> On 4 Jun 2002 at 11:41, A.J. Tufgar wrote:
>
> > Hello All,
> >           I have a project that I'm working on that runs of two D cells
> > in series.  I'm using a 16C711 that works from 2.5-5.5V.  On the ADC
> > port I have a 3VDC pressure transducer with a output of 0-60mV.  When a
> > certian pressure is reached a solenoid valve opens.
> >
> > Here's my problem.  I want this to be effecient on battery power so I'd
> > like to run both sensor and pic at 2.5V trouble is even LDO's will
> > dropout at about 2.7V.  The solenoid works to about 2.4V.

> > Any thoughts?  Any ideas on a reg that would output 2.5V until the
> > voltage dropped below 2.5V?


It shouldn't be that hard to make a low-dropout
regulator with three transistors. What sort of
current does this circuit require? If it's only
<100mA etc than this should do the trick:

In 3.4v to 2.52v
    ------*---*-----------,
          |   |           |
          R   |  PNP      R
          |   E  lo-sat   |
          *--B            |
          |   C-----------|------*------ 2.5v out
          |               |      |
          |               |      |
          C               |      R
           B--------------*-,    |
          E                 |    |
    any   |                 C    |
    NPN   |           any    B---*
          |           NPN   E    |
          |                 |    R
          |                 |    |
 gnd -----*-----------------*----*------ gnd


I've tested lo-sat transistors here that will do
better than 20mV C-E at 50mA, the bad news is that
they are TO-220 pack power trannys. I'm sure you
can find a decent lo-sat tranny in a small pack
like standard TO-92 etc. A fet will do the low
drop but will be almost impossible to drive with
only 2.5v as your input voltage. :o)
-Roman

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