Since the programming is the harder part, I'd recommend getting the ultrasonic sensor while you are at it and have it programmed at the same time. The part is really easy to install.
You can get the tilt sensor too, but the motion sensor seems more useful to me.
The ultrasonic motion sensor can usually be found on e-bay for about $20.
Last edited by Cazz; 06-03-2012 at 08:47 PM.