Hi,

Just wanted to share my experience with sensing if a PC is turned off:

I have built many so-called powerdown modules for friends of mine. I use a circuit I developed which senses the +5V line on the USB port. When this line drops to 0V, the PC has shutdown.

Is this the same way that a CarMP3 unit detects when to shutdown the power to the motherboard? What if the PC "hangs" during shutdowm, does the unit stay on eventhough the car is off?