Angry wasp in a box
The Pi could almost certainly do all the work but the Arduino is a very robust way to connect to stuff that is 5V friendly, can source a passable amount of current and it's cheap.
It's quite tight in there and the Pi is overclocked running flat out so gets very hot. Hot enough I think to affect stability as the thing tends to hang, which similar code didn't cause before I crammed it all in there.
This evening I hot glued a small fan to the composite socket just above a CPU heatsink I fitted thinking it would help with this. It does a fantastic job but sounds like there's an angry wasp in there. A VERY angry VERY large wasp, so loud I could hear it downstairs. I was expecting it to be a little noisy but it's so horrendous I've just ripped it out again.
Obviously a larger fan would be less noisy but there's really not much space.