>    Developing the idea I'd suggest not "backup" but rather
> "on-line" idea with two DC/DC converters.
>    First DC/DC converter - Step-up DC/DC converter from
> batteries' 3V to PIC 5V.
>    Second DC/DC converter - Step-down DC/DC or Buck converter
> from external (12v or any, even mains(Viper, TOPSwitch)) to
> batteries' 3V.

The original poster said nothing about a 3V battery.  He did say he was
using a 7805 regulator, which implies 7V or so minimum, and also mentioned
a 12V battery.  I believe he is linearly regulating the existing 12V to 5V
for the PIC and other circuitry.


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