Actually, this is not the complete problem, but I think I can adapt this to my needs. I had a different post for another part of my problem (
Ramping Up DC Power), but have not received a complete answer yet--still don't know component values to use. After I wrote that post, I got the bright idea that if I held in the power button for several seconds (like on my notebook computer), I could also turn the system off with the same momentary push button.
I'll try to create the combined circuit to provide the power on/off functionality I need.