Blinks button 0 in 1 second interval.
Simple 1-joystick Atari adapter for learning purposes.
Easy to modify for your own DIY-joystick (like arcade cabinet) which based on on/off-switches.
There is also keyboard version, which acts like USB keyboard.
For testing you don't have to have real joystick connected. You can just connect IO-pin to GND with piece of wire or something.
If you program Arduino sometimes to keyboard and sometimes to joystick, COM-port might change in Windows. Just select new COM-port and upload again.