HID lights have a major power consumption and lumen output advantage over halogen lights. Unfortunately the ballast component isn’t very cheap. [brian]’s been building his own ballasts for his mountain bike. We couldn’t find any more info on his design, but we did find some interesting stuff. This document lays out the requirements for a ballast. Here’s an older HID ballast design to get rolling. Keep in mind that the lamp requires high voltages – don’t play with it unless you know what you’re doing. If you’d rather use off the shelf components, check out the HID dive light we wrote up a while back. It uses a fairly inexpensive ballast/lamp combo.