Feb 28, 2013


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.
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.

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