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GENERAL
INTEREST
Soil Moisture Tester
for houseplants
Design by A. Baur
For those of us who don’t like to get their hands dirty, this simple tester
quickly checks the state of their plants and how much attention they need.
Recommended for all plant owners!
Plants seem to have something in common
with pets. They are usually acquired or given
with the best intentions, but not everybody
seems to be able to look after them properly.
Of course we do not expect everybody to
have green fingers, but when plants are not
watered enough they simply die. In any case,
too much neglect usually has fatal conse-
quences. Cactuses seem to survive
such a careless treatment the
longest and we have to admit that
these are the only plants that man-
age to survive at our offices.
So what can we do about it? It’s
simple really. All it needs is for regu-
lar checks to feel if the soil in the pot
has become too dry. But what is ‘too
dry’? Some people just don’t seem to
have the right fingers for this task. A
little electronics can be used to rid
us of this problem forever.
We’ve previously published a
Houseplant Buzzer in
Elektor Elec-
tronics
, but this was designed to
16
Elektor Electronics
4/2001
GENERAL
INTEREST
stay in the pot permanently and it
would beep when the soil became
too dry. It’s a wonderful idea, but it
could turn your plants into a group of
howling babies, which would get on
most people’s nerves!
This time we’ve used a different
approach. The circuit described here
might be very simple, but it’s a very
useful soil moisture tester. Two elec-
trodes are stuck in the soil and the
moisture level is shown on an LED
display. The LEDs have been
arranged into three colours: green
LEDs indicate that the soil is damp,
yellow LEDs that it’s getting a bit
dry and red LEDs warn that immedi-
ate action is required!
barely more complex than the block
diagram. It’s only really the supply
that is extra. Even this is very sim-
ple, consisting of only a small mains
transformer rated at 6 V/200 mA, a
single rectifier and smoothing capac-
itor (D1/C1) and a voltage regulator
which provides a stable +5 V.
The AC supply fed to the elec-
trodes is obtained in a very simple
manner: by taking it from the supply
just
before
the rectifier. The preset
used to set the sensitivity can be
found as P1. D2, R1 and C3 rectify
the moisture dependent AC signal,
which is then fed to pin 5 of IC2, the
heart of the circuit.
This IC used here is an old
favourite, the LM3914 bargraph dis-
play driver. This 18-pin IC converts
an analogue input to drive a 10-LED
(linear) display. The IC contains 10
comparators, which each are con-
nected to a reference voltage via a
precision resistor network. The
inverting inputs of the comparators
are connected to the analogue input
LM3914
000099 - 11
Figure 1. Damp soil is conductive. Some simple
electronics are used to give an indication of its
conductivity.
Principle
The operation of the tester depends
on the property of water to become
electrically conductive when it con-
tains dissolved alkalines, oxides, etc.
Damp soil can therefore be consid-
ered as an electrical conductor. The
drier the soil, the less conductive it
becomes and the higher its electrical
resistance.
Figure 1
outlines the working of
our circuit. Two electrodes, which
are just lengths of wire, are used to
measure the resistance of the soil. A
small current is made to flow
through the electrodes, which cre-
ates a voltage across them which is
dependent on the soil resistance and
hence on its moisture content. This
voltage is rectified and used by an IC
to drive a 10-LED display, which rep-
resents the voltage level. Since the
minimum required moisture level
varies between plants, it’s been
made variable by use of a preset.
The reliability of the electrodes is
of the utmost importance for this
design. In a damp environment, even
a small direct current through the
electrodes would cause one to oxi-
dise and the other to dissolve. To
prevent this electrolytic breakdown
from occurring, we pass an alternat-
ing current through the electrodes,
rather than a direct current. The con-
tinuing change in polarity prevents
this breakdown from happening.
via a buffer stage. The LEDs are driven
directly by the comparator outputs.
Pin 9 is used to set the display to bar-mode
or dot-mode. In the first case JP1 should be
+5V
C4
D3
JP1
100n
D4
3
D5
9
5
SENSOR
MODE
SIG
D2
10
L10
11
L9
L8
L7
L6
L5
L4
L3
D6
1N4148
12
IC2
6
7
RHI
13
D7
REFOUT
14
R2
15
D8
16
P1
500k
lin
LM3914
17
D9
8
4
18
REFADJ
L2
1
RLO
L1
D10
2
D11
R1
R3
C3
D12
10µ
16V
IC1
LM7805
+5V
F1
D1
Tr1
1N4001
200mA
C1
C2
Simple schematic
470µ
16V
10µ
16V
220V / 6V
200mA
000099 - 12
A quick look at
Figure 2
is enough to
ascertain that the full circuit is
Figure 2. The full circuit is barely more complex than the block diagram.
