AdaFruit pn532 NFC/RFID detecting multiple tags?

Question

I am working off an Arduino UNO with an AdaFruit pn532 NFC/RFID shield. The Goal is to have a shoe box, with a false bottom. Under that false bottom would be my prototype, which hopes to be able to tell every mifare tag (up to 6) that is in the box, above the false bottom.

I started with one shield, and had it detecting up to two tags with in range..

If i placed one tag it logged that one tag over and over again in the loop() of my sketch.

If I placed two tags above the shield it logged the two tags in an alternating pattern. ("tag1","tag2","tag1"....)

But when I placed three tags, it only logs the third tag.. This is essentially using the adaFruit mifare example.

I then set up the UNO with two shields and in the loop() checked both.. worked exactly the same. Once there were three tags, regardless of which pn352 they were placed on (2 on one, and 1 on another, or all three on one) it only logs one tag.

Has anyone tried to create anything that would detect up to 6 tags in range? If so could you share your discoveries?

New to Arduino..

thanks

Answer 1

The answer to your question leads into the technology of RFID. The reader emits radio waves at the operating frequency (usually 125kHz or 13.56MHz). When you bring an RFID tag to the reader - it accumulates the energy of the magnetic field of the reader and use this energy to transmits the ID at the same frequency back to the reader. The key point is that the RFID protocol does not provide for work with several tags at once.

So, if you bring 2 or more tags to the reader - they simultaneously start to generate RF signal, each with its own ID, thus "interrupting" each other. As a result, your reader gets garbage instead of the correct ID payload.