About
Felix HellerFreelance Mobile & Frontend Developer
Mobile Engineer · Frontend Engineer
Hi, I'm Felix — a freelance mobile and frontend developer. I shipped my first Android app while I was still in school, and more than a decade later I'm still hooked: my own apps have passed 2.5 million downloads, and I hold an M.Sc. in Business Informatics (GPA 1.35). Away from the keyboard you'll find me reading (books+coffee=), at concerts, at the cinema, or on my bike.
Got a project that could use an experienced pair of hands? Let's talk — I'm always up for a new challenge.
- Open to new projects
- Remote across Europe, on site in the Rhine-Main area
- Based in Darmstadt, Germany
Services
See If We Match
Programming Languages
- Kotlin
- Java
- TypeScript
- HTML
- CSS
- Dart
- GraphQL
- SQL
- JS
- PHP
- Python
Mobile Frameworks
- NativeScript
- Flutter
- ReactNative
- LibGDX
Frontend Frameworks
- Angular
- Vue.JS
- Laravel
- React.JS
Software
- AI Agent Development
- Git
- JIRA
- WordPress
- Docker
- UNIX
Principles
- Clean Code
- Software Design Patterns
- Scrum
- TDD
Works
Projects
A selection of things I've built — from personal apps with millions of downloads to client projects. Where an NDA applies, you'll find a work reference instead of a link to the final product.
AlcoTrack
Blood alcohol (BAC) calculator, consumption tracker, and drinking diary — all rolled into one app that helps its users keep an eye on their drinking habits.
- 500,000+ downloads
- Translated by its community into 17 languages
- 4.6 stars average rating on Google Play
- 49,000+ lines of Kotlin
DetoxDroid
Digital detox for Android: DetoxDroid strips away the attention-grabbing features of your smartphone — use your phone, rather than letting your phone use you.
- Fully open source
- 480+ stars on GitHub
- 3,000+ lines of code
CheatDroid
CheatDroid is a developer's toolbox for viewing, editing, and debugging the shared preferences and SQLite databases of Android apps.
- 2,000,000+ downloads on Google Play
- 9,000+ lines of code
M-Trust API Guard
Automated semantic versioning for Dart and Flutter packages: API Guard analyzes a package's public API, detects breaking changes, bumps the version, and writes the changelog — no more guessing whether a release is a patch or a major.
- Co-developed in a two-person team
- Open-sourced by EMD Group / Merck
- Static analysis of Dart API signatures
Own apps
SunMate
Sun protection built on the live UV index: SunMate turns the forecast into a personal burn time for your skin type — and reminds you to reapply when the real UV says so, not a fixed timer.
- UV index, burn time and vitamin D worked out on the device
- Home and lock screen widgets, UV alerts, morning briefing
- No account, no ads, no tracking
BMI Calculator
BMI, body fat and weight tracking for Android — with the metrics that go beyond BMI and real growth percentiles for children and teens.
- BRI, waist-to-height ratio and body fat on top of classic BMI
- CDC and Kromeyer-Hauschild growth references for kids
- No ads, no tracking, no account — everything stays on the device
Client work
Smart Caravan App
Companion app for GPS trackers and sensor systems that lets caravan owners keep an eye on their vehicle from anywhere.
- Maintenance and bugfixing of the legacy app (NativeScript + Vue.JS)
- Ground-up development of the new Flutter app
Felix helped us fix a broken app at very short notice. He works fast and carefully and got into the codebase quickly. At the same time he brought in improvements of his own and implemented them. We are very grateful for his work and can recommend Felix at any time.
Translated from German. The original wording is on the German version of this page.
Meeting Of Styles Website
Meeting of Styles is an international network of graffiti artists and supporters, which sponsors graffiti mural creation events in over sixteen countries. In 20 years, there have been around 400 events, featuring thousands of artists and attracting thousands of spectators.
- Re-design and development of the WordPress-based website
- Setup of the online shop
Tenant App
From-scratch development of a whitelabel app for housing-association tenants: report damage, retrieve documents — all in the association's own branding.
- Form UIs generated dynamically from backend questionnaires
- UI development and GraphQL API integration
- Component generalization for whitelabeling
Felix is an outstanding developer and was a pillar of my team during his time with us. He worked extremely independently and looked for solutions and possible improvements proactively. With his likeable manner, his helpfulness and his humour, Felix contributed a great deal to a collegial, dynamic and motivating working environment. His work was always on point and of high quality. I am very happy to recommend him!
Working with Felix was professional and a lot of fun. He works independently and takes on responsibility quickly. His eye for detail, many of his improvement suggestions and refactors improved the product and made our work easier. His fast and clean way of working let him get through his tasks quickly. I would work with you again any time and am happy to recommend you!
Translated from German. The original wording is on the German version of this page.
Inventory App
A lean Android app that speeds up warehouse logistics — built with a modern Jetpack Compose UI.
- QR code scanning for order picking
- Batch photo transfer to custom folders over corporate WiFi
You are genuinely impressive: your ability to get things done quickly and without fuss is simply great!
Translated from German. The original wording is on the German version of this page.
E-Book & Audiobook App
Continued development of an Android app for borrowing, reading, and listening to e-books and audiobooks.
- Co-development of the EPUB e-reader component
- Integration of app components with server APIs
- Refinement of the app's UI
RESQSHIP Website
RESQSHIP is an association committed to civilian sea rescue in the Mediterranean. Its website informs about the mission and helps collect donations.
- Volunteer work (pro bono)