I'm building (yet another) stopwatch as my first Arduino project. I do a lot of public speaking and have always wanted to build my own countdown timer that has different colored LEDs as "warning" indicators (10% of time left, 5% of time left, etc).
The software won't be hard - I'm a programmer by trade and have already prototyped the software on my Diecimila board. I'll need to interface it with a four digit 7-segment display, but I think that will also be fairly straight-forward. I'm planning on prototyping it with the Diecimila board and breadboard, then taking the 168 out and installing it on a pre-printed PCB to put in my Altoids tin.
The big question I have is: how do I power the thing? The tin won't allow enough room for three AA batteries (and the display, board, etc). A watch battery would be perfect, but I don't know if there is one that will supply enough power, etc. I can install a cutoff switch so that it powers down when the tin is closed to save battery. But I'd like it to be able to use it for at least 20-30 lectures each varying 15-45 minutes without replacing the battery.
I've seen several posts here that suggest LiPo batteries - but they also all have warning about blowing up my house if I do something wrong. I'm too new to take that risk :-X