I remember my dad yelling at my sister and I since we took long showers. My sister had the hot water turned off more than once on her although I never did.
Now it is my turn to yell at my two adolescents who like to take hour long showers. It is summer here, and we have an oil furnace which heats only hot water this time of year, and in three months, they have managed to go through a 220 gallon tank of oil which is just under $900 here. That tank under normal usage in previous years had lasted well into November and maybe even early December.
I have asked nicely that they be reasonable in their hot water usage, and now it is no more Mr. Nice Guy. So, I want to be able to sense water flowing in a pipe and if the shower is longer than 15 minutes, to turn the hot water off. I found something that will go on the end of the shower head that would work for $140 but figure I can come up with something cheaper than that. I have a good idea on how to turn the valve off on my hot water tank (limit switches, a 6v electric screw driver), but what I need help with is sensing when the water is on.
I found this online:
but was unable to get this to work. I thought it ideal since it requires no plumbing change. The exact piezo used in the circuit was unavailable so I found another one at Radio Shack so I didn't have to wait for it to arrive.
The piezo worked well with the op amp circuit documented in the tutorial and easily sensed pressure, large vibrations (I used it on my tablesaw and it sensed those vibrations), taps etc, but didn't pick up the low hum of water running in the pipes.
I had thought about using a temperature sensor on the copper pipe, but I was concerned it might hold heat and erroneously think the water was on longer than it was and not allow hot water to anyone in the house. The pipes from the back of the shower are in a tiny closet that is always very hot, so in summer it could be 105 degrees in there. I could use two sensors, one for ambient air and the other attached to the pipe. A substantial difference in temp indicates hot water is running. I am not sure if this is a good approach though.
I was thinking of trying some vibration sensors from Sparkfun to see if those would work. Would anyone else have any suggestions on how best to approach this? Has anyone done anything like this and what was your approach? Am I going to have to hire a plumber to put in a water flow sensor?