Introduction: Household DIY FLOOD Detector

About: Technologist (41st year)


I have constructed Water Alarm detection by altering Dollar Store intrusion Alarms.

Previously, I had built the same design, with a low side FET switch and some Alarm sounder. I purchased commercial alarms from RadioShack (now known as TheSource in Canada).

N-Channel FETs are fairly good devices for this water detection task. Water is a good conductor and a FET can saturate upon very little gate current flowing. The device is so cheap, you can use multiple devices to prevent flood damage in your home, cottage or boat.

I do acknowledge the 2015 article { } for the modification of the $1 Intruder Alarm. That article contains a main schematic drawing that has a major error. The authors Priest & Conaway have not corrected that error (in almost 3 years) , and the error has been pointed out in several comments that accompany the article.

This commonly available "Window and Door Alarm" uses a reed switch with a separate magnet to signal when a door is open. It is a low cost "sounder". I discard the Magnet and the internal reed switch, or use them in other projects. I open the Intruder Alarm, and add an inexpensive BS170 MOSFET in TO92 package to create a sensitive water detector. Two wires protrude from the existing case and when water bridges across those wires, an annoying warble alarm is heard all over the house.

I mount the case with "hook and loop" (aka Velcro®) attachment tape so that once I discover water, I can remove (or switch off) the device, and then mop up the mess without the alarm noise. The existing ON OFF switch is used to silence the alarm.

This project requires some soldering skill.

This design leverages a $1 store device with an inexpensive 50 cent MOSFET and (one or two) large mega-ohm resistor(s), and two wires protruding from that case, to act as sensors. I included a remote probe idea. The project is really a $2 sensor.

Any water that "bridges" the two external wires will activate the loud alarm.

Most of my sensors have been running 24 months on the same set of three LR44 button Zinc-Manganese 100 mAhr 1.5 Vdc batteries. The three LR44 batteries and original case of the "Window and Door Alarm" are retained, the reed detector will be removed, and a BS170 FET and resistor(s) are added.

Step 1: Concept (Why)


I built several units, out of necessity, from around 2010 to present, based on a well known FET method documented in 1996. Half the struggle was to find a cheap but loud sounder. The other half was to find an enclosure, switch, battery holder, and then find out buying a commercial unit was about the same as the parts + bench time equity. Now, the commercial $20 units are no longer available, nor sold.

While flood incidents can be human related accidents, some are serious failures that can result in high value property flood damage. I have dodged several high cost failures by catching issues early.


I have one under the Dishwasher in the kitchen, under the Washer drain tube at the basement slop-sink, at the top of the condensate pump of my basement Gas Furnace, on the floor beside each of the bath tube/showers, and at the Cottage in at least four other spots. I have two commercial Radio Shack (now called The Source in Canada) detectors on the Hot Water Tank, and another right over the house master sewage drain vent. I have used a similar design in a device on the Boat for monitoring the "auto" bilge pump. A rag can cause a lot of flood damage when it blocks a bilge float, or blocks a drain. leaks can turn into floods really quickly (and in silence).

These detectors should sit quietly for 2 years on LR44 in stand by. There are many uses for this type of detector. I am always thankful when one alarm goes off.

The first image is of that alarm when its used as a Door Alarm; separate the magnet from the main body and a loud annoying warble is heard until the small On Off slide switch is used, or the magnet is returned to that side of the alarm case.

The second image is of a modified alarm without the probe wires not connected.

Step 2: Parts (WHAT)


"Window and Door Alarm". Actual item shown.

In Canada, you can obtain them at Dollarama. In the USA, I have observed these units at DollarTree. First image.

In my Country, $1.25. The three LR44 are included. One screw holds the case together, inside the battery door.

Note: you will see that there are several versions of that Window & Door device; encapsulation and IC .

I will post a link if I find the same China Item on the web.

