Tracker PCB

Slim Sleep Tracker

A low-profile wrist wearable that measures motion, optical heart rate and SpO2 and streams them over Bluetooth LE to a PC visualizer.

3D renders from the board file: top side on the left, bottom (skin) side on the right. The optical sensor U1 has no 3D model and shows as bare pads.

What it is

The tracker is a 31.75 x 15.25 mm board worn on the inner wrist. The motion sensor, the optical heart-rate and SpO2 sensor and the skin temperature sensor sit on the bottom side, against the skin. The top side carries the nRF52840 microcontroller and radio, the USB-C charging port, the charger, two side buttons and a chip antenna. The firmware streams sensor packets over Bluetooth LE, and a Python visualizer on a PC shows orientation, the optical (PPG) signal, heart rate, SpO2 and battery level.

It is part of a sleep project and pairs with my Dual-Zone Bed Climate System, BedCooler. The link between the two is not part of this firmware.

This is not a medical device. The heart-rate and SpO2 values are experimental and have not been validated against a reference.

Block diagram as SVG. More views: one-page overview, angled render, top render, bottom render, signal map, functional layout, and the full schematic as PDF.

Key specifications

Values come from the design files (schematic, board and firmware), not from measurements.

Repository layout

hardware/          KiCad 9 project: schematic (4 sheets), routed board, project-local
                   symbol/footprint/3D-model library (libs/), board STEP model
hardware/production/  Gerbers and drill files (Tracker_PCB.zip), IPC netlist, BOM, placement
firmware/          Zephyr application for the nRF52840 (see firmware/README.md)
firmware/prototype/   earlier bring-up firmware and test scripts, kept for reference
tools/visualizer/  Python Bluetooth LE visualizer
docs/              schematic PDF, block diagram, firmware diagram and board renders

Hardware

Open hardware/Tracker_PCB.kicad_pro in KiCad 9. The parts library is inside the project (hardware/libs/easyeda2kicad), so no global library setup is needed. The 0402 resistors and capacitors use KiCad's own 3D models.

The schematic here is a redrawn copy of my original five-sheet design with the same parts, values, footprints and connections. SDA and SCL are drawn as two separate nets, as on the fabricated board; my last save of the original schematic had joined them into one net by mistake.

The board file was routed against that original schematic. Its copper matches the Gerbers, but SDA and SCL still carry one shared net name there (/MCU NRF52840/P0.26), and its footprints point at the original sheets. Before editing the board, run Tools > Update PCB from Schematic with "Re-link footprints to schematic symbols based on their reference designators" turned on.

Bus and pin assignments:

• I2C on P0.26 (SDA) and P0.27 (SCL), 4.7 kohm pull-ups: MAXM86161 at 0x62, MAX30208 at 0x50, BQ25120A at 0x6A.
• SPI to the LSM6DSV16X: SCK P0.29, MOSI P0.30, MISO P0.31, CS P0.28, INT1 P0.03.
• MAXM86161 LDO enable P1.03 and interrupt P1.10; charger disable P0.12 and interrupt P0.20; side buttons P0.02 and P0.17.
• Programming and debug through a Tag-Connect TC2030-NL footprint (J1): SWDIO, SWDCLK, reset (P0.18).

The Gerbers in hardware/production/Tracker_PCB.zip (2025-12-26) are the set the board was ordered from, and their copper matches the board file. bom.csv and positions.csv are in the JLCPCB format from the KiCad fabrication toolkit. The battery BT1 is a schematic symbol only and has no footprint.

Firmware

firmware/ is a Zephyr application that started from Nordic's Peripheral UART sample. Build and flash steps are in firmware/README.md.

• Bluetooth LE peripheral with the Nordic UART Service, bonding and one connection; advertises as nrf.
• Register-level I2C drivers for the MAXM86161 (FIFO, LED sequencing, green heart-rate mode, red and infrared SpO2 mode, 100 samples/s with 4x averaging) and the BQ25120A (charge settings, status, faults, battery level). The LSM6DSV16X runs through Zephyr's sensor driver on SPI.
• A 32-byte binary packet (sync bytes 0xAA 0x55): accelerometer and gyroscope, raw PPG, battery percent, charge status, timestamp, SpO2 estimate, signal quality, temperature, mode flags and a checksum. Heart rate is computed in the visualizer, not on the board.
• Text commands over the UART service: START, STOP, STATUS, HR_ON, HR_OFF, SPO2_ON, SPO2_OFF, TEMP, HELP, and the experimental IMU_MODE / HEALTH_MODE pair (a 100 Hz motion-only packet).
• Holding both side buttons puts the board to sleep.
• MCUboot with firmware updates over Bluetooth (MCUmgr); logging over Segger RTT.

The firmware diagram also shows the golf-swing tools I used with IMU_MODE; they are experimental and not included in this repository.

Visualizer

tools/visualizer/ble_visualizer.py connects to the tracker, sends HEALTH_MODE and START, and plots live data: 3D orientation (Madgwick filter), the PPG signal with heart-rate detection, SpO2 and battery level. On-screen buttons switch the heart-rate and SpO2 modes.

cd tools/visualizer
python -m venv .venv
.venv\Scripts\activate          (Linux or macOS: source .venv/bin/activate)
pip install -r requirements.txt
python ble_visualizer.py

It needs Python 3.9 or later and a Bluetooth LE adapter on the PC, and it looks for a device whose name contains nrf. ble_scan.py lists nearby Bluetooth LE devices if the tracker is not found.

Project status

This is the first board revision and the project is partly unfinished. The firmware reads the motion sensor's ID over SPI and finds the optical sensor on I2C. The skin temperature sensor is fitted but not yet supported in firmware; the temperature field in the packet is currently the optical module's die temperature. There is no battery-life, current or accuracy measurement yet.

Open items for the next revision

From a review of the rev 1 design files against the data sheets:

1. Skin temperature sensor U6: SDA and SCL appear swapped at pins 8 and 9 (from the data sheet pinout; not confirmed on the bench).
2. Board file: SDA and SCL carry one merged net name (see Hardware); update the board from the corrected schematic.
3. Charger settings: the firmware's register values give an 18 mA fast charge and a 1.75 V buck output by the data sheet, while the code comments say 100 mA and 1.9 V. Confirm the intended values.
4. Board checks: two small clearances (D1 ground pad to a D- via, 0.087 mm; TP1 to a ground pour, 0.061 mm) and silkscreen overlaps; the battery BT1 has no footprint.
5. The firmware builds for the nRF21540 development-kit target. A board definition for this board would keep the kit's front-end driver off P0.17 and P0.20 (not tested).

License

CC BY-NC-SA 4.0: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International. You may share and adapt this work for non-commercial purposes with attribution, under the same license. Firmware files that carry a Nordic Semiconductor copyright header (they started from Nordic's sample code) keep Nordic's 5-Clause license.