James Brown's Resin Smart Ring Put a Touch-Sensitive Raspberry Pi RP2040 on Your Finger

Maker James Brown has put together a smart ring wearable with display and capacitive touch input, using compact foldable PCBs and a 3D-printed mould for encapsulation.

"It started as a test PCB for a battery charger," Brown explains of his build, "but then I decided to try making a foldable PCB (without paying for rigid-flex.) There are wide copper traces across the folds on the back, and a split in the copper on the other layers which I hoped would cause it to crack neatly along the crease. Turned out not to work very well at all. But it boots."

That semi-flexible PCB design is split into five functional segments: a USB Type-C board, barely bigger than the connector it houses; capacitive sensors as an input device; a Raspberry Pi RP2040 microcontroller as the brains of the gadget; a power regulation and charging board; and a tiny battery.

Once folded — a process which Brown found easier by employing a laser to etch the seams prior to burning — the circuitry is placed into a 3D-printed jig for encapsulation. Resin is injected into the mold and allowed to set, fully encasing the electronics and allowing them to retain their curved ring form factor — with the USB port exposed for charging and data transfer.

"I do like the little 12×12mm [around 0.47×0.47 inches] modules it's made up from," Brown says of the project. "Part of me wants to make a whole bunch of those, with a defined bus of castellated edges. You'd stack them up vertically into tight little volumes of functionality.

Brown is well-versed in cramming tiny electronics into plastic. In June last year he unveiled a working version of the LEGO computer brick with tiny OLED display, a project which he later expanded to play a more-or-less recognizable version of Id Software's Doom, using the brick as an external display for a more powerful computer located elsewhere.

Doom, in fact, played a part in Brown's latest project too: the first attempt at assembling the device proved more challenging than expected, with the resin forming voids and breaking one of the traces. "I fixed it so it will at least charge," says Brown, "but I don't think I'll be able to reflash it [so] it's stick running a non-playable demo of Doom.

"The capacitive touch pad in the ring is just four copper pads each connected to a GPIO [General Purpose Input/Output pin], no external components," Brown adds of the ring's input device. "A PIO [Programmable Input/Output] program times how long the pad takes to pull high using the internal pull-up resistors."

More information on the project is available on Brown's Mastodon account.