AVR-Chat

On Thu, 18 Oct 2007 23:15:06 -0000
 "Graham Davies" <Yahoo37849@ecrostech.com> wrote:
> --- In AVR-Chat@yahoogroups.com, "Teo" <teodorstv@...>
> wrote:
> 
> > I would like to develop a sistem
> > based on Atmel to measure the moisture 
> > level of soil. Can you recommend me,
> > please, one good and cheap sensor ?
> 
> Use Google to look up "soil moisture sensor". To make
> your own, 
> fasten wires to two small stainless-steel machine screws
> (with 
> washers and nuts) and embed them in a small block of
> gypsum (plaster 
> of Paris). The gypsum will absorb water or dry out with
> the soil and 
> the resistance between the screws will vary accordingly.
> 
> (If you're the person that asked the same question on the
> ZiLOG 
> Z8Encore! group, then you might as well know that I'm the
> same person 
> that posted the above answer over there.)
> 
> > I would also like to communicate via USB.
> 
> As other posters have noted, using an FTDI USB to serial
> converter 
> chip is going to be the easiest way to go.
> 
> Graham.
> 
> 
Soil resistnce depends on a lot of things. Mineral and ion
content are strong factors. But, a simple resistance
measurement won't tell you anything about what is causing
the observed resistance. That said, there are inexpensive
"soil moisture gauges". The ones I have tried have never
been very consistent over even a few months and I never
bothered to open one up to see how they are supposed to
work. The ones I have seen are for potted plants but you
add fertilizer and the reading changes, if I recall.

Gypsum blocks have been used for soil mosture sensing for a
number of years. They are fine for short term but soil ions
diffuse into the gypsum and upset the "calibration".
Typically, a few months in temperate dry-season conditions
is about max.

The best commercial soil moisture devices consist of a
probe with a porus ceramic tip. The probe is partially
filled with water, then it is sealed. there is a vacuum
gauge to measure how much water has been pulled out of the
probe through the porus tip because of dry soil. When the
soil is more moist, water is pulled back into the probe due
to the relative vacuum.  I am quite sure that one could use
a semiconductor pressure/vacuum gauge in this application.
It is simple apparatus except for the probe tip.

Jim Wagner
Oregon Research Electronics, Consulting Division
Tangent, Oregon, USA
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