At 14:57 2/8/2002 +1100, you wrote:
>
>Are then any repositories of sensor circuits for the f87x analog inputs
>anywhere, such as soil moisture, air temperature, light, humidity anywhere?
>I haven't been able to find any (except for some play circuits).
>
>John
>
>
Hi John:

Look at:
http://www.sowacs.com/

We use the granular matrix sensor driven with a 'C711 PIC in our products.
It works great in ag. fields. It really measures the soil water suction.
The plant can extract water from the soil when the water suction is between
1/3 and 15 atm. Soil surveys show the relationship between water suction
and gravametric water percentage. Soil is a little hard to measure it's
electrical resistance because of dissolved salts and ion migration to the
charged metal of the sensor.

If you just want an automated way to water a garden a couple of stainless
steel nails, with wires attached, held about 1/4 inch, 6mm, apart, stuck in
the soil will work. You can calibrate it with some soil, same type, in a
flower pot if you have a balance, or good kitchen scale to determine the
percent water. But, if you fertilize or have high salt content in the soil
solution it won't be repeatable. Of course, you have have a series resistor
to form a voltage divider and use ac to excite it. We use two PIC pins to
the ends of the voltage divider to get some ac. First we set the one on top
high and clear the other low, then vice-versa.

To eliminate the salt problem encapsulate the nails with plaster of Paris.
The CaSO4 will keep the solution around the nails saturated eliminating the
variability of its effect on resistance. Actually, what you have now is a
gypsum block, see the above site.

The sowacs site has a lot on information on soil water content sensors.

Good luck

Chuck


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||   Chuck Mancuso           ||     i      AEC Systems     i    ||
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