Up front: darkroom timers exist, they work, they're cheap. This post therefore has no economic justification. Onward.
I wanted a timer with the right red numbers — that safelight red that won't fog the paper — and buttons big enough to find in the dark with fixer on my fingers. On the market: either ugly or backlit white. I opened the parts drawer.
The easy part
Some microcontroller, a red seven-segment display salvaged from a dead clock radio, three industrial buttons bought by weight. The firmware is the usual countdown with presets: I wrote it in an evening, it worked in two.
The real part
Then three weekends fixing what a real product already solved: button bounce with fixer on my fingers, a buzzer too shy under the fan, the enclosure misprinted twice because the display wasn't square. The third enclosure is straight. Almost.
Now it sits under the enlarger doing its job: it counts, it beeps, it doesn't fog. Every so often, with the paper in the tray, I look at it with a fondness no thirty-euro object ever earned.