Also, what type of soil and depth are you planning to use the VH400. In my experience this device was totally useless in North Carolina clay. It seems to work reasonably well in potting soil, however.
Tom
Was planning for about 4" down, and the soil around here is pretty sandy. Testing in my local soil so far it's performed reasonably well.
But I was hoping the sensor would work well in other soils too so I could build these for some interested friends. How did it behave badly in clay soil? Did a tiny drop of rain send it straight to full scale output? Did you find another sensor that worked better in clay?
I started a test in October where I planted it in the vicinity of a Watermark sensor that has been working well for about a year; just to compare results.
The area was a landscaped section (shrubs & perenials) next to a strip of lawn and the Watermark is buried about 8". The VH400 could only go from the surface to about 5-6" deep. It was a wet fall and winter so both devices were reading 'wet' for most of the time (the VH400 ALL of the time).
Finally, this Spring, the Watermark started to show signs of water uptake by the surrounding plants and eventually read "It's time to water." but the VH400 barely waivered from reading 'wet'.
I finally gave up on it and moved it to a pot where we've planted some Basil in potting soil just to see if it worked at all and it seems to be behaving in an expected, albeit uncalibrated, way. Sandy, loamy soil where the roots are shallow (ie no more than 6" deep) is probably ok. I wouldn't use this in denser soil or for shrubs/deeply rooted plants, and, frankly, won't buy another one.
Tom