Oliver, that is *extremely* clever!
I have saved your email for future reference.
I think you could even use a high voltage
FET to measure high (100v+) DC voltages.
Brilliant. And has total turn-off for
low power. :o)
-Roman



Oliver Broad wrote:
>
> 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.
>
> ----- Original Message -----
> From: "Roman Black" <fastvid@EZY.NET.AU>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: 12 January 2001 15:00
> Subject: Re: [PIC]: Unknown Voltage on PIC pin.
>
> > M. Adam Davis wrote:
> > >
> > > 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)
> >
> >
> > Sorry no prize! ;o)
> > It doesn't totally switch off. And the current
> > draw will be identical to the 2R voltage
> > divider, as at 30v the top resistor will
> > still drop 30v-5v and I =E/R
> > I do agree that at lower voltages approaching
> > 5v it will draw a bit less current though.
> > -Roman
> >
> > --
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> >
> >
> >
>
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