>
> 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 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.
>
> i'm wondering if there's not somethign i could do with a known RC circuit and the PiCBasic POT or RCTIME commands to
> determine what that voltage is .. with this setup i wouldn't be drawing current except when i execute one of those
> commands.
>
> i'll only need to check this voltage every 30 seconds or so.
>
> thanks for any ideas you may have !
>
> david dunn
>
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>
> 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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>
> Use a resistor, cap, and zener (no a/d):
>
> 0-30vdc+-----/\/\/---+----------+-------PIC I/O
>                      |          |
>                      |          |
>              +--|>|--+   +--||--+
>              |           |
>              |           |
>              |           |
> Gnd----------+-----------+
>
> To take a reading, drop the pin to gnd for a few ms, then float the pin
> and check it to see when it goes high.  Put it on an interrupt pin or
> interrupt on change and you can just use a timer.  Put it on the CCP pin
> in capture mode and you'll get the interrupt and the exact time when the
> pin changes.
>
> If the voltage is over 5 volts then there will be some current draw, but
> only very little (much less than a regular voltage divider)

>
> OK Usual disclaimer, this is an untested circuit but...
>
> Take an N-type logic level FET. Connect the gate to PIC VDD and the source
> to the measuring pin. Put the capacitor between the pin and ground, the
> resistor between the drain and the voltage. Drive the input pin high and the
> FET turns off. No drain at all. Drive the pin low then float it and measure
> the charge time. Obviously the pin will not rise above the Vdd-FET Vth but
> this should not be a problem.
>
> Oliver.
>