Embedded Hardware + Firmware Engineer — Wearable Posture Sensor (nRF52 BLE, IP68, miniaturized)

We are developing a wearable posture sensor — a small device worn around the neck (sensor unit sits at the nape). It measures the neck angle and gives a subtle vibration alert when the user slouches for too long. Connected via BLE to an iOS app, it produces a daily posture score (Whoop-style: the device logs offline and re-syncs). We already have a working proof-of-concept (off-the-shelf sensor + a first iOS app) and a 3D enclosure model. We now need a production-minded engineer to design the custom PCB AND the firmware to turn this into a compact, functional, IP68 prototype — built with later mass production and EU CE certification in mind. An enclosure STL (rectangular, approx. 10 x 5 x 55 mm, slightly curved) is available and will be shared with shortlisted candidates. SCOPE OF WORK / DELIVERABLES Schematic design (component selection + review). PCB layout for a small, curved enclosure (rigid-flex may be required). Firmware (see below). BOM with LCSC / JLCPCB references (we source via LCSC). Production-ready output: Gerbers + assembly files, ready for prototype manufacturing. Support during fabrication of the first prototype run (approx. 10 units). PRODUCT REQUIREMENTS Measures neck angle; vibration alert on sustained slouch (subtle haptic reminder). Must work FULLY AUTONOMOUSLY (vibration) even without BLE. BLE sync to an iOS app for a daily posture score; offline logging + re-sync. Auto-sleep when lying flat. Battery life target: 3-5 days. MECHANICAL Enclosure: rectangular, approx. 10 mm wide x 5 mm high x 55 mm long, slightly curved (follows the nape). STL provided. IP68, sealed — ideally no openings. Internal space approx. 8 x 3.3 mm, so a narrow battery is required. Eyelet on both ends (two-point attachment); a chain to the front carries a small tungsten counterweight. Because of the curvature a rigid-flex PCB may help, otherwise keep the rigid PCB in the straight section. HARDWARE (proposed — open to your recommendations) BLE MCU: nRF52 — we prefer a PRE-CERTIFIED MODULE (e.g. Raytac MDBT42Q) over a bare chip + antenna, to simplify FCC/CE and avoid RF tuning. Your input welcome. IMU: LIS2DW12 (3-axis, low power) — accel-only is enough for tilt/posture. Haptics: LRA + DRV2605L preferred (finer/efficient), or coin ERM. Onboard SPI flash (e.g. 16 Mbit) for offline logging. Fuel gauge: MAX17048. Hall sensor: DRV5032 (wake/clip detection; doubles as magnet input). Charging: clip / pogo-pin contacts — NO Qi wireless for v1. LiPo charger + protection. Battery: narrow LiPo pouch, max 8 mm wide, approx. 3 x 8 x 28 mm (50-90 mAh); pick an in-stock LCSC cell that fits. No buttons (input via IMU double-tap + Hall magnet). FIRMWARE (must be included in your quote) Own BLE GATT protocol (greenfield, no legacy). On-device posture loop: IMU interrupt-driven, low ODR, compute angle on-device, vibrate on sustained deviation; calibration ("stand straight") routine. Offline logging to flash + re-sync to the app. Sleep on lie-flat + activity detection to suppress false alerts. Battery / fuel-gauge reporting. OTA/DFU updates. Low-power design is critical (3-5 day target on a small cell). REQUIRED SKILLS / EXPERIENCE Nordic nRF52 BLE hardware + firmware (nRF Connect SDK / Zephyr or SoftDevice). Low-power wearable design, miniaturized / rigid-flex PCB layout. IMU integration and sensor-based algorithms (tilt/posture). IP6x sealed-enclosure electronics, battery + charging design. BOM sourcing via LCSC; DFM for JLCPCB/PCBWay. Plus: prior shipped wearable / consumer device; EMC/EMI best practice for later CE/FCC. PROJECT DETAILS First prototype run: approx. 10 units. Roadmap: beta (approx. 100 users), then Kickstarter, then CE. Please keep mass production + certification in mind from the start. Fixed price preferred, milestone-based. Please propose scope, milestones, price and timeline. SCREENING QUESTIONS Please share 1-2 examples of nRF52 / BLE wearables you have designed (PCB + firmware). Have you delivered IP67/IP68 sealed devices? Briefly, how did you handle charging + sealing? Do you recommend a pre-certified module or bare chip here, and why? Rough estimate of timeline and fixed price for PCB + firmware as scoped above? Have you worked with rigid-flex PCBs in tight curved enclosures? (An enclosure STL is available and will be shared with shortlisted candidates.)