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  • Author Author: Fred27
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Combined NFC reader / keypad

Fred27

So, the board is back from the fab. It looks really nice - especially as it only cost $1 (+$8 shipping) for 3 from Seeed. It's been pasted, populated and toasted and ready for test. Well - it would be ready for test if it wasn't missing just one component - an out-of-stock 32.768kHz crystal.

image

The board will eventually be the way into my house. There's a small NFC antenna that I'm hoping will be tuned well enough for the tiny NFC implant in my hand. There's a capacitive touch keypad (hence the MSP430FR2633 microcontroller) because even I don't think getting my kids chipped is a great idea.

 

Unfortunately without the missing crystal it will be a pain to get the capacitive touch or the SPI comms with the TRF7970A NFC transceiver working. I'll wait for it and work on other stuff for now.

 

I can confirm that the reverse mount RGB LED that shines through the circular gap in the soldermask seems to work really well though. The FR4 diffuses it nicely. Blinky is always a good start to prove the MSP430 is running happily. I'll have to wait to see how my QFN soldering went. It looks borderline under the microscope. This time I decided to solder down the thermal pad first and check alignment and then use some paste and a hot air gun to do the leads. That should minimise the chance of any shorts under there that I can't see. At least I hope it does - I've never tried it that way before.

  • Thanks for the advice about bodging in another crystal. I did that, adapted some basic NFC reading code and what do you know - it works! image Despite my questionable QFN soldering and guesswork at antenna tuning it actually reads my implanted NFC tag. I'm still always surprised when something works. I think the range / sensitivity could be better but I'm happy for now. I'll leave the capacitive touch keypad for another day.

  • in reply to BigG

    The design is based on one that I did previously using a wirewound inductor as an antenna.

    https://0xfred.wordpress.com/2016/03/04/nfc-login-2-1/

     

    I don't have the equipment (or the skills if I'm honest) for tuning an antenna so the PCB coil should be about 1uH to match tuning circuit values from other commercial designs. I've also knocked up another PCB with various attempts at trial-and-error antenna design. Nobody else seems to have done any work trying to tune a PCB antenna to an implanted NFC tag. The small size and the fact that the centre line of the coil will be aligned parallel to the board make it difficult to couple.

  • Hi Fred,

     

    It's looking good. Looking forward to more updates.

     

    Luis

  • in reply to Fred27

    It looks pretty good from the photo : ) there are no bulging blobs on every resistor like I often have! (I've solved my problem recently by getting thinner solder).

  • I love the inclusion of the NFC antenna.

     

    Did you refer to any design guides to make this. What trace width did you use, for example.

     

    I'm very much a noob here, so was wondering how did you "tune" that antenna, as in choose the correct resistor & capacitance values for the antenna.