A bipolar transistor and 3 transistors provides a "soft" threshold.

Add a few more R's to get some +ve feedback for sharper edge.

Go to 2 transistors and a few R's for classic Schmitt trigger.

Super sharp cheap enough is TL431 or TL:V431 + 3 R's (2 to set level, one
in Cathode output - could be uC internal pullup if desired.
The 2.5V TL431 needs about 500 uA to regulate.
The 1.24V TLV431 needs about 85uA max.

Then there are a whole host of battery and voltage level monitoring ICs.

The TLV431 can be used with goodish precision with a 100k input resistor to
the ref divider and at decreased precision with 470k input. If you are
willing to select-on-test the 40k input is OK.

Even a 100k input R draws only  (3.1V - 1.25v)/100k ~=3D <=3D 20 uA.
If left on it will draw about 4 mAh/week of battery capacity (20 uA x 168
hours =3D 3.36 mAh.)

The excessively desperate can provide a divider which disconnects at ultra
low battery voltages - to stop the divider loading the battery very long
term.


       Russell

LRVALT1G =3D TLV431 =3D $US0.035 / 10000 China.
20c to $1 /1 in US.





On 17 July 2013 15:59, <lee@baudalign.com> wrote:

> I'm making a li-ion (18650 battery) powered board with mostly 3.3V PIC an=
d
> devices on board, with a 3.3V buck-boost, but I realised in my last
> version that I'd left out any sort of battery monitoring.  I don't
> actually need/want much, I don't necessarily need to read the
> voltage/current all the time, just need some way of noticing the battery
> is getting low and setting a flag in the PIC to display a "charge battery=
"
> indicator?
>
> I had been thinking of using a voltage divider, MOSFET, and an ADC channe=
l
> to get a rough reading of the voltage every now and then, but really I
> only need to have a threshhold point, say about 3.1V?  Any thoughts on th=
e
> cheapest, lowest parts count way of detecting this and feeding a spare
> PIC24 pin, be it digital or ADC?
> Would be nice to give end users at least some sort of warning/indicator
> before things start misbehaving.
>
> Lee
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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