XIAO06.08.26

Uno/Nano — basics: blink LED, button, potentiometer, temperature sensor, buzzer/haptic
2. Seeed XIAO nRF52840 (~$8) — same Arduino IDE, but adds BLE + low power + small size. Build the "smart pendant" here: BLE notify, haptic motor, coin-cell sleep mode
3. Custom PCB — when the prototype works, shrink it to a round board for production

1. Redesign around a module, not a dev board
Don't hand-solder the raw nRF52840 chip. Use a BLE module like the Raytac MDBT50Q (nRF52840 inside, antenna already tuned + certified). On your round PCB you then add only: the module, haptic motor + driver, LED, coin-cell/LiPo + charge chip (BQ25101), button, and Qi receive coil.
2. Draw the schematic + round layout in KiCad (free)
- Copy the circuit from your prototype's schematic (the module datasheet gives the reference design: power, crystal, decoupling caps)
- Draw the board shape as a circle in the Edge.Cuts layer (e.g. Ø20mm)
- Put components on one side, battery on the other (like the ECG necklace: front = electronics, back = battery)
- Keep the antenna zone clear — no copper, no ground plane, no components under/around the module's antenna area
3. Manufacture + assemble at JLCPCB / PCBWay
- They build the PCB and do SMT assembly — pick parts from their LCSC catalog (module, motor, chips) so there's no manual soldering
- Prototype cost: ~$2-5/board for qty 5-10, a few days lead time
- Use ENIG (gold) finish — better for contacts/coil
4. Test with your existing code
Same Arduino code runs on the module (Arduino nRF52 core). Flash via the SWD header you keep on the board.
5. Waterproof it
Then pot the populated board in clear epoxy, or drop it into a sealed metal cavity (O-ring + screw cap). The Qi coil can sit just under the resin surface.
Cheap trick to validate the layout first: design the round PCB around the XIAO module footprint so you can solder a XIAO onto the first round board and test the mechanical fit before committing to the MDBT50Q version.

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XIAO06.08.26

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