Gardening made easy with a soil moisture meter

Plant watcher ensures your plants won’t go thirsty

plant-watering-watcher.jpgPlant watcher ensures your plants won’t go thirsty

This is the gadget green thumbs across the globe have been waiting for. Italian designer Flavio Dellepiane put together a plant watering watcher that flashes an LED at a low rate when the soil in the pot plant becomes too dry.

Adjusting the 50k 10mm cermet trimmer allows the plant watering watcher the flexibility to adapt to different soil and pot combinations. With extremely low 3V power consumption, there’s really no reason for your plants to get anything less than five-star treatment.

Put your feet up this summer and let the watering watcher take the guess work out of your plant’s next drink.

Device Purpose

This circuit is intended to signal when a plant needs water. A LED flashes at a low rate when the ground in the flower-pot is too dry, turning off when the moisture level is increasing. Adjusting R2 will allow the user to adapt the sensitivity of the circuit for different grounds, pots and probe types.

Circuit Operation

IC1A and related components R1 and C1 form a 2KHz square wave oscillator feeding one gate input of IC1B through the voltage divider R2/R3 made variable by adjusting the trimmer R2.

If the resistance across the probes is low (as when there is a sufficient quantity of water into the pot) C2 diverts the square wave to ground, IC1B is blocked and its output will go steady high. IC1C inverts the high status to low, thus keeping IC1D blocked: the LED is off. When the ground in the flower-pot is becoming too dry, the resistance across the probes will increase and C2 will be no longer able to divert the square wave to ground. Therefore, IC1B output begins to transfer the 2kHz signal to IC1C which, in turn, passes it to the oscillator built around IC1D.

No longer disabled by a low level on its input, the IC1D oscillator slowly pulses Q1 base low causing the LED to flash, signalling the necessity to water the plant. The short low pulse driving the base of Q1 is actually a burst of 2kHz pulses and therefore the LED flickers about 2,000 times per second – appearing to the human eye as if the LED was steadily on for the entire duration of the pulse.


• A square wave is used to avoid problems of probes oxidization.

• Probes are made with two pieces of bare, stiff lighting cable of 1mm diameter and should be about 60mm long.

• One end of these probes is soldered on the underside of the board, directly to the tracks marked with P1 and P2 on the PCB layout. In this way, the probes result about 32mm apart, as shown in the photographs. Please note that all these parameters regarding probes material, dimensions and spacing are not critical.

• The probes should be driven fully in the pot’s ground.

• Current consumption: LED off = 150µA; LED on = 3mA for 0.1 sec. every about 2 sec. allowing the battery to last for years.

• The quiescent current consumption is so low that the use of a power on/off switch was considered unnecessary.

Build instructions

You can mount all the parts on a printed circuit board having the dimensions and layout as shown above in full size. Or you can use the suggested layout to accommodate the parts on a pre-drilled board of about the same size with holes spaced 2.54mm, as shown in the prototype photographs.

In this case, the connections of the components can be made on the underside of the board using pieces of wire (e.g. resistors, diodes and capacitors leads) cut to the right length and carefully soldered to the appropriate pins. In any case, you must at first insert and solder the diode D1 after bending its leads at 90 degrees, paying attention to the polarity: the coloured band indicates the Cathode.

Then you should insert the six resistors in the same way of the diode. There is no polarity to respect, but you must pay attention to the colour code of the main three coloured bands, indicating the value of each resistor. R1 and R4 have yellow-violet-yellow bands plus a golden band indicating the tolerance (5% for all resistors used in this circuit). R3 has brown-black-yellow bands plus the usual golden band. R5 = orange-orange-red, R6 = brown-green-orange, R7 = brown-black-brown.

Next step will be the insertion of IC1: please note that the IC must be inserted with pin 1 at the top left, viewing the circuit from above as shown in the Parts Layout drawing. Some ICs have an indentation notch on the body near pin 1, or a semicircle notch at the top of the case, as shown in the Parts Layout drawing. The trimmer R2 is easy to install: the arrangement of its three pins will prevent a wrong positioning. C1 and C2 insertion is straightforward: their value is clearly stamped on the case and you can insert them in any way. C3 and C4 are polarized capacitors, so you must pay attention to their polarity.

The Parts Layout drawing shows where the positive lead of these capacitors must be inserted. Please note that most electrolytic capacitors currently have the negative lead stamped on their body instead of the positive. Q1 transistor has three pins but, due to the particular shape of its case, it should be an easy task to insert the pins correctly.

The LED D2 should not be inserted flush to the board, but its leads should be cut at a length of about 10mm, in order to left about 7mm of room between LED bottom and board after the device is soldered in place.

Next, the two holders for the N cells will be soldered to the board, respecting polarities and the Probes are soldered directly to the board tracks as explained in the Notes above.

Finally, after encasing the PCB into the plastic box, a hole of about 5-6mm diameter must be drilled on the box lid, exactly above D2. Two small holes of about 1.5mm diameter must be drilled on one of the shorter sides of the case to allow the probes to pass through.

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