Anyhow, water is not nearly as conductive as you would think.  Rudimentary testing of water droplets indicated about 200k-500k of resistance.  My first, simple attempt at a water detection circuit did not take this into consideration.

Simplistic water detection circuit

Here’s a snapshot of the test circuit I built.  It works great using that little tactile switch.  But it doesn’t have the capability to pull the S input on the S-R flip flop to ground when running through water.  That’s simply because the path to ground for the VCC pullup has too high a resistance ( 200k-500k through water ) to drop the voltage below 0.8V, necessary for a TTL low signal.  I didn’t bother putting together an eagle schematic for this design since it didn’t work.    The astute will notice I don’t actually have it wired up to an R-S flip flop ( 74279 ).  I didn’t have one on hand, so I built one with a 7400 I had in stock.

At his point, I figured that I needed some amplification of the current I was able to push through a drop of water, so obviously a transistor came to mind.  A standard NPN transistor ( 2N2222 ) wouldn’t do, though, since it doesn’t provide adequate amplification of the base current produced by 5V pushing across 200k-500k resistance.  What I needed was a Darlington transistor!  Amplify the amplification!

Working water detector!

Here’s the circuit I put together using a Darlington NPN transitor to amplify the current that could be pushed through a drop of water at 5VDC.  When the power turns on, the microcontroller needs to reset the S-R flip flop by cycling the Reset line low then hold it high.  We have the S-R flip flop to “remember” when we have a moisture condition for the sleeping microcontroller.  The S input has a pullup resistor R1 just because it’s bad practice to leave a TTL input floating.  We connect that same signal to one end of our sensor probe ( SENSOR1 ), as well as the collector on our Darlington, T1.  The other side of our sensor ( SENSOR2) connects to the base of T1.  If we can push a mere fraction ( think millionths ) of an amp through that sensor, the Darlington is going to open up and dump all of the collector current through it’s emitter into ground.  This will draw the S input to ground, flipping the water signal on.  The water signal will not turn off until the Reset line ( R  on the S-R flip flop ) is pulled low by the microcontroller. Speaking of which, R2 is a pullup to make sure we don’t accidentally reset the flip flop during sleep/wake-up cycles, because on some microcontrollers, like the Stamp used here, the pins switch to inputs for 18 ms until the interpreter takes control.

Working water detector prototype!

I put this design together and tested it – it couldn’t work better!   In fact, it’s able to detect the microamps that were able to push through my *body*.  The Hfe on the BC517 Darlington is 30k.  *Serious amplification!* Now I had a simple circuit that could remember a closed condition ( leak ) and could detect current flowing through megaohms ( necessary for detecting the presence of water ). Mission accomplished.

Messy, but functional!