Voltage regulator with alternate voltage source...

[tony duell]

> > > 1) I have a 12 volt DC supply.  12 volts seems to be
> > >    within the VIN range for the 7805s whose data
> > >    sheets I've now read.  Can I simply apply 12 volts?
> >
> > Yes, but that regulator might get mighty hot! I would
> > not do this for fear of cooking the poor thing.
> 
> That's what I figured to start with, before reading the
> datasheets.

If as your photos (in another message) suggest it will run
OK from 8V with no heatsink on the 7805, then I would be
pretty sure it will be OK at 12V if you bolt a reasonable
heatsink to the regulator. Give it a smear of silicone
grease, of course.

-tony