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Gadget Freak Case File #172: Sensor Signals Water Wasters

Gadget Freak Case File #172: Sensor Signals Water Wasters

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.

GF 172 Toilet Sensor


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.

Gadget Freak 172 Schematic

The bill of materials lists a 0.01 muF 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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Gadget Freak 172 Parts List - Excel Format

Gadget Freak 172 Parts Listing
Allied Electronics Parts
Part #

Magnetic Proximity Switch, SPDT, Cherry Electrical


Magnetic Proximity Switch, SPDT, Hermetic Switch

808-1005 Allied

NE555 Timer IC, 8-pin DIP

248-0292 Allied

Continuous-Tone Transducer

623-0027 Allied

100 kO Resistor, 1/4W

296-4745 Allied

1.0 MO Resistor, 1/4W

296-6331 Allied

22 muF Capacitor, Electrolytic, 16V DC

852-6516 Allied

0.01 muF Capacitor, Ceramic, 50V

507-0210 Allied

Alkaline Battery

737-0540 Allied

Battery Holder, 3 AAA Cells

839-2484 Allied
1 Serpac Plastic Enclosure
882-0636 Allied
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