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Thread: Unknown Voltage on PIC pin.
www.piclist.com/techref/power.htm?key=voltage
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face BY : Roman Black email (remove spam text)



Drew Vassallo wrote:
>
> >i have an unknown voltage that will vary from about 0 volts up to about 30,
> >and i need to determine what that voltage
> >is via a 16F877 pin.
>
> I don't know about the inputs of a 16F877, but I assume that 30V is a little
> out of spec.
>
> >i know that i could just do a simple voltage divider, put that on a A/D pin
> >and figure it up.
> >what i'd like to do is find a way to get it without drawing current all the
> >time.
>
> Then why not just put a transistor in to open the current path when not in
> use?  Turn it on, take your reading, turn it off.

Do both together, turn the transistor on and power up
your 2R divider (so about 30v==5v), then turn it off,
and a small C on the bottom resistor will determine
the time to discharge, and the PIC pin can measure the
time to reach the 0.2v at the schmidt input low
threshold, time will be relative to the amount of
volts on the 2R divider at the start. This will
work. It will have to measure from 30v to about
4.8v minimum, as the PIC schmidt pin needs to exceed
0.8v to give high level.

It would take two pins and two transistors. ie,
PIC pin drives npn, drives pnp to turn on 2R divider.
And it is only good for 30v to 4.8v range.

Now I challenge someone to do it with ONE PIC pin!!
Or less/no transistors! Or better range!

Requirements:
1. must measure up to 30v
2. must draw no current from measuring circuit
when not needed.

:o)
-Roman

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