Just got back from holidays with the parents and found my shiny new arduino waiting on the steps for me 
Here is what I'm building for the "is the car on" circuit
http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.html
(actually have a 7805 laying around from some other failed project, will that work smurf? you said 7905)
Thanks Kitty, I went to school with an electrical engineer, he told me the same thing