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Wearable

Open smart ring

A finger-worn health sensor. 2.4 grams. A 15 mAh battery. Everything fights for microwatts.

baseline: τ-Ring — open hardware, MIT firmware
mass 2.4 gPCB sub-1 mm flex, wrappedbattery 15 mAh Li-Poradio BLE 5.0 + 2.4 GHzfirmware MIT

The open equivalent of the litigious incumbent. We decompose a fully-documented open ring to its real BOM, then ask the only question that matters at the system level: what is already solved, what just refreshes on a clock, and what is worth designing.

5 of 7 parts are reuse — 2 are the slice worth designing. That ratio is the NRE bill. The descent below is how you find it before you pay it.

Exploded

Decompose the product into its stack. Each layer is colour-coded by what it is to the next build.

Titanium / resin shell
Optical window (PPG) Opportunity
Flexible PCB Common · stable
Sensor + AFE cluster Opportunity
nRF52840 MCU + BLE Common · cyclic
Curved Li-Po cell Common · stable

Bill of materials

stable ×3 cyclic ×2 Opportunity ×2

Every part is a real, sourced component from the open baseline — no invented part numbers.

nRF52840 Nordic Semiconductor Common · cyclic · refreshes each generation — reuse the current one
MCU + BLE radio — 64 MHz Cortex-M4F · BLE 5.0 · 1 MB flash · 256 kB RAM

The BLE SoC refreshes on a generation clock (nRF52 → nRF54). Reuse the current part; never design one.

https://www.nordicsemi.com/Products/nRF52840 →
GH3026 Goodix Opportunity · the slice openWafer designs
PPG / optical — 3-wavelength PPG · per-LED current drivers · 24-bit TIA ADC

A discrete optical analog front-end. The TIA + LED drive + ADC are exactly what an omni-modal biosensing chiplet absorbs.

https://www.goodix.com/en/product/sensors/health_sensor →
ICM-42688P TDK InvenSense Common · stable · reuse as-is — zero NRE
6-axis IMU — ±16 g accel · ±4000 dps gyro · 0.88 mA · 1 kHz ODR

Mature MEMS commodity. The proof mass can’t fold into CMOS — keep it discrete, reuse as-is.

https://invensense.tdk.com/products/motion-tracking/6-axis/icm-42688-p/ →
GXT310 digital thermistor Opportunity · the slice openWafer designs
Skin temperature — ±0.1 °C · 16-bit · sampled at 3 ring positions

A second discrete AFE for one slow scalar. Folds into the same biosensing front-end as the PPG.

GD55B01GFYIGR GigaDevice Common · cyclic · refreshes each generation — reuse the current one
Data storage — 128 MB serial flash

Flash density refreshes each generation. Reuse the current capacity point.

Li-156825 Constant Common · stable · reuse as-is — zero NRE
Battery — 15 mAh curved Li-Po cell

Commodity curved cell — the fixed energy budget. It’s precisely why moving inference on-die matters.

sub-1 mm flex PCB — Common · stable · reuse as-is — zero NRE
Substrate — flexible circuit wrapped to finger radius

Settled flex-PCB process. Reuse.

The slice openWafer designs

Three of these lines — the PPG optical AFE, the temperature sensor, and their analog conditioning — are separate ICs, each carrying its own front-end and its own idle current. openWafer’s slice is one omni-modal biosensing chiplet: a chopper-stabilised diff-amp + TIA + current-injection front-end that covers PPG, temperature, and bio-impedance on a single die. Fewer parts, lower idle power, and modalities the discrete stack can’t reach. Pair it with an always-on neuromorphic classifier on the same die and the ring stops streaming raw PPG to the phone — so a 15 mAh battery goes much further. That chiplet is the novel slice. Everything else in the BOM is reuse.

Open sources — decomposed and linked, not hosted