$15 (easy) Water Detector

After the slew of hurricanes and tropical storms that plagued the east coast this year, I think just about everyone became aware of 'flooding basement syndrome'.  I certainly did.  And, in accordance to Murphy's Law, the flooding MUST begin at 3am when you are soundly asleep in bed then continue to flood until you wake up the following morning to a brand new indoor pool.  The second night is even better.  You've learned your lesson and you're prepared for the worst.  Unfortunately, all this results in is you standing there at 1am staring at a sump pump float for hours, performing your own scientific field test of the adage 'what goes up....must come down' and each time praying that it does.

Around hour 3 of this, it occurred to me that this is the 21st century!! I should just be alerted to the fact that my basement is flooding while I'm sound asleep by some device with the voice of Pierce Brosnan!!

As with most good ideas at 3am, I completely forgot about it.  Then I recently saw this post on Make for a $25 water detector which reminded me.  I thought I'd take my own spin at a simplified method.  So here it is.....might not be a soothing voice to awake you (yet), but it will definitely get the job done...

What you'll need

  • A battery operated smoke detector ($7)
  • a large PVC coupler ($6)
  • a large sponge ($1)
  • A SPST switch ($1)
  • wire
  • a soldering gun
  • glue

How it works

It's pretty simple and should only take you a good 30 minutes to put together.  Basically, your highjacking the 'test button' that's present on any smoke alarm by attaching to leads to the PCB that go down into a sponge that's at ground level.  When water saturates the sponge it completes the circuit causing the alarm to sound (and yes, the smoke detector still works too...as I quickly found out while soldering).

(see the rest after the break)

  • Start by drilling a hole in the side of your PVC coupler to accommodate the switch
  • Mount the switch in the PVC coupler

  • Using a dremel, cut out sections at the bottom of the coupler to allow water to flow in instead of around it as it's sitting on the floor

  • Cut the sponge to fit in the bottom section (snugly) of the PVC coupler

  • Disassemble the smoke detector
  • Remove the battery and the PCB (you'll probably have to remove some battery clips as well)

  • On the PCB, find where the test button is
    (long flat piece of metal)

  • Flip the PCB over and find the two leads that correspond to the test button.
  • Solder a wire to each one

  • Run one of the wires to the switch then prepare another longer wire to run through to the bottom
  • Run the other wire coming from the PCB through to the bottom

  • Strip a healthy section of insulation off the tips and apply solder
  • At this point you probably want to touch the two leads together for a few seconds to ensure everything is working properly (make sure your switch is in the 'on' position).  If so, you'll be greeted by that all too familiar screech, if not your wiring  is incorrect.

  • If all is well, place the smoke detector in the top section of the PVC coupler and push each soldered lead through the sponge so that the ends are no more than an inch apart and just sticking out of the sponge.  Once tested, glue the smoke detector in place.

That's it!  Test it by putting it in your sink and filling it up with water.  The switch allows you to turn it off once you've had a successful alarm (otherwise, it'll just keep going until the sponge dries out enough to lose the connection).


  1. thanks...its good

  2. In step eight, where you say to touch the wires together "to ensure everything is working properly. If so, you'll be greeted by that all too familiar screech, if not your wiring is incorrect."

    I just want to point out that everything can be working fine and you still get no screech at this point; if your cutoff switch is in the "off" position!

  3. It would probably be a violation of your local building / fire code, but, if you have a centralised smoke alarm system, you could wire this into it, and all your alarms would go off if your basement gets wet (as you may not hear an alarm in the basement from the bedroom).

  4. Need to wire this up to an Arduino and have it send an email when a leak is detected!

  5. Also make sure your sponge is not conductive. I've found several that were unusable because they conducted just enough to set the alarm off. Also another idea, cover the top with an open 'roof' so if any water falls on it it will keep water off the smoke detector.

  6. This could easily be added to most home security systems along with descriptor and isolated zone.

  7. Arduino is the next step....send an SMS when the circuit is closed.

  8. Another option would be to have it switch on a light mounted above the basement door that reads "Flood Alarm"

  9. http://www.amazon.com/Leak-Frog-LF001-Water-Alarm/dp/B000WMSTUO

    $15 without the hassle of making it yourself, or the possibly of exposing yourself to the radioactive materials commonly found in most smoke detectors.

  10. I couldn't help but post this on my forum. So simple and potentially life saving. I've also shared your blog by placing it in our link directory.

    Keep up the good work!

  11. Huzzah - similar product to the Leak Frog, for $10 at Home Depot. In Canada, even!

  12. Helps if I include the link, eh? http://www.homedepot.ca/product/water-alarm/925817

  13. Thanks Anon - planning on buying a few from Home Depot today (in Canada).

  14. My basement has a furnace and the sump pump in the same room. I used a CO monitor instead that fit perfectly in the top of the pvc coupler (the 4 inch version) and then painted the pvc the color of the wall. The top of the alarm is flush with the pvc and looks like it was made for it. You can barely tell it's there.

  15. Instead of buying and wasting a smoke detector for no real purpose (smoke at the basement floor = everyone in your house has probably already died of smoke inhalation), couldn't you simply buy a piezo buzzer, battery, and your 'wet' sensor leads?

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  18. Great guide, thanks for sharing it.