This simple circuit was
originally devised as an add-on for the Water
Meter project, but can be used as a standalone
device or for general experimentation more of
As can be seen from the
schematic in Figure 1, the detector is built
around a quad 2-input Schmidt trigger NAND gate of
which only two gates are used, the two remaining
gates have their inputs wired together and taken
high. CMOS devices should never have their gates
floating and need to be tied high or low, looking
at the data sheet for the 4093B the IC consumes
less current if spare gates are tied high.
Gate IC1a is wired as a
traditional astable oscillator whose frequency is
determined by R1, C1 in this case around 1.3KHz.
The output from this
gate goes via DC blocking and wave shaping
capacitor C2,to one side of the sensor. This
capacitor also helps to compensate for stray
capacitance caused by the sensor. Under quiescent
conditions the square wave output gets as far as
terminal X1-1 and goes no further.
When water comes into
contact between terminals X1-1 and 2 the circuit
is made and the square wave now travels via C3,
through the charge pump level restorer C3, D1 and
C4,D2. After exiting D2 the square wave is now
just a DC high level (+5v), as IC1B is wired as an
inverter this causes the output at pin 4 to go
low. R5 is used as pin protection to stop
excessive current occurring if there is a short or
similar for the following uC circuitry.
This resistor can be
left out if using either of the output circuits in
2. This just leaves explanations for C4 and
R2, these two components help with the sensitivity
of the of the device and R2 also ensures that the
inputs of IC1b are pulled low in the absence of a
The sensor in its most
basic form is just two strips of copper that are
shorted out when rain covered by water. Photo 1,
shows two examples, one using Veroboard/stripboard
and the other proto board.
This particular proto
board is ready tinned, if using stripboard then a
layer of solder along the tracks will help to stop
the copper corroding as shown in the photo. In
both cases where the connecting wire is soldered
to the tracks some waterproof polymer sealant
should be liberally daubed around the connection
on both side of the board. This can be the left
over sealant from fixing leaking showers and the
like. The sensitivity to the type of rain can be
altered by experimenting with the distance between
the two terminals (the wider the gap the heavier
the downpour has to be).
Photo 2 shows two
stainless steel dough hooks borrowed from a food
mixer that I technically enhanced (in other words
it no longer works). These make excellent soil
moisture detectors and could be used with the
circuit to signal lack of moisture in a plant bed,
sensitivity here depending solely on how far apart
they are placed and the type of soil.
If not connecting
this to a microcontroller, then Figure 2a-c has
some suggestions for alternative outputs.
In 2a, the two spare gates
IC1c,d have been paralleled together and connect
directly to a LED. 2b shows a connection to a
relay, note in this circuit and that of 2c the
relay and the switched output may be at a
than that powering IC1.
IC1 can be powered from 3 – 15V.
There is nothing crucial about this
circuit and it can be constructed on stripboard.
Photo 3 shows the prototype unit constructed on IC
proto board available from Tandy (Radio Shack).
The board fits quite nicely into the case used for
the water controller, again the casing is left up
The Fun Bit
Experiments with this basic circuit.
An oscilloscope is useful to see waveforms but not
Is trying out any NAND,
NOR or INVERTER IC that you may have lying around.
They must have Schmidt trigger inputs though.
Despite theoretically the 74LS132 ought to work,
two samples I had refused to oscillate with the
components specified. However mine were from a
never built project from 1981 with more recent
versions you may have better luck. The 4093 must
be the buffered
The astable frequency is not that
crucial so C1 can be altered, R1 should not be
lower than 1K or higher than 1M.
With an oscilloscope
watch how the wave form behaves with C2,3 out of
circuit or with different values. Values should
not be more than 1uF or less than
Remove C3,4 and C1,
connect pin 3 of IC1 directly to the D1,2
junction. Connect the probe in place of
The probe would need to
be altered to two pieces of insulated cable with
their far ends waterproofed, the cable will need
to be about 12” or so long glued to a plastic
substrate – a plastic rule would be fine. Connect
one end of the probe to 0v and the other to the
input junction of IC1a and R1. You now have a the
basis for a capacitance sensor. Due to the
impurities of water when the two waterproofed ends
are immersed in water there will be an alteration
in the capacitance of the probe and the frequency
should change. A capacitance meter will show that
the capacitance can change by about 30pF if the
probes are dipped in water.
If you want the signal
output to go high in the alarm condition, then one
of the spare gates can be used to invert the
output , then you could change the PNP transistor
to an NPN equivalent.
The permutations for
experimentation is endless.