@Dan,
I am happy to have a go at this. I will accept any and all inputs from anybody but it is my belief that Ray kinda owns the hardware design for this whole project. I PM’d him yesterday to ask if he had any requirements (I2C bus, USB, etc.) for connectivity and if he had any definite no-nos for hardware connection. When I have his answer, I’ll post it here.
Meanwhile my two sensors have turned up and they work well. You can give them any voltage between 3.3 – 20 and they give a 0-3 volt output depending on how wet they get.
It will be easy to make something read these and to give an output. Initially I will use an Arduino because I have several available and they have 6 analog inputs. This should give me an output to the I2C bus. If I get no further guidance, that is the way I shall go until a better idea comes along. The I2C bus works well because it is available on both the Arduino and the Pi. I2C is also on the Beagle Bone.
There is one very simple way to do this and it is to make a stand alone system that simply manipulates to outputs of opensprinkler. This would then not require any changes to any opensprinkle and would be connected in series with each output. When the desired soil moisture was reached, it would simply cut off that channel until the humidity dropped. This method has the advantage that there is no hardware or software needed. The moisture threshold would be set by potentiometers on each channel The down side is, of course that moisture sensor management would be entirely manual which is not that great.