the request for a blog post! :) \$\endgroup\$ – Dave May 31 '11 at 13:20
  • \$\begingroup\$ I third this notion: really... do it! :P \$\endgroup\$ – Anonymous Penguin Aug 2 '13 at 23:38

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