Embedded Firmware Engineer (Nordic nRF5340 / Zephyr RTOS) for a Wearable BLE Sensor Device

About the project We're a venture-backed startup building a wearable that streams microphone audio and PPG (heart-rate/biometric) data over Bluetooth Low Energy to a mobile app. The firmware is functional and field-tested on a custom nRF5340 board. We're now hardening and cleaning it up ahead of a beta hardware release, and we're looking for an experienced embedded engineer to review the codebase, raise its quality, and keep building with us. What you'll do - Review the existing Zephyr / nRF Connect SDK codebase; assess structure and quality and propose a concrete refactor plan - Clean up and modularize the application (the main file has grown large — split into well-tested drivers/services) - Harden sensor-acquisition reliability: optical AFE FIFO handling, stall detection and automatic recovery - Strengthen the BLE streaming path and the store-and-forward flash buffering - Implement automatic gain control / calibration so the optical sensor works across different users and skin tones - Add BLE link security (pairing/encryption) and improve FOTA reliability + battery-state reporting - Establish CI, unit tests, and coding conventions for ongoing development Stack & hardware - Nordic nRF5340 (dual-core), Zephyr RTOS, nRF Connect SDK, sysbuild + MCUboot - MAXM86161 optical PPG AFE over I²C; PDM microphone; ADXL362 accelerometer; external NOR flash - BLE 5 (2M PHY, Data-Length Extension, custom GATT service); SMP / mcumgr-bt FOTA - West build system; RTT-over-SWD debugging Required - Proven Zephyr RTOS firmware on nRF53/nRF52 taken to production - Direct-register sensor driver work over I²C/SPI (bonus: MAXM86161 or similar PPG AFE) - BLE GATT design, throughput tuning, and SMP FOTA experience - Strong C, git discipline, and the judgment to refactor a live codebase without breaking it Nice to have - Biosignal/DSP familiarity (PPG, HRV) - Audio streaming over BLE (ADPCM) - LittleFS / store-and-forward, low-power optimization Engagement Start with a paid scoping + code-review milestone, then ongoing part-time. Remote, async-friendly. To apply: Share 1–2 relevant Zephyr/BLE wearable projects and, in a few sentences, how you'd approach reviewing and refactoring an existing embedded codebase.