This project is an attempt to make an automatic waterer more efficient.
I think that these devices are used throughout the U.S. I have not been to a farm supply store that does not carry them, and we used them regularly when I was a kid. The draw back as I remember was that they always leaked, if the tank was not on level ground they wouldn't work, and they bring the water line up so near the top of the tank that there is little room for error if they fail. There might be a mechanical solution for the problem that someone could come up with using a 3D printer, but most of my thoughts have revolved around the use of an electric valve and an ultrasonic sensor.
Pressure balanced valve from a Melnor water timer. The failure types described were from dry rot in the diaphragm and the LCD on the timer would go out, but usually after more than a year, and the unit would run daily for approx. 1yr. on a 9v battery. Sadly I can't find this valve type commercially available.
The drawbacks as I see it for using an electronic solution is that 110vac is required at the tank, but in most instances I believe that this is present. These devices only work in the summer (at least for most of the U.S.) unless the hydrant is wrapped in electric heat tape or there is an electric heater in the tank (these present their own problems that I hope to address in a future project).
This is usually the source of the leaking. If it begins to leak you can reach in and turn the little piece of plastic to a new spot, but this only works so many times. There is also no feed back to whether the device is working or not. While there is no reason not to check the tanks regularly it can be useful to have some feedback, a lot can happen in 24 - 48 hours.