Thomas, thank you very much.
I'm a newbie in PIC & electronics, these suggestions are very nice!
mmm.., at the moment I have only RA4 free on 16F84, but i'm starting to port
the logger code on 16F628 with a new design, so i think that it will be
possible free two or three pins.

Marco


----- Original Message -----
From: "Thomas McGahee" <tom_mcgahee@SIGMAIS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 25 febbraio, 2002 17.19
Subject: Re: A cheap A/D interface


> 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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> >
>
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