You can build a simple voltage to pulse-width converter to do the job.
You can even have multiple "channels" quite easily. The basic idea is
to create a triangle wave that goes from 0 to +5 volts. This is fed
into the (-) input of a comparator or op-amp. The voltage you want to
measure
goes into the (+) input. 1 volt will generate a 1/5=20% duty cycle,
2 volts will generate 2/5=40% duty cycle, etc..

The one triangle wave can feed into any number of comparators, so
you can easily generate multiple channels.

******

The triangle wave method above is self-repeating and so requires only
one PIC i/o line. If you have two i/o lines available, then you
can make a single-slope converter instead.

A single slope converter consists of  a capacitor driven by a constant
current source, and a comparator. The capacitor is discharged, then
allowed to charge. The ramp is connected to the (-) input of a comparator,
and the voltage to be measured is connected to the (+) input of the
comparator. When the output of the comparator goes LOW, the time is
proportional to the voltage. It is useful to have the constant current
source adjustable, so the scaling can be adjusted via a pc pot rather
than having to do it via software.

Note that if the compliance voltage of the constant current source is
less than +5 volts, then you may have to use a simple voltage divider
at the (+) input of the comparator to reduce the max input voltage
to something less than the compliance voltage.

Multiple comparators can be driven by a single ramp circuit, so
each additional analog input requires one i/o pin and one comparator.

Fr. Thomas McGahee

>
>
> > I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> > have a voltage output, but in my board I have only one free pin (RA4),
so
> > I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> > For a lot of reasons, I would like a really cheap solution, preferably
not
> > over 2-3$.
> > I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
> > haven't never used it: can it be an acceptable solution for my specs or
> I'm
> > going in a wrong way? Have anyone any suggestion about 4046 VCO use or
> other
> > low cost alternatives?
>
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