>     When sleeping, the PIC16F628@4MHz consumes about 42uA @ 5V. If I suply
> it with some AA Alkaline batteries, it would mean rougly:
>     2850mAh / 42uA = 67857 hours = 2827 days = 7.74 years without use.

I know that this was only a calculation to determine how heavy the load was
compared to the battery's capacity but do note that an Alkaline battery has
a shelf life of around 3 years (some manufacturers claim longer). If you
produce equipment with very low current drains then the reduction in battery
capacity with time can make serious inroads into the actual operating time.
When that happens, Lithium batteries are a possible solution. In this
particular case it's not an issue as your expected  operating lifetime is
far shorter.

If a regulator IS required there are much lower quiescent current drain
versions than the 7805. For example the beautiful (but excessively
expensive) LM2936 draws about 20 uA with no load in real world use and
others draw even less.

Turning the processor off completely can also be useful in some
applications - eliminating the 42 uA off current MAY allow you to use
smaller batteries (but PLEASE don't use AAA's :-) ) and a more compact
design in some applications. As long as the startup time is quick enough for
your application you can achieve true zero current when off. There are ICs
that do this for you but you can achieve the same result with 2 (maybe 1)
transistors.



        Russell McMahon

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