RFID Card Detector

Not satisfied with the "griddipper" type detectors I built an RFID head that just sends out a REQA command to an ISO14443 tag and detects the returned response. This is the ATQA, the Answer To Query A. I'm not really interested in whatever answer the tag returns, only that it answers. Whether it's ATQA, ATQB, ATQZ I couldn't care less. The original idea was to verify the destruction of a RFID device. So far I haven't been successful in achieving this, apparently there is some excess energy protection built-in. The trick with the modified flash from a disposable camera hasn't been tried, I did try an industrial heater (which of course runs at 13.56 MHz) but that just melted the plastic. A microwave oven I didn't try either, I'm afraid it will produce similar results...

Back to the hardware. It turns out that a card will work on anywhere between 9 and 16 MHz (maybe even further) so getting a 13.56 MHz crystal is not necessary. I had a 12.00 MHz crystal in my junkbox so I used that one. As the detector is rather picky in its tuning I could not use a free-running oscillator. The oscillator feeds a divider that makes the clock for the PIC. The PIC outputs the REQA string which goes to a modulator and then to the driver of the detection coil. From the coil the envelope is detected and the 750 kHz load modulation signal is extracted and amplified. Finally it is detected. The PIC also provides a blanking signal that prevents the detector from detecting the transmitted signal.

The PIC is a straight-forward bitbanger. It just sets and resets the OOK (On/Off Keying) port, banging out the REQA bits, waits awhile, then starts anew. Really boring. It's all it does. And the BLANK port. That too.

The scope shows how the ATQA signal looks. The problem with a reliable reading of the card is that the simple circuitry doesn't include any gain control, so when the card is held close to the reader the detector saturates thereby destroying the recovery of the actual data. As a detection device however it is perfectly usable.


















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Last Update: 1 December 2007

This software is licensed under the CC-GNU GPL.