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.

My initial plan is to use diodes between each of the two power sources and
my 5v regulator.  I would then use a resistor, a 5v zener, and perhaps
decoupling capacitor so that one of the PICs input pins could monitor the
external power supply.  In the event that external power is lost, program
logic would invoke a low power operation mode (dont try to run relays, LEDs,
etc until external power returns).

Can I expect this scheme to work flawlessly, or did I miss something?

Incidentally, the application is a PIC based temperature monitor/heater
controller for my wellhouse.  I want it to keep the water temperature in the
hydropneumatic tank at 38 to 39 degrees, which is just below the temperature
that the water comes out of the ground.  I also want it to log the amount of
time during which power is down, and the minimum air and water temperature
during those incidents.

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu