This whole thing started with the idea of monitoring the moisture of the soil in my garden so I would know when to water it. (It's actually my girlfriend's garden, but it's in my backyard... don't ask.)
I figured I could measure the resistance between two probes and get an idea of the moisture content from that. So I set up a little "Test Unit" by filling a flower pot with moist soil and measured the resistance between two nickels (I chose nickel because it doesn't corrode easily) and got about 2K Ohms. Wondering if temperature would affect the measurement, I put the "Test Unit" in the refrigerator overnight and the measurement went up to about 3.5K Ohms (did the soil loose some moisture overnight?) Next I put it in direct sun light for several hours to heat it up and the measurement went down to about 1.5K Ohms. Sure enough, not only does temperature affect the reading by approximately 7.5% per Degree C, but it appears that moist soil (at least the sample I'm using) has a negative temperature coefficient! (Almost all known conductive materials have a positive temperature coefficient where the resistance goes up with temperature, moist soil, however, goes down making it somewhat of a rarity.)
So a few brain cells fired and I got the idea that I could put a little Microprocessor out there in my garden with a wire running in to the house and remotely monitor the soil. Then I figured that as long as I'm doing that, I may as well add other sensors too. Eventually (after even more brain cells fired) the idea blew out of control to become a Multi-Drop Network with an array of Addressable Microcontrollers each scanning an array of Sensors and reporting the data back to a master PC which will archive the data and graph it. What started as a single soil moisture measurement became an automated network measuring...
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