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My father recently brought home an old identification card, and after cracking it open I found that it consisted of simply a large coil of wire, connected to a small electronic chip (which was so small that I sadly lost it - it looked something like this but a lot smaller. Incidentally, it was so small that when I was cracking open the ID tag it actually got a bit damaged).

here

It works as an identification card, in that when you swipe it close to a detector, it registers your identity. I could (and have) made some guesses as to how it works (probably involving a magnetic field?) but many times my guesses turn out to be wrong, and I can't seem to be able to find a good physical explanation online as to how these identification cards function, and how they store your identity.

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The coil connected to a chip is basically how all passive RFID cards are constructed.

The coil (together with some small capacitance on the chip) is tuned to the frequency of the reader. When the coil is excited by the reader, it powers the chip. The chip then signals it's ID by repeatedly shorting and thus detuning the coil, which changes the load at the reader end.

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