This plant watering monitor 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.
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.
Some notes on this plant watering monitoring system
R1,R4 – 470K 1/4W Resistors
R2 – 47K 1/2W Trimmer Cermet or Carbon
R3 – 100K 1/4W Resistor
R5 – 3K3 1/4W Resistor
R6 – 15K 1/4W Resistor
R7 – 100R 1/4W Resistor
C1 – 1nF 63V Polyester Capacitor
C2 – 330nF 63V Polyester Capacitor
C3,C4 – 10µF 25V Electrolytic Capacitors
D1 – 1N4148 75V 150mA Diode
D2 – 5mm. Red LED
IC1 – 4093 Quad 2 input Schmitt NAND Gate IC
Q1 – BC557 45V 100mA PNP Transistor
P1,P2 – Probes (See Notes)
B1 – 3V Battery (2xAA, N or AAA 1.5V Cells in series)
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.
Some notes on this plant watering monitoring system
- 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.
R1,R4 – 470K 1/4W Resistors
R2 – 47K 1/2W Trimmer Cermet or Carbon
R3 – 100K 1/4W Resistor
R5 – 3K3 1/4W Resistor
R6 – 15K 1/4W Resistor
R7 – 100R 1/4W Resistor
C1 – 1nF 63V Polyester Capacitor
C2 – 330nF 63V Polyester Capacitor
C3,C4 – 10µF 25V Electrolytic Capacitors
D1 – 1N4148 75V 150mA Diode
D2 – 5mm. Red LED
IC1 – 4093 Quad 2 input Schmitt NAND Gate IC
Q1 – BC557 45V 100mA PNP Transistor
P1,P2 – Probes (See Notes)
B1 – 3V Battery (2xAA, N or AAA 1.5V Cells in series)
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