Or you could even use a power socket with an NC switch built into one of the
contacts, as most fo them have.  Much easier, it's what the vast majority of
mains/battery appliances use.  (Although their long term reliability is not
terrific)


Mike Rigby-Jones
mrjones@nortelnetworks.com



> erik wrote:
> >
> > Hello,
> > I'm finishing up my first (?real?) PIC project. A chess clock. I'd like
> > to power it with either a 9V battery or a little wall transformer.
> >
> > I'm guessing there is an IC out there to switch between supplies. Or
> > maybe there is some other trivial way to do this.
> >
> > Any help would be much appreciated.
> >
> > Thanks,
> > Erik
>
>   One way would be to use diode "Or"-ing (use a power supply that has
> over 9V output - which would include most 9V wall warts!) - Common
> cathodes, an anode to +9V and +WallWartIn, and the system picks the
> higher voltage automagically.  Cheap, too.  (Do lose a diode drop's
> worth of power - an FET or BJT circuit might be better if switching
> between 2 battery sources.)  One project used this with a bridge
> rectifier they had scads of, as a reverse voltage protection/spike
> protector (connect the common anodes to ground, though Battery & Wall
> Wart then need to share grounds.)  "Think Devious" <G>
>
>   Mark
>