Wrist Health Tracker

A wrist-worn health tracker for scoring sleep quality.

• What it measures: a Nordic nRF52840 BLE module reads heart rate, heart-rate variability, blood oxygen (SpO₂), steps and skin temperature from three sensors on one I²C bus.
• What it does with the data: it logs everything to on-board flash and syncs in batches over Bluetooth Low Energy to a home server.
• What the server does: the analysis — HRV statistics, sleep/wake and sleep-stage estimation, temperature trends and desaturation events.

Hardware

Top (skin side)	Bottom (battery side)

The circuit has six blocks:

• Dock and charging: four pogo landing pads (VBUS, D−, D+, GND), USBLC6 ESD protection, and an MCP73831 Li-Po charger at 83 mA with a red charge LED.
• 1.8 V power: a TPS62840 buck with 60 nA quiescent current runs the whole system. A 1 M / 470 k divider feeds the battery voltage to the ADC.
• MCU and radio: an Ebyte E73-2G4M08S1C (nRF52840) with a ceramic antenna at the board edge, under a copper keep-out on every layer. Plus a 32.768 kHz crystal.
• Sensors:
  • a MAX30101 optical sensor (green, red and IR LEDs);
  • a BMI270 IMU with an on-chip step counter;
  • a TMP117 ±0.1 °C skin-temperature sensor.
• Data log: a W25Q64JW, 8 MB of 1.8 V QSPI flash.
• User interface: a side-push button and a white status LED.

Layout.

• In1 is a solid ground plane, and In2 is the +1V8 plane.
• Signals run on the outer layers, each with a ground pour.
• Buck converter: its input loop is 0.31 mm².
• Thermal and current: a four-layer current and thermal solve puts the peak at 27 A/mm². The hottest part is the charger, with a 20.5 °C rise while charging.
• TMP117: it sits in a no-pour area, so it reads the skin rather than the board.

Further reading:

• docs/design.md: the power design, the full MCU pin map, the stack-up, the analysis results and the bring-up order.
• hardware/schematic.pdf: the schematic.
• docs/routing.svg: a plot of the routed layers.

Enclosure and dock

The case is a 3D-printed tub with a skin-side back plate. The stack, from the wrist outward, is: back plate, board (parts facing the skin), cell, case top.

Optics.

• Two 3 mm acrylic light pipes carry the PPG light. They sit in an IR-opaque holder with a barrier between the LEDs and the photodiode.
• The holder is part of the back plate, on a boss 0.65 mm proud of the skin face.
• An aluminium post conducts skin temperature to the TMP117.

Assembly.

• Four M1.2 corner screws close the case.
• The board is located by crush ribs against datum pads.
• The side button is a printed cap over the TS24CA switch.

Dock.

• The device drops into a recess in the cradle, which has a channel so the band can stay on.
• A tongue on the cradle enters a well in the back plate and carries four pogo pins onto the dock pads. Each pin compresses 0.6 mm.
• Two pairs of magnets, with opposite poles, pull the device down and let it seat only one way round.
• Grooves in the cradle carry the charge LED's light out beside the device.
• A 100 µF electrolytic across the dock's VBUS and GND pins damps the hot-plug ring.

Fit check. mechanical/check_fit.py checks every printed and cut part against the board's 3D model. It covers:

• interference;
• the board's insertion path;
• the button travel;
• a maximum-size cell;
• the battery lead route;
• the rework pry slots;
• the wall thickness round each screw;
• the device seated on the dock, including pogo alignment against the pad positions in the .kicad_pcb;
• the LED windows and the magnet pockets.

The current design is CLEAN. mechanical/README.md has the parts list, the materials and the build order.

Skin side	Dock

Verification

Ordering

Board (JLCPCB).

• Upload hardware/gerbers_jlcpcb.zip, with these settings:
  • 4 layers, 0.8 mm thick, ENIG finish;
  • minimum track and space 0.09 mm;
  • minimum via hole 0.2 mm, tented.
• For assembly, use standard PCBA, top side only, with hardware/bom_jlcpcb.csv and hardware/cpl_jlcpcb.csv.
• Every BOM line has an LCSC part number.
• Parts cost about $58 for two boards, plus a $30 loading fee for the extended parts.

Enclosure.

• The body and back plate are printed in MJF PA12 nylon; the back plate must be black.
• The button cap is printed in SLA resin.
• The light pipes are laser-cut from 3 mm clear cast acrylic.
• The cradle can be printed in any light-coloured filament.

Firmware notes

• All I/O is 1.8 V. A debug probe must support 1.8 V targets, such as an nRF52840 DK or STLINK-V3MINIE.
• Programming is over the SWD pads on the battery side, then over USB through the dock.
• The PPG is disabled below a battery voltage of about 3.4 V, because the MAX30101's LED supply comes straight from the battery.
• The full pin map and the I²C addresses are in docs/design.md.

Repository

README.md
docs/
  design.md                hardware design: power, pin map, stack-up, analysis, ordering, bring-up
  board-top.png, board-bottom.png, routing.svg, hero.png
hardware/
  schematic.pdf
  health-tracker.step      board 3D model (KiCad export, origin at the board's top-left corner)
  health-tracker.glb
  gerbers_jlcpcb.zip, bom_jlcpcb.csv, cpl_jlcpcb.csv
kicad/
  health-tracker.kicad_pro / .kicad_sch / .kicad_pcb
  lib/                     project symbols, footprints and 3D models
mechanical/
  README.md                enclosure and dock: design, parts list, build order
  enclosure.py             parametric CadQuery model of every printed part and the dock
  check_fit.py             interference, build-path and dock checks against the board's 3D model
  assembly.py, render.py   full assembly (GLB) and renders
  out/                     STEP and STL for each part, full_assembly.glb, renders

adom-wiki repo clone tylerk/wrist-health-tracker