Hacking with Claude on a $27 Smart Watch

By Mike Kasberg
Husband, Father, and Staff Software Engineer at Strava

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⌚ The Project

The PineTime is an affordable, $27 smart watch designed for those who love open-source firmware. For a couple of years, mine had been gathering dust in a drawer. However, after seeing a tweet from @steveruizok regarding the use of Claude to hack ESP32 devices, I felt a surge of inspiration to dust it off and test the limits of AI-driven development.

"if you know what a coding agent is then go buy this or something very similar..." — Steve Ruiz

The Goal: A Custom Aesthetic

I found the stock watch faces to be adequate boring. After seeing @levelsio create a custom Casio-style face for the Apple Watch, I wondered: Could Claude help me replicate that classic look on the PineTime?

🛠️ The Technical Approach

While I refer to the AI as "Claude," the bulk of the heavy lifting happened within OpenCode using various open-weight models.

The Workflow

The process followed a specific sequence of events:

Development Steps

• Clone the InfiniSim repository (including the InfiniTime submodule).
• Establish a working build process via Claude.
• Feed the AI a reference photo of the Casio face.
• Use existing watch face code as a template.
• Orchestrate sub-agents for specific development tasks.

📉 Iteration & Refinement

The first attempt was... rough. The AI struggled with spatial awareness, leading to a layout where text elements overlapped and were largely illegible.

To save tokens and time, I stopped relying on the AI for every pixel. Instead of having the AI code every static line and border, I decided to create a fullscreen background image. This meant the code only had to handle the dynamic elements (like the time).

⚡ Hardware Constraints

Once I moved from the simulator to the actual device, I hit the physical limits of the hardware.

1. Transfer Speed: Sending the $240 \times 240$ pixel image over Bluetooth took approximately 10 minutes.
2. Performance: Swiping to refresh the screen takes about $1 \text{ to } 2$ seconds.
3. Memory: The watch cannot load the entire image into RAM; it must stream it directly from the file system.

// Conceptual logic for the watch face
void updateWatchFace() {
    drawBackgroundImage(); // Streamed from filesystem
    drawDynamicTime();     // Updated via code
    drawBatteryLevel();    // Updated via code
}

🏁 Final Thoughts

I've uploaded the completed code to GitHub for anyone who wants to use it or build upon it. I also had Claude generate an AGENTS.md file to summarize the lessons learned, which should help others avoid the pitfalls I encountered.

Working in a "low-stakes" environment—away from my professional production code at Strava—was incredibly refreshing. It allowed me to iterate rapidly and experience the tangible reward of holding a physical device that I programmed.

Years ago, I lacked the motivation to dive into the InfiniTime codebase. Today, thanks to AI, I built a custom face in just a few hours.

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