Soundcard-RFID - Code for decoding 125KHZ RFID HID cards using your soundcard.

Copyright(c) 2011 by Christopher Abad

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You will need the following things:

   A function generator that can do 115khz sinewave output.
   A soundcard that can sample at 125khz 16bit mono.
   Audacity or something to record with.

And the following schematic:

                        L1        C1
 115khz SINEWAVE >-----nnnn---+---||-----| GND
                              |
                              +----------| LINE-IN

   C1 - 22pf capacitor
   L1 - 16uH inductor

It may make most sense to wind the inductor yourself,
and it seems to be extremely forgiving. I just wound some
magnetic wire around a rectangular box about the size of
a credit card about 25 times.

This schematic is very minimal and may be prone to issues
but generally the LC resonator circuit should be tuned to
about 115khz or so as I had better success with a slightly
slower signal to the inductor and sample slightly above
this at the soundcard.

Now you will have to sort of adjust the function generator
gain a lil at first just so you can get a decent sampling
at the soundcard. The next thing you may notice is that
the soundcard may have a relatively lower RMS than you expect
but were not concerned about absolute power.

It helps immensely to hook the output to an oscilloscope
first to see if you see something that seems like it
looks like it might be data when you bring an rfid card
near to the inductor. If you see this magical action
you are most likely in business.

Also, visually you may not be able to see the data in
its analog form very easily if at all because it relies on
a very small variance in the FSK frequencies.

Place your RFID tag over the inductor that you made.
From here just record the data with audacity at 125khz if
you can, save it out to a 16-bit mono WAV file and then
run it through process.sh after building the few programs.

Now you should find a 16-bit 5 digit number on the back
of your card and this should show up in the decoded binary.