>I want to implement battery backed operation, wherein a PIC based device
>ordinarily runs from a 9v to 12v wall wart, and switches to a 6 volt
battery
>pack (AA alkalines) when the external power supply is unavailable.

If it is acceptable to only see when external power is plugged in, jacks
are available that close (or open?) contacts when the wall wart plug is
inserted.  This doesn't tell you anything about the "validity" of the
power, however...

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