This is a speed climbing chronometer's schematic. You can find source code for attiny2313 here : https://github.com/Lezh1k/ClimbingChronometer . Here I'm using 2 type of buttons : "release" and "closing". "release" means that contacts are connected when button ISN'T PRESSED. And "closing" means that contacts are connected when button IS PRESSED. I don't use any common wire because of interferences. Also I use twisted pair for connecting buttons and lamps because usual climbing wall is about 18-20 m. For connection with PC you can use any USB->USART connector. I use CP2102. Good luck, have fun!