Nick Knoll's running-toilet alarm uses a magnetic switch and a floating magnet in the toilet tank to signal when the tank has filled. A metal bracket holds in place a plastic tube that serves as a guide for a foam float with an attached ring magnet. A magnetic switch within the tube applies power to a timer circuit until the toilet tank fills. The timer circuit sounds an external alarm intermittently when the tank fails to fill within 45 seconds. A basic NE555 timer integrated circuit and four passive components create the timer circuit. This alarm should get an apartment dweller's attention so he or she can jiggle the toilet handle and correct the problem. And they can call Nick, too, if the problem becomes a nuisance.
Construction
The schematic diagram below shows how to connect the components. You can build the circuit on a small piece of perf-board or create your own printed circuit board. For the original circuit we used magnetic proximity detector 59135-502, but for this feature you can use a Hermetic Switch proximity detector available from Allied, part number 808-1005.
The bill of materials lists a 0.01 µF ceramic capacitor, the equivalent of the 10 nF capacitor shown in the schematic diagram.
Nick created the foam float from a piece of closed-cell foam tubing, cut to length sufficient to float a ring magnet.
Choose a ring magnet with a center-opening diameter sufficiently large accommodate the outside diameter of the clear acrylic tube. You must experiment with the placement of the proximity switch in the clear tube so it will change state when the water in the tank gets close to the full level. If you do not have enough height, flip the ring upside down so the magnet is mounted on the bottom. The bill of materials lists K & J Magnetics as a source of magnets. A Google search will help you locate other suppliers.
You can mount the "beeper" on the outside of the plastic case. Mount the battery holder as appropriate for your installation situation.
The original design called for a piece of series-304 stainless-steel strap to hold the acrylic tube in the tank. You might find brass easier to work with, although it might corrode slightly in the tank water. You can coat the (clean) brass with lacquer or several coats of Krylon clear spray. Another simple alternative: Use electric-fence wire and bend it to the shape you need- it's already stainless steel, readily available, and inexpensive.
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