Senior React Native Developer for a Bluetooth-connected fitness hardware app (iOS + Android)

ATLETICA is a fitness equipment brand with operations in Germany and the United States. Alongside our hardware business we run a strength training mobile app for our customers, live on iOS and Android with an active user base. We are now building the companion app for a smart strength-training machine. A working proof-of-concept already exists, so you would build on something that works rather than starting from scratch. The goal is to turn it into a polished, well-designed app that people enjoy using. The Bluetooth layer that talks to the device is already built and maintained by our hardware team, so you do not need to be a Bluetooth expert. What matters most to us is a clean, intuitive app and a strong user experience. What we need A senior React Native engineer who takes ownership of the app side and grows with the product over time. You will work directly with our product lead and align with our in-house designer on the look and feel. The heavy lifting (motor control, sensor calculations) runs on the machine, not in the app, so the app configures the training and shows the live values the device sends back. What you will be doing - Building a clean, well-designed app on top of the existing control functionality: connect to the device, set and adjust the weight (presets and increments), and show the live values it sends back - Device setup and pairing - User accounts and a small, lightweight backend for login and preferences, so a user gets their setup back when they switch phones - A basic progress view (think Garmin or Polar as a reference) - Turning wireframes from our designer into a smooth, good-looking app across iOS and Android You should have - Strong React Native, one codebase for iOS and Android - A solid UI/UX sense: you can take wireframes and make an app that looks and feels good, not just functional - Comfort building a small backend yourself (auth, a simple database, a cloud deployment) - Clean, maintainable code, and the willingness to grow with the product over time - Fluent English, written and verbal, since most communication will be in English Strong plus - At least one shipped app that talks to a real Bluetooth device - Experience with fitness, health, or training apps - TypeScript in a production React Native codebase How we work - Async-first communication, video calls when needed - Realistic estimates: you tell us what you don’t know - Documentation is part of the job, not an afterthought - Self-starter mentality: we will not micromanage; we expect you to take ownership, close loops, and flag blockers Engagement model - Hourly contract with a weekly hour cap, paid via Upwork - You get the existing source code and a starting spec; our hardware team can test your builds against the real device and answer any Bluetooth questions How to apply Please reply with: 1. An app you have built that talks to a real Bluetooth device in real time - how did you handle the live data stream (reconnection, dropped packets, edge cases)? 2. How do you build a first version so that larger features later (multiple training modes, device variants, analytics) can be added cleanly, without rebuilding everything? 3. A short example of taking wireframes and turning them into a smooth, good-looking app, and what made the difference. Skip cover-letter templates. Direct communication, grounded reasoning, push back when we’re wrong - that’s how we work. Looking forward to hearing from you.