I’m sure most people that have built a CarPC are already familiar with shutdown controllers, but I think this hack is pretty clever. Computers don’t like being turned off suddenly so you need to figure out how to power down the PC safely. This circuit has a serial connector and appears to XP as a generic UPS. When the ignition is turned off it breaks the 5V line to COM pin 8. XP responds by hibernating. Once the computer shuts down the relay for the power inverter opens. When the ignition is switched on the inverter powers up and the computer turns on.