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> Howdy..

Ayoop

> I want to create a "low battery" warning LED, to warn me when the
> voltage drops below say 4V (from 4.5 supplied from 3xAA)...

I use this circuit on 6V gel cells. There's a little more capacity available
than AAs but the TLC or any low A op amp should run at a few uA. Also
in the circuit is an LDO regulator (Vreg), which may not be suitable in
your case, but a zener might do. Depends on what lifetime you expect
from the batteries. B1 in the diagram is power supplied by a PIC pin,
so normally the only current consumed is across the resistors. Depending
on the input current of op amp used you could probably increase those
resistors considerably to reduce quiescent current. The unmarked
resistor sets the trip voltage

There are some special function ICs that have a low battery warning
as a secondary function, but that probably won't do you

You could try a low current comparator, like some of Microchip's

http://www.microchip.com/1000/pline/analog/anicateg/linear/linercat/comp/ind
ex.htm

Some are sub-uA

===========================

Guess 1

Rudimentary low voltage detection by measuring the time it takes to
charge a capacitor. IF you use an ST input and use a source voltage
that just exceeds the ST threshold at the battery voltage you wish to
detect. As V+ drops, so will the charging source and the the ST threshold
will be later in the charge curve, thus taking longer to switch. I have a
feeling though that the ST threshold is a function of V+, in which case it
might not / probably won't work

Guess 2

A resistive divider used slightly above the ST threshold. Based on same
assumption as above

Guess 3

Measure period of a VCO (VCO powered by PIC pin). Can use a
reasonably high current as it's needed only occassionally

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