4/2001
Elektor Electronics
17
GENERAL
INTEREST
shorted, in the second it can be left open cir-
cuit. Obviously the dot-mode gives the least
current consumption of the IC.
Potential divider R2/R3 sets the reference
voltage. The total value of both resistors also
determines the brightness of the LEDs.
C4
sensor
JP1
D2
R2
HOEK4
P1
IC2
R3
D3
D4
D5
D6
D7
D8
D9
D10 D11 D12
K1
Construction
~
TR1
D1
000099-1
C2
R1
~
C1
Due to the small number of components used,
it’s unlikely that the construction of the tester
will give you sleepless nights. The use of the
PCB shown in
Figure 3
makes things even
easier, but unfortunately the PCB is not avail-
able via the Readers Services so you would
have to etch it yourself.
Start the construction with the lowest
components (resistors); that tends to be eas-
iest. You should preferably use a socket for
IC2. Take care that you get the polarity right
for the diodes, electrolytics (C1, C2 and C3)
and the LEDs (short leg = cathode).
The small mains transformer (Tr1) is
mounted onto the PCB last. Make sure that
you use a sound and well isolated cable (with
a strain-relief) between the mains and the pri-
mary of Tr1. Carefully check the finished PCB
before applying mains power and never work
on the circuit when it’s plugged into the
mains!
The circuit should be mounted in a safe
plastic case, with a label stuck on the bottom,
stating the mains voltage and the value of the
fuse. A pair of sockets for banana plugs is
mounted on the case for the connection to the
electrodes.
The electrodes are made from two lengths
of stiff, isolated copper wire, about 10 cm long
and 1 mm thick. 4 cm of insulation is removed
from the ends, which are then tinned. This is
to prevent the copper wire from oxidising.
The connection between the electrodes and
the circuit could be made with two lengths of
flexible stranded cable.
F1
IC1
C3
HOEK1
0,2 A
Figure 3. Printed circuit board layout for the moisture tester. There is also room to
mount the small mains transformer (PCB not available ready-made).
COMPONENTS LIST
D3,D4,D5 =LED, green
D6-D9 = LED, yellow
D10,D11,D12 = LED, red
IC1 = LM7805
IC2 = LM3914-N
Resistors:
R1 = 100k
Ω
R2 = 2k
7
R3 = 2k
Ω
2
P1 = 500k
preset H
Miscellaneous:
Tr1 = mains transformer, secondary 6
V 200mA ( e.g., Monacor/Monarch
VTR-1106)
F1 =fuse, 200 mA, with PCB mount
holder
JP1 = short-circuiting jumper
K1 = 2-way PCB terminal block, lead
pitch 7.5mm
Capacitors:
C1 = 470µF 16V radial
C2,C3 = 10µF 16V radial
C4 = 100nF
Semiconductors:
D1 = 1N4001
D2 = 1N4148
Calibration and usage
Once the supply has been switched on and
the electrodes have been connected, the
tester is as good as ready for use. But first
preset P1 needs to be adjusted. All you need
for this is a glass of tap water. The electrodes
are inserted into the glass of water and
should be kept between 1 and 2 cm apart.
This corresponds to the maximum moisture
level, so we have to adjust P1 until the top
green LED (D3) just lights up and D4 just
extinguishes. When the electrodes are
removed from the water, you should see one
of the red LEDs (D10, D11, D12) light up.
Since this absolute maximum level will not
occur very often, the tester could be cali-
brated more practically. A pot plant
should be watered liberally, after
which the electrodes are inserted
into the soil, again keeping them
between one and two cm apart. P1
is then adjusted until one of the
green LEDs lights up. You will prob-
ably find many opportunities to
check that one of the three red LEDs
lights up when testing a plant that
hasn’t been watered for three weeks.
And that’s it!
After the previous description it
should be clear how the tester
should be used. The electrodes
should always be kept the same dis-
tance apart (between one and two
cm), perhaps using a spacer, and the
tinned ends should always be com-
pletely inserted into the soil.
(000099-1)
18
Elektor Electronics
4/2001
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