A fresh set of alkalines will give you a pretty constant voltage that you
should be able to calibrate to upon power up and provide useful accuracy.
Once the batteries reach the end of their useful life, the voltage will take
a steep plunge and there is no real advantage of tracking or calibrating
your pic reference to it. Two aging cells in series providing 2.5 volts
means it is already on a rapid avalanche to zero. A fresh set of cells is
the only reliable fix.

Rick

"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.
>
> What I was thinking of doing is not using a regulator so the voltage
> would vary over battery life about 1.6-1.25.  Vref (I have it set at
> about 70mV) should drop with respect to battery and so should the output
> of the sensor (it's a wheatstone bridge configuration).  I also have the
> pic calibrate on boot.  So these three attributes combined I would
> believe would be enough to not have to use a regulator.
>
> Any thoughts?  Any ideas on a reg that would output 2.5V until the
> voltage dropped below 2.5V?
>
> Aaron
>
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