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Original question:

NFC loop using PCB

TRF7960A: How to pack multiplexed NFC readers closer together.

Kevin Hoos
Prodigy 10 points
Part Number: TRF7960A
Other Parts Discussed in Thread: TRF7970A

I'm looking for some help designing a custom electronic hexagon game board. The game is fully developed, but I'm still prototyping the electronics portion. Each hexagon cell side is 1 inch. The board is also a hexagon, with 11 cells per side. There are a total of 331 cells. 18 of which are unused. So that means there are 313 usable cells. The dividing lines between cells is about 1/16 inch wide. I'm using https://smile.amazon.com/gp/product/B07MKWHYQK/  (ntag215) to identify the game pieces. There are 36 kinds of pieces.

I liked the article http://www.ti.com/lit/an/sloa167/sloa167.pdf to describe how to multiplex out to 16, but obviously I need to go a lot further. For ease of construction, I'd like to use a pcb trace for my "antennas" (Yes, I know this isn't the proper term) as described in http://www.ti.com/lit/an/sloa135a/sloa135a.pdf. I did read over the http://www.ti.com/lit/an/sloa231/sloa231.pdf report that suggests using the TRF7970A reader.

I know there will be a trade off in speed verses cost if I only use one reader chip. My thought was design the game board in slices of 52 cells per circuit board, with one reader per slice. One circuit board would have 53 cells, to include the center cell of game board. 

Part of the challenge is that I need a LED in the middle of each cell. The board will be covered with a sheet of acrylic (1/8 - 1/4 inch... not sure yet). I would like the antennas to be hexagon in shape. I'm only looking for a read distance of around 1/2 in. My bigger concern is not getting readings from other nearby cells. Could I use a 2 sided PCB? one side for the antennas, and the other side for all the antenna switching & LED control? If I can't use a double sided PCB due to interference problems, how much separation do I need?

Any guidance would be appreciated. 

  • 0
    TI__Guru*** 131560 points

    Hello Kevin,

    We are primarily device experts and while we have a lot of expertise about RF in general, this sort of specific system implementation goes beyond what we have knowledge for.

    The descriptions here are of a fair compact game board, and you are trying to do very precise measurements of the pieces based on this. HF RFID is not well suited to make such accurate measurements. The antenna design portion is going to be difficult and we've never done hexagonal antennas. I would recommend squares or circles personally. I don't know how to model a hexagonal antenna to get the approx inductance so you could be able to tune it.

    Keep in mind every antenna will need tuning.

    PCB trace antennas will be fine as long as routed well. The challenge is that it will be very hard to keep antenna paths from crossing, so you need to be careful about which antennas are active and how they are impacted by adjacent antenna paths.

    Read range of 1/2 an inch is no problem. If anything, you will have too much read range. The solution presented will have issues if the tag is placed between two cells. Both will register it, and there is no way around that with NTAG215. The RF field pattern will detect it as it will be close enough to both.

    One solution would be to use a smaller tag. For example, http://www.ti.com/product/RF37S114

    That tag would be harder to get multiple readings on, it is more likely to require being better centered on the antenna itself.

    You can make an antenna that is small, 15mmx15mm, and still read the tag. This would help keep the antennas further apart too. See this app note for details on read ranges: http://www.ti.com/lit/an/scba035a/scba035a.pdf

    With that setup, you may have more success and then the challenge will be the routing of all the antennas.

    Hopefully this gives you some level of insight, there is not much else we can offer for such an abstract application due to lack of experience with such an implementation. It's a cool idea though, and I hope you are able to make it work!