The Pine Time is a $27 smart watch that runs open source firmware. I’ve had one sitting in the back of a drawer for a year or two. When I saw @steveruizok’s Tweet about hacking on ESP32 devices with Claude, I found some inspiration to pull my PineTime out of the drawer and see what Claude might be capable of!

I knew I wanted to start by building a watch face because the default watch faces that come with the watch are pretty plain and boring. I’d also recently seen @levelsio’s Tweet about building a custom Casio watch face for is Apple Watch.

I wondered if I could build something similar for the PineTime (with a little help from Claude). As it turns out, the PineTime is a great watch for hacking with Claude!

• It’s cheap ($27).
• It runs open source firmware (InfiniTime, or any other RTOS you want).
• It’s very well-documented. Claude thrives on this.
• It has a great simulator (InfiniSim). That gives Claude a fast feedback loop on your computer.
• The firmware is simple, so you kind of need to add what you want.

Although I’ve been saying “Claude”, I actually did most of the work in OpenCode with some of my favorite open weights models. Kimi K3 & K2.6, and DeepSeek v4 Pro & Flash. I started by cloning the InfiniSim repo (which has a git submodule for InfiniTime, and asked Claude to get a build working. It was quick and easy on Ubuntu! Then I got pretty ambitious: I gave Claude the photo from @levelsio’s Tweet, and asked Claude to replicate the watch face, copying the code from an existing InfiniTime watch face as a starting point. And I asked it to orchestrate and use sub-agents as needed for well-scoped development tasks.

It built a pretty good rough approximation, but it was very rough. It seemed to just guess at text sizing and positioning, which led to things being sort of in the right spot but overlapping each other and unreadable. Still, it was a good starting point that we could iterate on. I think Fable would have probably been able to finish the work and get it pixel-perfect without supervision, if I gave it a feedback loop to screenshot the simulator. But since I was doing this with open weight models on OpenCode, on a limited budget, I opted to save some tokens by getting a little more involved myself, and gave it specific feedback with isolated tasks and concrete next steps. I fixed one thing at a time – getting each text element right before moving on to the next one.

I noticed we were spending a lot of effort trying to build some static parts of the screen, and realized that might not be necessary. I wondered if I could just make a fullscreen background image with all the static parts, so we only had to program the dynamic parts of the screen. We tried this, and it worked on the simulator! After installing the firmware on a real watch, I found that we were pushing the limits of what this little watch was capable of. It took about 10 minutes to transfer the 240x240 image over bluetooth to install it on the device, and it takes 1-2 seconds to refresh the whole screen when you swipe. The watch can’t hold this whole image in memory at once; it has to stream it from the file system. But that’s totally fine for a watch face I built in a couple hours! I might come back at some point to build more of the background in code, so it can update instantly. But for today, I’m happy with my working prototype!

I pushed all my code up to GitHub, and you’re welcome to use it directly, or use it as a starting point for your own project if you want. I also had Claude summarize the things we learned along the way into an AGENTS.md. That should help you get started quickly, and avoid some of the hurdles that I ran into, if you want to try this with your own watch!

I loved working on this project with an AI! The low-stakes environment (not production code at my day job) makes me feel like I can try whatever I want and iterate quickly, and it’s really rewarding to work on firmware for a physical device that I can hold and use when I’m done building it! I couple years ago when I bought this PineTime, I ran out of time and motivation to learn how to work in the InfiniTime codebase, and the watch sat in a drawer for a couple years. But today, I was able to build a new watch face in a few hours, and had fun doing it! The future is bright!