Hey there, this would be a device for coffee machines. But as the solenoid valves are quite versatile, tool can be used in many ways. I imagine that some reasonable protection from water/heat/physical movement should be there, but again, this is not a tool you would use in field, rather in controlled environment.
Agreed about ATTiny - the functions required are super simple - love the idea of "more universal" tool.
The digit topic - you are absolutely correct - I mis-labeled tenths of seconds as milliseconds. I believe 4 digits would be good then.
Original thought is that tool is super straight forward, so I do not intend to have any buttons. Maybe power switch on the main box would be good, just to keep it safe but thats about it.
Feel free to PM me if you feel you can take this project on