At the moment I have used two little plates cutted from an old AT PC board
connector (gold plated). I use the circuit only in water, not soil. It isn't
the best choice,  two platinum thread would be much better but the price....
I have used this probe for a month in water (continuous measure) and the
results are good, but I don't have any idea for a longer period: a problem
may be sea-weeds prolificity. Another cheap solution (for water) may be
graphite sticks. Inox? I haven't tested yet.
Note: however, if the probe is used for water conductivity, I think is
mandatory that the electrodes will not be "free" around them but inserted
into a confined vane (with open sectors for the water-flow). If not, the
relation between water resistance and frequency isn't too linear.

regards
Marco


----- Original Message -----
From: "Phil" <phil@PKELLER.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 18 aprile, 2002 3.18
Subject: Re: [PIC]: SOIL MONITOR


> Marco,
>
>   Interesting design.  What are your probes made of ?
>
> Phil
>
> marco genovesi wrote:
> >
> > Hi Gary, this is a very simple and cheap circuit (I don't know his
limits on
> > accuracy). Really, I use a similar layout to check water conductivity
with
> > my PIC16F84, but probably may be useful also for a not too-precise soil
> > moisture check. It is 4093 - based: note that frequency output
(squarewave)
> > is sensible to V+ variations, so a regulator will be nice. You can
change
> > the C1 cap. value in the schematic or distance between the probes to
obtain
> > higher or lower frequency output. There are similar circuits on the web
(the
> > most part  555timer - based  but often a lot of these not have a really
"AC"
> > voltage to prevent probe oxidation).
> >
> > good luck
> > Marco
> >
>
>
>   SNIP
>
>
> --
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