FAQ: Do-It-Yourself Startup/Shutdown Controllers
A startup/shutdown controller (SDC) is a device that automatically starts your computer (usually when the car is started or the key is turned to the ignition or accessory position) or shuts it down when the car is turned off. This prevents you from having to switch the system on using a manual switch or relay and also from having to select "shutdown" from your Start or Apple menu.
There are several purpose built commercial startup/shutdown controllers (SDC's) available. The mp3car.com store carries 3 types (here) and there are others available as well. In addition, most commercial power supplies and regulators include startup and shutdown features in their packages.
The reason you need a shutdown controller is that your PC may hang on shutdown, even after receiving the command to shut down (or hibernate). If this occurs, the PC will continue to run and may drain your battery. Most SDC's will shut power off after a certain amount of time (say, 45 seconds) to prevent this from occuring.
Commercial SDC's have a variety of features that often allow the user to specify how the sequence and timing of the startup/shutdown commands occur and they range in price from around $40-$70 USD.
There are other alternatives, however. Several members have come up with solutions to build your own SDC. Here are links to some of those threads:
1. Home made SDC
2. Ghetto SDC
3. Schematic based on UPS monitoring.
4. Controlling your PC from a key fob
I'm way over my head here but how hard would it be to design a small circuit that when it senses power off, a small memory chip would send the Tsshutdn command via serial port (or USB?????) and if the Bios had the option, wake on ring?
Seems like a breadboard would be cheap enoug.
I know this might be simplistic and possibly covered before but I searched
on "Tsshutdn" and got no hits.