Quick overview

Step 1. Hardware time preset

Step 2. Start timer

Step 3. Timer is over

To stop timer I want to use the same button but hold it for 3 seconds.

No pause function needed

Power requirements

Ideally to make it work from tiny battery like CR2032

But we need to discuss and do some calculation. Maybe I will have to run it from 2-3 AAA batteries

Enclosure requirements

No enclosure or enclosure build from PCB

Size requirements

I want to have it somewhere 85 mm x 50mm