[geeks] 9v battery meter

[Dave Kimmel]

On Sat, 11 Jan 2003, Dave McGuire wrote:

>    That sounds reasonable, though 9V batteries driving 7805s tend to go
> through batteries pretty quickly...dropping nearly half the available
> voltage in a linear regulator is pretty inefficient.  Better to use
> something like an LM2940 low-dropout regulator (which is a drop-in
> replacement for a 7805 in most cases) then power it from 6V...like four
> AA batteries in a cheapie battery holder.  (6V isn't enough input for a
> 7805)

True enough, those linear regulators aren't very efficient.  I measured it
once and I think it turned out that the regulator was using 5mA (@ 9V) for
itself.  The main advantage, for me anyway, of the 7805s is that they're
easy to find locally and dirt cheap.

Another alternative is the MAX631.  This is a step-up power supply which
gives 5V from a 1.5V-5V input, the only external components are an
inductor and a capacitor (to filter the output).  I think I saw this, or
something very similar, in Poptronics a while back.  Unforutnately, I
haven't been able to find these locally.  Maxim ships out samples though,
so for a one-off hobbyist thing it might not be a bad choice.

-- Dave Kimmel
   crisco_kid at shaw.ca