Plants Watering Watcher Electronic Circuit Diagram

Plants Watering Watcher Electronic Circuit Diagram

Reviewed by: Lila

On: 13 Jun, 2016

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Category: Alarm

Plants Watering Watcher Electronic Circuit Diagram Rating:
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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 ...

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.


R1 470K 1/4W Resistor
R2 3K3 1/4W Resistor
R3 100K 1/2W Trimmer Cermet
C1 1nF 63V Polyester Capacitor
C2 47µF 25V Electrolytic Capacitor
D1 1N4148 75V 150mA Diode
D2 5mm. Red LED
IC1 4093 Quad 2 input Schmitt NAND Gate IC
P1,P2 Probes (See text)
SW1 SPST Slider Switch
B1 3V Battery (2 AA 1.5V Cells in series)


This little gadget encountered a long lasting success amongst electronics enthusiasts since its first appearance on this website in 1999. Nevertheless, in the correspondence exchanged during all these years with many amateurs, some suggestions and also criticism prompted me to revise thoroughly the circuit, making some improvements requiring the addition of four resistors, two capacitors and one transistor.

This resulted in a more stable and easy to setup device, featuring a more visible flashing indicator with no resort to ultra bright LED devices.

Extensive tests were also carried out with different flower-pots and probes. Although, as can be easily imagined, differences from various pots and probe types proved to be exceedingly high, typical resistance values across two 60mm long probes driven fully into the pot's ground about 50mm apart measured around 500 to 1000 Ohm with a high water content and about 3000 - 5000 Ohm when the ground was dry.

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 hight. 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.
The probes should be driven fully in the pot's ground about 30 - 50mm apart. Please note that all parameters regarding probes material, dimensions and spacing are not critical.
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. In any case, to switch the circuit completely off, you can short the probes.

Plants Watering Watcher Schematic

plants watering watcher 1.jpg

Using a Veroboard mother-board about the same size as the battery holder, a daughter-board was added to hold the remaining parts.


plants watering watcher 2.jpg

Parts list

IC1 -. CD74HC132
B1 -… Two AAA alkaline cells, with holder
C1, C3 - 1nF (0.001uF) or 2.2nF (0.0022uF)
C2 -… 100uF/16V electrolytic
C4 -… 220nF (0.22uF)
R1 -… 470K (all resistors 1/4W, 5%)
R2, R4 - 100K
R3 -. 3.9M
R5, R6 -.. 680
R7 -.- 15
R8 -. 47K
D1 . 1N4148 or 1N914
D2 . MV8191 or HLMP-D101A
Q1, Q2 .. 2N4403 or 2N3906

Circuit Description

IC1D is a CMOS Schmitt trigger oscillator at about 2KHz. It starts and continues to oscillate with a supply down to 1.24V (the lowest output voltage of my LM317 variable power supply) or less.
IC1A is an inverter.
IC1B is a Schmitt trigger NAND gate. Its output is low only when both inputs are at, or higher than the upper Schmitt trigger threshold voltage. With 47 ohms or less between the probes, an input is always low, so the output is always high. With a resistance of only R8 between the probes, the voltage across C3 is high most of the time, so the gate's output is low for ½ the oscillator's period. With a resistance that is halfway, then C3 is charged high by that resistance when the oscillator's output is high, then is discharged when the oscillator's output is low. When C3 is being discharged, then pin 12 of the gate is high, and pin 13 is also high until the discharging voltage of C3 reaches the lower Schmitt threshold voltage. During this time, the gate’s output is low. So the low time of the gate's output depends on the value of the resistance between the probes. This is Pulse-Width-Modulation of the low output of the gate.
IC1C is another CMOS Schmitt trigger oscillator at about 2Hz. D1 and R4 discharge C4 quickly so that its output is low for only about 15ms with a 3V battery, and about 25ms with a 2V battery.
The series connection of Q1 and Q2 performs like a NOR gate, so that the LED lights only when both inputs to the transistors are low.
R7 is a current-limiting resistor for the 1.8V LED. With a 3V battery, the LED current is about 35mA.

Circuit Operation

When the soil is very dry, the LED flashes brightly, since the soil's resistance is very high.
When the soil has been watered a few days before, but is drying, the LED flashes dimly,
When the soil is damp because it has been recently watered, the LED is off.

Note that different soils have a different resistance. Also, sometimes, watered soil will continue to have a high resistance until the soil absorbs the water, a delay of about one hour.

Although the LED's current is 8mA with a 3V battery, it is lighted for only a maximum of only about 1/64th of the time, so its maximum average current is only 550uA. The remainder of the circuit draws 200uA. The total is 750A for new batteries, and about 250uA for run-down batteries. Therefore the exponential current of 300uA will continue with 1000mA/hr batteries for 2000 hours, or about 4.6 months. The LED's current is logarithmic with the soil's resistance, so that when the resistance is one-half, then the LED's current is one-tenth. If you water the plants when they need watering, then the average LED current will be very low, and the batteries should last for about one year.

Project Assembly

Try to obtain the very bright and wide-angle LEDs that are listed. Samples are available from Fairchild.
Use tinned copper 1.5mm diameter buss-bar wire about 8cm long for the probes.
Use silicone caulking to attach and seal the Veroboard to the battery holder, and to seal the battery holder's contact holes.
Perhaps the project can be mounted in a plastic bottle for pills, available from a pharmacy (chemist?), with the probes sticking out of its lid.

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