Thoughts on building an NFC reader for the Framework laptop

The Framework laptop has several little slots which can be used be used to expand the functionality of the laptop. They convert the internal USB-C ports into a different sort of port.

For example, Framework sells an official HDMI card and Ethernet card:

But the community have built their own SD card expansion pack as well as 3D printed snack drawers:

Brilliant!

I have an NFC-only authentication token which I want to use when I log in with my laptop. I currently have a USB-C NFC reader which I could use, but it is a bit big and bulky.

Is it possible to fit an U2F capable NFC reader into a FrameWork expansion slot?

Size of expansion cards

This was a bit tricky to find. Based on this GitHub page, I think the maximum internal dimensions of a circuit board are about 26mm x 29.7mm. Tight!

According to this video from Framework about the cards shows the maximum size of the physical cards.

All the cards use a USB-C connector which sits proud of the card itself.

Potential Hardware

First up is this tiny NFC reader. It is designed for phones, and costs about a fiver. The dimensions look like it would might fit if the plastic shell were removed. But the description says:

Output 4/7 bytes of chip serial number, UID only

So no good for doing U2F. But it does at least show that reading an NFC card ought to be possible in a small form factor.

There are component USB-C readers

That one would easily fit - but the description says it is for reading only. So I don't think it can send the cryptographic challenges necessary.

Then there's the ACM1252U-Z2 - Small NFC Reader Module which costs about £60.

It is a much more capable reader. It uses micro-USB rather than USB-C, which isn't insurmountable using a flexible male-male cable. What about the dimensions?

Certainly thin enough! But about twice as long as we need. Could we chop off the end? What's on the other side of that empty space?

Ah! A honking great antenna. If it could be bent at 90°, it would "only" stick up 2.5cm. Which would be ugly, but not disastrous.

Then there is the ACR122T which is in thumb-drive format.

That might work. The antenna is probably in the bulbous end. The official schematics don't give much clue.

But if we look in the FCC Database there are some internal photos!

Ignoring the USB jack, the board is about 50mm long. That would leave about 2cm sticking out of the side of the laptop. Which isn't too bad. The width is within what we need. There's even an updated version with a slightly different spec.

I could buy an A-to-C converter which would add unwanted extra length. I wonder how hard it is to unsolder the connector and mount a USB-C one?

This video makes it look relatively simple.

But, given the weirdness with resistors, it might be easier to use an OTG Shim.

I could rip that apart and solder it on to the board. Or perhaps just solder on a USB-C OTG board. Maybe?

I guess it is time to buy some components and try not to break them! If you have any advice or suggestions, please let me know!