House plants in general often have a
pretty hard time of it compared to their garden bound cousins, which
seem to get more than their fair share of watering, even if their owner
forgets, thanks to the British weather. With so many other things to
think about, the first reminder that many people get to water their
plants is when it is noticed that one or two are wilting or the leaves
are turning brown and dropping off! Modern central heating also ensures
that the soil in pots dries out much faster, making regular watering
more important, so that a little electronic help in remembering to do so
should be most welcome.
Circuit Diagram
The circuit suggested here, and shown in Fig.20, drives a piezo sounder, WD1, to
provide
a timely warning that the soil in the plant pot is almost dry.
Hopefully, the plants will be watered regularly so the alarm will remain
off but it may become active at any time and it is unlikely that the
plants will be watered immediately as the owner may be out. It may
therefore continue to sound all day before the plants are watered. To
avoid having to replace the battery too often, it is important to ensure
that the current drain in either condition is as low as possible. To
minimise the current drain during the alarm condition, a complementary
astable circuit built around transistors TR2 and TR3 is used. Its
operation is beyond the scope of this article, but it oscillates with a
frequency determined by resistor R2 and capacitor C1. With the component
values given the frequency will be about 2kHz, producing a fairly loud
sound from piezo sounder WD1. This device has a very high impedance and
so a load resistor, R3, is provided for TR3. Since both transistors
switch on and off together and remain off for a relatively long period
(dependant on the value of R2) compared to the time when they are on,
the average current drawn from the battery is very low, at about 1mA.
The output consists of short positive going pulses which turn on the
piezo sounder WD1. The operation of the oscillator is controlled by TR1.
When this transistor is on, the base of TR2 is held low and the circuit
cannot oscillate. The circuit relies on sensing the resistance of the
soil between two metal probes which are inserted into the pot close to
the plant. Completely dry soil will have a relatively high resistance
but this will fall as the moisture content is increased.
The series resistance of the probes, resistor R1 and potentiometer VR1
form a potential divider across the supply. With the soil moist, the
resistance of VR1 can be adjusted so that the voltage at the base of TR1
is at 0·6V, ensuring that this transistor is switched on and so
disabling the oscillator. As the soil dries out, the base-emitter
voltage of TR1 falls to a point at which it switches off sufficiently to
allow the oscillatorto function, producing an audible warning.
As
described earlier, this circuit produces short output pulses and
therefore draws only a small current when it is oscillating (about 1mA).
In the stand-by condition when the oscillator is switched off, the
current drain on the battery is only 10mA, so the battery should last a
long time.
Construction
The
circuit is built on a piece of stripboard having 7 strips × 15 holes,
as shown in Fig.21. Only one strip cut is required and there are no link
wires. Care should be taken to ensure that the transistors and the
sounder are connected the correct way around. The probes consist of two
stiff metal wires the length of which is not
particularly important
and will depend to a large extent on the size of the pot into which the
unit is placed. Copper is perhaps the easiest wire to get hold of (and
to solder). In the prototype, two 10cm lengths of 2·5mm diameter rigid
wire of the type used in house wiring were used. These were soldered
directly to the tracks at the positionsshown, the wire being too thick
to pass through the holes in the board.Since these are liable to break
off if the probes are pushed into hard earth, it is probably best to
solder the wires directly to the copper tracks straddling several holes.
This may then be strengthened by covering the joints and an adjacent
area of the board with epoxy glue. Alternatively, the wires may be
mounted a few millimetres apart on an insulating surface, such asthe
plastic box in which the unit is to be placed, and connected to the
board by flying leads.
Soundless Alarm
When
completed, place the probes in moist soil close to the roots of the
plant. Set VR1’s wiper to a fully anti-clockwise position, and then
adjust it until the circuit just fails to oscillate. Should the alarm
sound as the soil dries out but it is still judged to be too moist to
require watering, VR1 should be turned further clockwise. In some
situations, an audible alarm may not be desirable, in which case the
sounder can be omitted, and an l.e.d. plus ballast resistor of about 470
can be wired in place of R3, with the anode (a) connected to transistor
TR3’s collector, and the other side of the 470ohm resistor on the 0V
line. Omit R3 itself. Do not use the l.e.d. without the ballast resistor
as the current through it cannot be guaranteed to be within its limits,
even though the current is pulsed. The sounder and l.e.d. may both be
fitted, although this will result in a slightly increased current
consumption and a slightly reduced sound output, but should still be
adequate for most situations.
