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)
Suggested by Mihai, Radu RO.