Thanks wanderson! I was confused why it said AC but didn't seem to switch the direction. At least that is cleared up! Also thanks for showing me towards the frequency measurement technique. Very helpful!
Docedison, good points. I have no idea what I am doing :). I am glad to have you chime in with your experience in the very topic I am trying to do.
I am trying to measure soil moisture with the arduino. Actually I want to measure many things, but I'm taking it one step at a time. The temperature sensor part was much simpler and only took a few hours to work through. This is much more difficult (at least for me!). I would prefer to use AC for the reasons you mention, and was under the incorrect impression about the other circuit.
I should be able to build the "sensor" fairly well utilizing stainless steel strips spaced evenly, set in sand, inside of of an old sediment filter for an reverse osmosis filter, sealing the ends with plaster of paris and pvc.
The reason I don't think I need the voltage regulator (right term?) is that the arduino can put out 3.3v which I thought was the goal of the part I cut out. I certainly do not know more than the designer of the original circuit, which is why I posted here. I am not embarrassed at all, I am very knowledgeable on certain topics, this is not one of them :). Got to start somewhere.
I do have a high level plan, but do not understand all the requirements so it's hard to be detailed:
An arduino that can measure 4 sources of soil moisture, switch 4 mains relays, switch 6 soleniods, see positions of 3 different switches, and measure the temperature of 7 different locations. This will all be communicated via the wifi shield I have to my web server. I have successfully gotten the temperature sensor portion to work and have switched relays as well. I do not want to spend $60+ dollars on a circuit to measure one soil moisture sensor + $30 for a sensor.
I plan on building each of the small "sections" on a breadboard, than combining them onto a pcb board. Each of the sensors, switches, and relays should be able to be plugged in to ports of some kind so that I can easily remove them if I am moving the arduino to a new location.
Very long term, I would like to be able to understand some of the conductivity measuring circuits I have seen for measuring the conductivity of salt water. For now, that is a pipe dream.