-- [ From: Quentin Lewis * EMC.Ver #3.0 ] --


> seems like a good method.  However, I wanted to add a battery backup
to the
> PIC, as this chip will normally be powered by a 9V DC power supply
that is
> connected to AC.  The 9V will go through a 5V fixed regulator before
the PIC of
> course.  My question is, what kind of circuit can I build that would
switch in
> or out a 9v battery as a backup?  I

How about just diode isolating each supply....that is, a diode from each
supply to the regulator....then when the battery voltage is higher then
the 9v AC supply, it supplies the regulator.

Ok....so you might want to ensure that the 9v AC supply is always going
to be higher than the battery, so that the battery only supplies when
the AC supply is out.

So two diodes on the battery....or a schotkly on the AC supply and a
silicon diode on the battery.

Just a thought,
-Quentin