Ron Bean has a fascinating post on Milwaukee Makerspace’s wiki about his project to plumb in the hackerspace’s water cooler.
In addition to the obligatory refrigerator full of soda (or sometimes empty of soda, depending on whether anyone’s filled it recently), we also have a water cooler that takes the big 5gal water bottles. They also sell coolers that can be plumbed into a water line, and a while back I discovered that they’re actually bottle-type coolers that come with a conversion kit. The kit consists of a float switch and a solenoid valve, and a mounting bracket that takes the place of the bottle holder.
There isn’t a conversion kit for the cooler we have, but it looked simple enough, so I made one from scratch. I don’t like float switches, so I used two other methods of sensing the water level (I wanted a backup sensor because certain failure modes could result in an unlimited amount of water on the floor, which would be a Bad Thing). Instead of trying to find the “best” way to do it, I used the components I was interested in learning about.
Ron ended up using a pair of stainless steel probes that measure conductivity between them (and therefore can sense if there’s water in the tank) as well as a proximity sensor, triggered by a relay, serving as a backup sensor.