Sorry to bring up this old thread. I really like your idea, sound simple, is there any chance you can write up how to put all these parts together in dummies term please. Thanks in advanceI've been reading this forum for a good 3 years now and I think I finally have something interesting to contribute. I've designed a SDC I feel is simpler to build and pretty effective. I'm using it on my CarPC now. For power up, it simply relies on ACC/IGN voltage to determine if the car is turned on and toggles the PWR signal on the mobo. The power down part relies on the behavior of the ATX power supply.
Normally, an ATX supply keeps all the voltage rails off and has one 5V rail for standby. When the mobo pulls the PS_ON# line low, the PSU will turn on all the voltage rails (+12V, +5V, +3.3V, etc). When the mobo shuts down, it releases the PS_ON# line and the ATX turns all the voltage rails off again. I'm using this voltage rail off event to tell my controller that the the power can be switched off.
There are two relays and a small circuit involved in my SDC. One relay is controlled by the ACC/IGN lines, and turns on when the car starts. The relay provides power to the PSU and the 5V standby voltage comes online. My circuit detects this 5V and tells the CarPC to turn on. Once on, the +12V from the PSU turns of the second relay. Now both relays are on and the CarPC is operating.
When the ACC/IGN is turned off, the first relay turns off. But the PSU is still powered by the second relay. My circuit again detects the loss of ACC/IGN and signals the CarPC to turn off. While the CarPC is shutting down, the second relay is held closed by the +12V. Once the CarPC is done with shutdown, the PSU then turns off all the rails including the +12V. This basically releases the second relay and system power is cut.
The good thing is that I never have to guess how long it take for the CarPC to shutdown. The power will stay on as long as the shutdown process is happening. This brings up the question, what if the PC crashes and doesn't shutdown. I believe this is a system maintenance issue that should be resolved anyway. A well put together XP system running with EWF should not have problems with shutdown. EWF gives a layer of failure protection that should make a big difference.
And if I don't want the CarPC to shutdown while I fill gas, I can always keep the ACC on and it won't shutdown. I'm actually curious why there are so many timer based SDCs. The only timer I use is a 555 timer to time how long to pulse the mobo power btn.