Hokay. Finally took some pictures of my water shutoff. This is the one on the water heater.
At the top is a triangular aluminum bracket that clamps to the incoming water line and a line that tees off from it. The clamps are cut from delrin. You can't see them in the picture. The long white thing is the heavy-duty screen door closer cylinder, and on its bottom end is a machined aluminum cup that has a setscrew to keep it on the cylinder, and two bolts go into tapped threads to carry the strain-relieving/latching mechanism. This avoids any constant load on the plumbing, and allows a latch that is easily held by light tension on the cable. The valve is open in this pic, with the cylinder extended. Its internal spring is under compression.
Two cables run from the mechanism. The tight one on the right goes to the water detector on the floor. The other, with slack in it, is for shutting off the gas burner. Both run through a crude guide cut from delrin.
Here the mechanism has shut the water valve off. The cylinder is rigged so that it bottoms just as the valve reaches full closed, avoiding, again, mechanical stress on the plumbing. The hard water here (calcium and magnesium salts) eat the copper, weakening it, so I didn't want to bust things prematurely.
Here I am reopening the valve and getting ready to relatch the mechanism. I opened the valve using the blackish lever, which stretched the door closer, and when I put the camera down I will stick a big cotter pin into holes that line up in the struts and latch, to hold it while I get down and reattach the cable to the water detector. The pin has a red streamer on it so I don't forget to remove it immediately after the detector is reattached. Aircraft maintenance practice. Long red streamers on stuff that MUST be removed before flight.
The hook angle is critical. You don't want it to self-hook. It needs to let go if it isn't restrained by the trigger cable, so the angle and surface finish has to be right.
The fancy, hi-tech water detector. A lever cut from aluminum plate and pivoted between two brackets of aluminum, which are screwed to the concrete floor via cheap drywall plastic anchors pounded into 1/4" holes drilled in the concrete. The second bolt is both a stop and has a spacer that keeps the brackets lined up. The important part of the detector is a sugar cube you can see under the left end of the lever. It's right on the concrete, and a few drops of water will cause it to fail and release the lever, which has a headless bolt in its other end that retains the triggering cable. The bracket's angled ends guide the water to the cube. You can see aluminum dams I made and screwed and sealed to the floor to catch and direct the earliest moisture toward the detector. More drywall anchors there. The dams go all the way around the tank. Just out of sight on the left is a floor drain that goes to the sewer line via a trap.
Another delrin guide and an aluminum lever machined to fit the gas valve. You might recognize the #12-10 yellow crimp terminals as cable ends. Not much tension on the cable, as folding over the end of the cable a half-inch or so, sticking that into the terminal and crimping it holds it really well. The small aluminum cable clamps I made for rigging purposes.
Here, the mechanism has closed the gas valve. It only goes to the pilot position, since pilot heat won't be any problem even on a drained tank, and because that knob has to be depressed to turn the valve completely off, further mechanism would be necessary to do that. More work that it was worth.
The crimp terminal here grabs the spring wire I unwound and doubled for the purpose. I see that my yellow insulator has fallen off. The spring is to prevent any undue strain on the valve knob. It is plastic, after all. The clamping screw in the lever has a knurled head, and finger-tight is all that is needed.
The other shutoff I made is on the house's incoming water line, which comes up from the concrete floor. The water meter is just above that valve, so the mechanism is inverted, cylinder on the bottom, pulling on the lever positioned on the appropriate side of the valve. The cylinder bracketry mounts on the wall.
Does it all make sense?