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By TestPoint

Water Alarm: Priceless Protection, Crazy Simple

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Ever come across water in your home where you didn't want it?  Basement drain stopped up?  Toilet overflow?  Sink drain pipe fail?  Spring a leak under your dishwasher?  I've had all of these happened and I wish I had an immediate warning.

5 parts. Really.

My quest to make a water alarm came from finding the main drain had failed (tree roots) and had 3 inches of water throughout our basement.  Not fun.  While I know that life happens (pipes crack, and trees get thirsty), I sure would have liked to learn about the disobedient drain, the moment the water was above its normal height.


Behold, the MOSFET

A MOSFET (metal-oxide-semiconductor field-effect transistor aka FET) is our switch and the brains of our water sensor alarm.  As of this writing, modern processors can employ more than 23 billion transistors, we’ll only need one; the common 2N7000 (or 2N7002) as they are cheap as dirt cost effective.

Since this FET only requires a voltage difference of 3V or more between the Gate and Source…

N-Channel MOSFET Symbol

…we can use the conductivity in water to easily fight the wimpy 10M pull-down resistor, turning the FET on, allowing current through to connect our ground to the negative wire on the buzzer. 

Like this…

Simple Schematic

A 9V, 12V, or 15V power supply will work fine (if your buzzer is rated for it).  They can handle a voltage range (i.e. the 458-1259-ND buzzer from Mallory Sonalert will work from 3 ~ 28V) and are louder with more voltage.  The supply need only be able to offer enough current to drive the buzzer as this circuit uses almost nothing in stand-by.  (It draws so little, I couldn’t read it on our 6-digit bench meter or Dave Jones’ uCurrent).  This makes it work well with batteries in remote locations where wall power is not handy or reliable.  (Lithium’s have some of the best self-discharge-rate, like 10+ years.  Pricey yes, but not compared to water damage.)

! - If you use a voltage beyond what the FET’s Gate can handle, you’ll kill it.  Our 2N7000 will fail shorted if more than 20V is applied.

Because FET’s can’t handle much voltage on the Gate, that brings us to our need to add another component…

Full Schematic

By adding a small value capacitor (but not too small) across our Gate and Source, it protects the Gate from short static hits by absorbing them.  I learned this the hard way (FET’s tend to fail on (Source and Drain shorted).  You could use a standard TVS (Transient Voltage Supressor) like a Zener, MOV, etc. but after adding the cap, I have yet to have an issue. 


Having only 4 nets…


 …you don’t really need a PCB, but a small piece of perf-board would work well (instead of dead-bug style). 

For those who don’t own a soldering iron, you could use a simple 4-position Euro-style screw barrier block, like the ED2997-ND.

Step 1)  Bend the Leads of the MOSFET

FET Bent Leg.png

Step 2)  Trim the Middle Lead

FET Clipped Leg

Step 3)  Space the Leads of the Capacitor and Resistor

the 2 apart

Step 4)  Do the Twist

the 2 together

Step 5)  Twist in the FET (! Careful- Be Sure the Flat Side is Up!)

the 3 together

Step 6)  Screw onto the Block (Positions #1, #2, and #3)

the 3 on block

Step 7)  Screw Down the Negative Power Wire (Black Wire on Position #1)

Step 8)  And Twist the Red Wires Together

with Battery Connector and Buzzer

Step 9)  Screw Down the Remaining Wires (#3 and #4)

with Battery Connector and Buzzer on Block

Step 10)  Connect Your Sense Wires (#2 and #4)


 * Note * Again, you can use a wall power supply in place of a battery if you have access to power.  Just cut the connector off, strip the wires, check the polarity and connect.

Time to Test it

Stick the wires into some water and it should scream.  If unfamiliar with buzzers, you can hold your finger over the sound hole to dampen the volume while testing.  A pulsing buzzer may be more expensive, but it will catch your ear far better than a continuous tone (hence the repeating beep with smoke alarms and large equipment when in reverse.)

Feel free to stick it into a plastic box to finish it up proper (or, if you feel like going full red neck, you can simply run a nail through one of the holes like this…

hole on block

…if it’s going to be under a sink, or next to a drain in your dingy unfinished basement, etc.)

You can either use a wall power supply or battery power, the alarm cares not.  I hope these save you loads of money and stress like they have for me.


Oh yea, we cut down that darn tree.

Key Parts and Components

Add all Digi-Key Parts to Cart
  • 2N7000FS-ND
  • 458-1259-ND
  • BC5132-ND
  • 10MEBK-ND
  • 36-84-4-ND
  • N723-ND
  • P687-ND
  • 364-1759-ND