This build was a bit of a washout overall, but since the boards are made up of distinct modules connected by wide traces it’s very easy to break it down and reuse bits. Here’s the usb segment & battery charger segment from one of the casualties charging a little Lego clip-on battery.

So, another go.

This one survived the process. It's even almost practical.

The capacitive touch pad in the ring is just 4 copper pads each connected to a gpio - no external components. A PIO program times how long the pad takes to pull high using the internal pull-up resistors.

This worked well. The capacitive sensor works by discharging the pad through a resistor and timing how long it takes. Previously I was doing this a few thousand times in a loop and returning the accumulated time. Here I'm doing it the other way round - spending a fixed time in the loop, and counting how many charge / discharge cycles it can do. My aim was to make it easier to tune the sample period, but it turns out noticeably smoother and less noisy this way too.