Open Source Developer

Charles Hartmann

I build Android apps, AI-powered tools, and automation platforms — all free, open-source, and published for anyone to use, study, or improve.

My work lives under Hartmann Studios on the Google Play Store. Whether it is a chess game, a Bible study companion, or a developer utility, every project I ship is designed to be genuinely useful and completely transparent about how it works.

View all projects GitHub profile

About me

Who I am

I am a self-taught developer with a focus on Android, AI integration, and open-source software. I got into programming because I wanted to build things that I actually wanted to use — apps that respect user privacy, run well on modest hardware, and do not hide behind paywalls or subscriptions.

Over the years I have shipped several Android applications, built automation scripts to streamline repetitive workflows, and experimented with self-hosted AI models to add intelligent features without sending user data to third-party cloud services. I enjoy the constraint of working with self-hosted models: it forces you to think carefully about latency, prompt design, and what AI can realistically do well.

When I am not coding, I am usually reading, playing chess, or looking for new problems worth solving. I believe the best software comes from developers who are also genuine users of what they build.

What I build

Areas of focus

My projects fall into a few categories. Each one reflects a genuine interest — I build what I want to exist, then open-source it so others can benefit or contribute.

📱

Android Apps

Native Kotlin apps for Android that prioritize speed, clarity, and a clean user experience without unnecessary permissions or tracking.

🤖

AI Integration

Applications that use self-hosted AI models for intelligent features — summaries, search, and suggestions — while keeping user data private and local.

⚙️

Automation

Python scripts and GitHub Actions workflows that automate repetitive tasks: content rotation, deployment, data fetching, and more.

🔓

Open Source

Everything I build is published on GitHub under open licenses. Other developers are welcome to study the code, fork it, or contribute improvements.

Featured project

📱

Dreamloom

Dreamloom: private on-device AI dream journal for Android

Kotlin

Website Source

Projects

Apps I have shipped

The following projects are available on GitHub and, where applicable, on the Google Play Store. Each one is free to download and open source.

♟️

Knightfall

Knightfall is a fully-featured chess application for Android. It offers standard chess gameplay with a clean, responsive board interface built in Kotlin. The app is designed for players of all skill levels — whether you want a casual game or a more serious match. Because it is open source, players who are curious about how the move validation or game logic works can read every line of code on GitHub.

Kotlin Android Chess

Source

📖

Path — Bible Study

Path is an Android app that combines traditional Bible reading with AI-powered study tools. It lets users read, search, and navigate Scripture, then uses a self-hosted AI model to generate passage summaries and study notes on demand. Because the AI runs on a server the user controls, none of the reading history or study data leaves the user's own infrastructure. Path is built with Kotlin for Android and connects to a lightweight backend designed to be easy to self-host.

Kotlin Android AI Self-hosted

Website Source

Skills & technologies

What I work with

I work primarily in the Android and Python ecosystems, with experience in web tooling and CI/CD automation. My preference is for straightforward technology that is easy to understand and maintain over time.

Languages

Kotlin
Java
Python
JavaScript
HTML & CSS

Platforms

Android SDK
Firebase
GitHub Actions
Google Play

Interests

Self-hosted AI
LLM prompt design
REST API design
Automation scripts
Open source licensing

Philosophy

Why open source

Every project I publish is open source for a simple reason: I believe software you use should be software you can understand. When an application is closed-source, you have to trust the developer blindly. When the code is public, you can verify what it actually does — how it stores data, what it sends over the network, and whether it respects your privacy.

Open source also means that if I stop maintaining a project, someone else can pick it up. The work does not disappear when I move on. Other developers can learn from it, adapt it for their own needs, or contribute improvements that benefit everyone.

I keep all my apps free to download because I want them to be as accessible as possible. If you find my work useful and want to support continued development, you can buy me a coffee — but there is no pressure. The apps will remain free regardless.

Find me