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I recently had a magnetic stripe on my bank card fail and I was curious to find out the exact nature of the corruption which would require scanning the surface of the stripe and determining its magnetic characteristics.

I was wondering if there is any simple setup that would allow me to just use a probe and make a map of the magnetism of the strip? Note that I do have a cast iron X-Y table which has dials calibrated in thousandths of an inch. So, it is easy for me to locate the probe to that degree of precision.

I assume that there is way to do it, because I know in the old days there were these things called hard drive "microscopes" that had some kind of needle and you could literally read the bits off the disk platter, bit by bit, with the microscope. But, what I don't know is how easy it is to make the "needle" part of such an apparatus. Is this really simple, or would it be a complicated project?

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  • \$\begingroup\$ q-card.com/products/magnetic-developers/magnetic-developers/… \$\endgroup\$
    – jsotola
    Sep 18 '18 at 15:20
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    \$\begingroup\$ I'd say, buy a generic card reader and 'scope the signal from that. But @jsotola 's idea is intriguing. \$\endgroup\$
    – rdtsc
    Sep 18 '18 at 15:46
  • \$\begingroup\$ you can find ferrous particles in river sand or in lake sand (probably any sand) ..... just run a magnet through the sand to pick them up ..... you may find particles that are fine enough to sprinkle on the mag stripe \$\endgroup\$
    – jsotola
    Sep 18 '18 at 17:25
  • \$\begingroup\$ Possible duplicate of electronics.stackexchange.com/questions/94615/… \$\endgroup\$ Sep 18 '18 at 20:34
  • \$\begingroup\$ @jsotola you could just use printer toner powder. No need to go gold panning ;) \$\endgroup\$
    – Passerby
    Sep 19 '18 at 0:37
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The simplest diy magnetic probe is a cassette tape reader head, wired to a 3.5mm audio plug. A computer or even cell phone audio jack can then be used to see the waveform output of the mag card. You simply need to position the head at the right height to read each of the 3 tracks possible on a standard credit card.

Some of the first generation of cell phone audio jack card readers like square and PayPal used, were just that. Prior to it being found out and exploited, forcing them to add hardware to stop it. https://hackaday.com/2012/04/18/reading-credit-cards-with-a-tape-head/

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The simple version would be to get a card reader that allows you a "raw" view of the bits, and then look for those that fail checksum. Possibly over repeated swipes.

More complex approaches: well, a high-localisation magnetic detector is available in old hard disk heads. I've no idea how you'd use one and it would almost certainly require building your own analogue front end.

Hall sensors are capable of fine detection of the magnetic field but aren't particularly localised. You'd have to build a thing similar to a hard drive head with a U-shaped ferrite core with a tiny gap in it.

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