Go ahead and try it. A couple things you might go back to Ray’s page and look at again:
1) Inrush current. Ray mentions this on the page. The solenoid needs a spike of high current to get the slug of iron in the solenoid moving. For a very long run of wire at 18ga it’s not going to be reliable because for the few microseconds that it’s pulling 20A or more the voltage is going to be way too low. Which is why I said I’m pretty sure the commercial systems are using a boost/buck system.
2) Regular cat-5 ethernet is 24ga not 18. Also, the jacket of regular ethernet will not remain intact in water. I’ve seen enough of it pulled out of pipes that had water in them to know what happens to regular PVC ethernet cable jackets. They make underground rated ethernet and it has the same tough PVC case that the 18ga sprinkler wire has. I think you will find underground rated ethernet to be the same cost as sprinkler wire. You are also ignoring that the largest cost in ANY sort of large distributed network is laying the wire. NOBODY in charge of a large system is going to put in an untested system that specs DIFFERENT cabling than all the other existing competitive 2 wire sprinkler systems on the market.
3) The reason sprinkler systems don’t use motorized ball valves is so that if power is lost the sprinkler valve shuts off. Otherwise you have your sprinkler going and there’s a power failure or the controller craps out and then the sprinkler is running all night long and you just lost $200 worth of water and caused a flood. Of course that then means that the valve has to be held open by continuous current. And if you have 5-6 valves actuated at the same time you run out of current carrying capacity on the wire.
4) Sprinkler system plumbing is organized as follows, valves are generally star topology to the water source, zones are daisy chain from the valve. In small home systems the valves are usually placed next to the water source. In large systems the valves are out in the field. Think about what happens to a daisy chain system that is mapped over a star topology – for every valve you are going to be running TWO pairs to it – one from the hub out to the valve and one from the valve back to the hub then out again to the next valve. This is because they are only going to dig 1 trench for the water pipe and the cable for the valve it’s a lot cheaper. That’s why the cable length in theses systems is so long.
You did mention the issue of powering the controller at each valve so I’ll give you that.
You are also ignoring the inherent flaw in a daisy chain config which I mentioned already which is if there’s a break in the system then all the valves after that are dead. If you had any experience in this sort of thing you would have known that this could be solved by creating the daisy chain in a loop with both ends of the loop terminating at the sprinkler controller.
If I was running a golf course for me to send a tech out to replace a failed valve in a sprinkler is 2-3 hours of labor which dwarfs the piddly amount of money I might save on a cheaper node. If I buy your cheaper nodes and they have a higher failure rate my savings will be eaten up.
I am sure that if you were to keep at this after a string of failed hardware behind you and a pile of wire pulled out from underground and a long enough time you would be able to create a running system. But in half the time a lesser schooled guy could have probably reverse engineered the most popular 2 wire commercial sprinkler system on the market and produced a compatible controller that works with all their valves, and that way the greenskeeper can buy all -their- valves and the cheapo controller and been watering his lawn by the time you get something working.
If you absolutely must design your own then why not use the cheapest wire possible – 28ga telephone wire – and run a high frequency on it like 600 Hz AC at 200volts and then you can use tiny toroid core transformers at each valve to drop it down to 20v AC, and it’s also easy to rectify that and get as much current you want to power a controller and a valve. And you can make frequency shifts to carry data to the controller. That system probably will carry the signal a few miles….