A little story: When I was growing up (which was a very long time ago) my room was in the basement. The problem was that whenever we had a hard rain, the city's storm drains would fill up and water would back-up through the basement floor drain, flooding the basement (and me). If we knew the storm was coming, we would install a stand-pipe in the floor drain. In this way the water would rise in the pipe, instead on flooding the basement. But, surprises do happen and storms can come without warning. I needed a way to let me know the basement was beginning to flood so I could get out of bed and install the stand-pipe. In the original version I used a large dry-cell and a door-bell. Today we have other ways, but those mechanical bells are sure loud.
I don't have a basement anymore, but I do have things that can cause a flood in the house. Water heaters, clothes washers, and air-conditioners to name a few. There are many products available to detect high water, but they tend to be very expensive. I've also seen some DIY solutions, but they seem much too complicated. Here's a solution that is cheap, reliable, and simple.
Step 1: Materials Required
A spring-type clothespin, some wire (I used 24ga), 3/8" heat-shrink tubing (shrink-seal type is best), some conductive foil tape, and a uncoated aspirin. Make sure the aspirin is uncoated (the real cheap aspirins).
Step 2: Tools Required
A Dremel tool or other drill. A very small drill bit, I used a #60 bit for the 24ga wire. A heat source to shrink the tubing. A soldering iron with solder.
Step 3: Prepare the Clothespin
Drill a hole through both sides of the clothespin. Try to center the hole on the flat area that meets when the clothespin is closed.
Step 4: Prepare the Wire
Cut two wires of appropriate length. Strip off about 3/4" of insulation off each wire. Twist the strands tightly then tin the wire using the soldering iron and solder.
Step 5: Insert the Wires Into the Holes
Insert the wires into the holes in the clothespins. Pull each side snug then wrap around the front to help secure the wire. If the hole was the proper size, the insulation should not enter the hole.
Step 6: Apply the Foil Tape
The foil tape helps hold the wire in place and provide a good contact surface. Cut two small pieces of foil tape and place them on the faces of the clothespin. Squeeze the clothespin to make sure the tape is well seated.
Step 7: Secure the Wires to the Clothespin
Cut two 3/8" long pieces of heat-shrink tubing. Shrink-seal is best. This is the tubing with the internal lining of hot-melt glue. If you just have the regular tubing, first secure the wires to the clothespin with dabs of hot-melt glue. Now slide the pieces of heat-shrink tubing up the wire and over the sides of the clothespin. Heat with a heat-gun or other source to shrink the tubing and secure the wires.
Step 8: Install the Aspirin
Uncoated aspirins dissolve quickly in water. When the aspirin dissolves, the two sides of the clothespin come together and make electrical contact. It is a good idea to scuff the aspirin on some fine sandpaper before installing. Today even uncoated aspirins seem to have a thin gel coating that would slow the dissolution. The resistance is quite low, so it can be used to switch several Amps. Of course, when you build this into a circuit be sure to use a safe source of power. The best source would be a battery. The wood and aspirin are both excellent dielectrics (when dry), so the battery would last a very long time.
Step 9: Alarm Circuit
Shown above is the detector, a 9V battery and a piezo buzzer. In this case the buzzer requires so little current the alarm will sound before the aspirin dissolves, but a louder source, such as an electro-mechanical ringer or a horn alarm, would require much more current. As you can see in the second photo, the aspirin dissolves quickly and the clothespin snaps together.