I had to order some cheap red LED strips from Amazon, and it's surprisingly hard to find plain and simple 5v ones. These arrived, and I added 2N3904 NPN transistor to drive them.
Coding it was fairly easy actually. I used the
blinkWithNoDelays() sketch as a starting point, and just ran through checking inputs and timers based on
A quirk is that you can't press down multiple timer buttons, or the timer start button, or enlarger button reliably because they are all in a voltage divider together. However, there's no need to press these buttons together, so that's not a problem.
One feature is that it saves the timers in the EEPROM, which means that they persist when the timer is switched off. Another feature is that I convert seconds to MM:SS time, which is more readable for minutes. There isn't a colon on my display to mark this, but I can live with that.
The whole code is in this gist: https://gist.github.com/MarcelloTheArcane/7be865507c2018faa79d0f57276a63ac.