(2) LR44; there are three in the above kit, *but* of five recently purchased units, two kits came with dead batteries. Nice; the items probably sat in the store for too long. Typical LR44 specification link here:

(3) BS170, N-Channel MosFET ,typical specification link here:

this small signal N-Channel FET costs $2-3 for quantity 5 in Canada.

(4) Gate resistors, original was 3M3 or 3.3 MegaOhm or 3,300,000 Ohms, 1/4W, 5%.

I have used some 4M7 with and 1M0 with same success. This project will use 1M and 10M.

(5) bare wire to act as sensor. I use Telecom solid copper (STP stripped of insulation) however, I have also used stainless wire in high humidity areas as the copper tends to corrode to a green coating, and may become less effective over time.

Check out the last image; I used a BERG computer jumper and pulled out the sockets. I used TeleCo "strap" wire 22 or 24 gauge solid, with copper stripped and exposed and then bent back; this is very effective for Laundry Tub, Sink, Bath Tub, or any application where detection will be away from the alarm case. I have one set of wires about 1 meter long; that length of wire has no effect on sensitivity.

Step 3: Construction (How)

The first image is the schematic I created when I reverse Engineered that Window and Door Alarm device.

This design uses two resistors, 10 Meg and 1 MEG in a branch, and I have used two 10 K resistors as "short probes". The second image is the MOSFET pin TO-92 orientation.

Water across the probe wires will cause the FET to saturate, completing the negative branch of the Alarm circuit, causing the oscillator and amp to drive the Piezo-element speaker in a loud "90 dB" frequency shift warble.The positive branch of the circuit is connected through the existing ON OFF slide switch (handy).

The third image is the interim build where the negative wire was removed and the FET is in series where that wire is removed. I many images, you will not see that I also removed the reed switch to use in other projects.

The images on this page are from 2016. I decided to take another dollar store unit and modify it and document that build in the next few images to consolidate my three designs to one.

Step 4: Disassembly, Cut Negative Wire, and Remove Reed Switch.

Images are in sequence of preparation.

I modified a unit on January 20th,

and the work took about 15-20 minutes on the bench, not including telephone interruption time.

Open the battery cover, remove the screw holding the case clamshell together.

Cut the negative wire from both ends, board and LR44 negative terminal.

Cut the reed switch (you can also leave that reed installed, but I re-use that part.

Step 5: Install the BS170 MosFET

Install BS170 FET, as shown previously and in these images.

SOURCE to battery negative terminal,

DRAIN to board contact where wire was previously connected,


GATE to 10 MEG resistor (BROWN/BLACK/BLUE) to switched POSITIVE on board (after slide switch).

Original black wire remains connected to the center of slide switch.

Install 10 MEG resistor as shown. Gate to switch positive on Board.

Install 1 MEG resistor as shown. Gate to negative battery terminal.

Heatshrink tube insulate the exposed resistor leads as shown.

Attach Probe solid wires as shown, from FET GATE and POSITIVE SWITCHED points.


I had to cut two wire notches in that white case to allow the probe wires to stick out "the bottom".

I also cut corresponding notches in the top case to match.

Last image shows probe wires in "radial" format and in parallel with *same length*.

1 small threaded screw holds the case together.

Step 7: Detection Probes

As previously mentioned, images 1 & 2 depict a probe constructed of 22 or 24 AWG solid copper twisted wire, fitted to a BERG jumper housing with shorting female sockets removed. A heat-shrink tube completes the probe.

Images 3,4,5 are a version with two 10 K resistors used as probes. This is the version for the dishwasher standalone detector where the probes protrude from the bottom of the case and you can see the case notches for those resistor lead "probes".

The 6th image is for a remotely wired probe (22 or 24 AWG solid copper pair). One wire of the probe is right on the FET gate, and the other is on the positive switched side. One resistor from gate to ground prevents the FET from latching up after the water "bridge condition" across the probes is removed.

The values should be high enough so that quiescent or standby current is very low, allowing for long battery life. I measured 85uA at rest, and 30mA when the alarm sounds, and the unit will likely alarm for 2-3 hours, depending upon battery condition.

