Be sure you add a diode in series with a + supply (a 1N4000-whatever). And it would be a good idea to put a Zener to ground (anything in the 20-30V range will do) after the 100 Ohm resistor. That'll help get rid of any noise spikes (which can be both positive and negative) from the bike's supply. Bikes are probably not as bad as cars, because they don't have lots of motors and solenoids for things like wipers and power windows/locks, but it's still not a "clean" electrical environment like your benchtop supply is.
Thanks for the explanation. I have read that the supply can be spiky but no sign of problems so far and i wasn't sure where to put the zener or what voltage it should be. There is certainly the starter and its solenoid that could cause spikes. In theory I think I should have zeners on the tach and neutral light lines as well as the power supply. I'll look at the diode and zener for the ps in the next layout. I've also thought about cannibalizing a cheap cigarette lighter ipod charger and just getting that away from my circuit altogether. The RBBB power supply has a diode in it by the way, where would that come in?
I'm working on two versions now codenames Brawny and Pikachu. - Brawny will be the rbbb version with a multi-function display and robust power supply. - Pikachu will be a mini version with basically just the atmega and a minimum of other bits. Pikachu's display will be something like a single LED or a bicycle speedo or a panel voltmeter which I think would be kind of fun - 1 to 5 volts for the different gears, 0 for neutral. I thought if I kept the current draw down to something really low I could get away with some really simple power arrangement - maybe even a coin cell.