PLANT MOISTURE DETECTOR
ELECTRONIC PROJECT
When we think of solar cells or panels, what springs to mind
immediately is producing power — only natural, given the primary purposes of
such devices; but we don’t necessarily think of using them in applications where
the fact they don’t produce power in the absence of light may actually be
useful. Yet this is just the case in the project discussed here.
The project, then, is intended for detecting moisture here
on Earth using solar power. It’s primarily aimed it at those of you who like to
brighten up their house or flat with pot plants, but are afraid of inadvertently
letting them die of thirst.
Using its two electrodes, formed from two stiff pieces of
bare copper wire, it can be stuck into the pot of any plant you want to monitor.
As long as the plant isn’t thirsty, i.e. the soil in the pot is moist enough,
it will just sit there and do nothing at all. But when the soil dries out below
a certain threshold (which you can adjust to suit the soil used and the plant
being monitored), it starts ‘squealing’ to tell you it is time to give the poor
plant a drink.
We obviously want it to work only during the day. This is
where the solar cell comes in handy: on the one hand, it is used to power the
circuit, making it totally stand-alone; and on the other, the lack of power
produced when in darkness means the circuit is automatically silenced at night.
Once we’ve adopted this principle, the circuit is remarkably
simple, using just a single 4093 CMOS logic chip, which contains four 2-input
Schmitt trigger NAND gates. The first gate, IC1a, is wired as a very low
frequency astable oscillator. When its output is at logic high, which occurs at
regular intervals, it enables IC1b, which is also wired as an astable
oscillator, but this time at an audible frequency. The signal from IC1b then
has to pass through IC1c, which can only happen if E1 and E2 are not connected,
allowing the corresponding input to be pulled up to logic High. You will have
realised that E1 and E2 are the electrodes stuck into the soil and so will not
be connected if the latter is not sufficiently conductive, i.e. when it starts
to dry out. The threshold at which gate IC1c turns on is obviously adjustable
using P1. Depending on whether or not the circuit is supplied from a voltage
greater or less than 3 V — which depends on the solar cell used, as we’ll be
seeing in a moment — the piezo sounder can be connected either directly between
IC1c output and the positive supply, or between the outputs of IC1c and IC1d, which
is wired as a simple inverter and so enables you to double the output voltage.
CLICK ON THE IMAGE TO ZOOM IN
The circuit is very simple to build, and you can just build
it on a piece of prototyping board. The sounder used must of course be one
without built-in electronics, as here it is just being used as a simple
transducer. If it’s a large-diameter flat type, you could, for example, glue it
onto the casing of IC1, while if it’s a small-diameter type with rigid pins, it
can be soldered directly onto the end of the PCB where its connection pads are
located. As for the solar cell, for the prototype Solems devices were used,
available for example from Selectronic France [2]; these are marked with a very
simple 3-figure code in the form NN/LL/WW, where NN is the number of elements
in the cell (each element producing around 0.5 V), LL is the length of the cell,
and WW the width, in mm. Equivalent cells from other suppliers may work equally
well though. Although in theory standard CMOS logic ICs only work above 3 V,
the majority of those we tried in our circuit did actually work with a lot
less, which means that if you’re on a tight budget (or have a lot of plants to
monitor!), you can use the cheapest cells.
If your budget is a little higher, and you don’t want to bother
selecting the 4093 CMOS ICs, go for a 07/048/016, or better still a 07/048/032,
which will allow the circuit to work under excellent conditions as soon as the
illumination reaches around 1,000 lux. You can also cannibalize such cells from
solar-powered garden lights, which can often be found at giveaway prices in the
big DIY stores.
Given the size of the suggested PCB, the Solems cells can be
soldered directly onto the copper side of it. But when connecting the cell up,
do take care to be very quick soldering the leads to the two silvered pads at each
end of it. They are actually metallised directly onto the glass of the cell and
so are pretty fragile.
As soon as the cell is connected, if the two electrodes E1
and E2 are ‘in mid-air’, the circuit should start ‘squealing’, as long as it is
getting enough light. You can then solder two stiff copper wires onto E1 and E2
(e.g. stripped offcuts of 1.5 mm² / AWG16 domestic wiring cable) and spike the
circuit into the plant you want to monitor. Then all you have to do is adjust P1
so that the circuit cries for help when the soil has reached the level of dryness
you have chosen. If the frequency of the sound produced doesn’t suit you, you
can change it by increasing or reducing C2 and/or R2. Likewise, if you don’t
like its repeat frequency, you can change that by adjusting C1 and/ or R1.