You should test your flood alarms regularly, twice a year, with the same schedule as Smoke and CO2 Detector battery swaps.

Step 8: Testing 1,2,FLOOD...

Pour some water into a glass or into a small flat container.

I use a plastic blister pack top in the images of testing.

Switch the unit ON.

If the unit is properly wired and soldered, alarm is silent.

Place or submerge (just the) probes in water; the Alarm warble should sound loudly.

Pull the probes out of the water; Alarm should silence, and not latch up.

pending: a short video

Step 9: Installations

1st and 2nd image: Condensate Pump for Gas Furnace. In the summer , Furnace condensate is from the Air Conditioner coils inside the plenum. In the Winter, a sealed high efficiency forced exhaust Furnace has condensate evaporation. The water is collected in a auto-pump; it has its own float switch. In both seasons, extreme weather builds up a lot of trapped water in the pump reservoir, and I put an alarm at the top of one of the rubber plugs of that cache. The pump switch may fail, or that small ball check valve will fail; the tank over-fills, and another safety interlock prevents heating and cooling operations with a fail-code. This alarm warns me well ahead of disaster ; a quick repair can resolve these simple Furnace maintenance tasks.

3rd image: The one time I had a Water tank leak, the damage repair was expensive. The tank was leaking over time without detection, and "let go" overnight. The result was costly high value damage, with some denied insurance claim losses. A second occurrence was with my DIY alarm, alarmed well in advance. and a loose $18 fitting was replaced next day. The Flood detector was placed at the low point, and a small puddle formed.

4th & 5th images; detecting under the Washer Machine drain hose, below the rim of the basement Tub basin by about 2 cm or so. On occasion, a sock or other laundry item has dropped into that tub sink, and blocks the drain. This simple fault has happened several times. This alarm cries out when the drain water backs up too high; Instead of a mop operation of a few hours, a simple fish out of the blockage cleared the problem(s) in seconds.

Again, the issue is continual leaking or flood without notification means late intervention.

6th image; the main drain has a "blocker" provided by our Municipality. There is no warranty, and on several occasions, serious external issues with City Sewage "back-up" were signaled by "black water" ( with aroma ) coming up. This was extra fun to clean up that first time. The incident motivated the one-way valve installation, and it does work, but there can be some minor pooling during the Spring thaw in Canada. This alarm gives me plenty of warning. I can clean up the flood without letting the grey water sit and smell.

Observe in the images for my electric water heater tank, and Sewer main drain are the commercial types with 2 x AAA power, low battery indication, and TEST LED features, because these areas (are: should be) covered by Insurance, and D.I.Y. detection methods are "not acceptable for Insurance Inspection". The Sewer detector is also the lowest point in the room and this one caught the washer loose hose clamp release incident early.

7th image; Under the dishwasher in the Kitchen; The door seal is another item that fails over time. Drip,Drip Drip. My small DIY $2 device screams when any water leaks down from either the dishwasher or the sink. This device has also saved me quite a bit of labor and clean up.


  • Hot Water Heater Tank; right at the bottom forward edge, probes touch the surface of floor or spill tray
  • right under the Dishwasher or off to the side, probes within 2 mm of floor
  • at the main sewer drain, but resting on the floor and not in the sloped "well"
  • about 2-4 cm from the top of the laundry sink-tub, somewhere away from the splashing of the drain; I put it right under the drain because the water rarely splashes up unless the tub is flooding
  • hanging down from a Bath Tub spout, so that the detection wires are at a point where you want the alarm to signal that your bath has filled and is "ready"; this kind has to be made portable with a hanging "sling" (not shown in any of my pictures)
  • in a Pool Pump House, under the pump to detect leaks
  • under the pressure tank for a Jet Well Pump (mine is in a drip tray that covers a wide area)
  • in a position to give an early warning above the *normal* level of a House SUMP well (coming soon)
  • just above the Bilge area of a boat, above the Bilge Pump to signal the system failure

or anywhere where you need an early warning of impending Flood.