I don't have anything yet, except what I want my program to successfully do. After thought a 3 segment LED display will be enough for my counting since I only want it to go to the hundreths place 0-999.
I do not have a RTC module yet. Any suggestions?
I am looking for them to be a bit larger than 1 inch, rather 2-3 inch.
Wouldn't it be simplier if the two counts were on seperate LED displays?
If you have the MAX7219 driving the displays, and you wanted larger 2-3" digits, then seperate common cathode displays is the way to go.You said you wanted time to hundreths, so MM:SS.TH + 99 would be one way to occupy 8 digits.
Easily.You can do it with SPI.transfer() commands to set up the MAX7219 and send it the 6 digits you want to display, or use some library to do it.This is a much larger version that uses a Promini for control and a MAX7221 (MAX7219 is identical but with faster signal edges) driving countdown timer display (and countup is easier) in the middle and 2 set of 00-99 digits at the upper sides.The '328P based promini does the timing, no external clock needed.I think your hardest part will be making it all watertight.Can make it rechargeable-battery powered with sealed connectors for the pushbutton for the lap counter and the external reset.Or go fancier and make the reset wireless, so the only connection is for the lap button.