Andrew, Great information. Thank you. I too had been think of using 1-wire. I am still waiting for my VH400s to be delivered so anything I say now is pure conjecture. My first priority when they arrive will be to simply make them work. After that, I will take any advice going.
My son is a member of the Charlotte NC Hackerspace and I told some of the members there that I was looking into this and I now have possibly more help than may be good for me. The software for this little project will be interesting since Once I make a bank of 8 of these things work, they will most likely provide a channel number (0-255) and a value (0-255) giving the moisture level. This will have to be made to turn any sprinkler zone on or off depending on a threshold set in software by the user. I do not think that OSPi currently has a per zone on/off capability.
My plan is to try and provide two types of sensor. Wired and wireless. I need wired so that VH400s can be buried under grass so that the grass may be mowed. Wireless won’t work underground and anyway you would need to find them and to dig them up to change batteries. Wireless sensors are needed for plants and beds where the VH400 can be simply poked into the ground leaving the battery and wireless parts above ground. Clearly the big advantage here is no cables need laying.
I originally liked using an Arduino because I know a little about it and it has six analog inputs so my plan was/is to serve up 6 x 0-255 channels and 2 x on/off. However if there is a better way then we can do that. However I also want to make this as cheap as possible. The VH400s are nearly $40 each when shipped so the 8 channel controller needs to be fairly low cost and, it also needs to be able to be daisy chained.