Parts:R1___________470K1/4W 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)　

　　Device purpose:

　　This circuit is intended to signal when a plant is needing water. A LED illuminates at maximum brightness when the ground in the flower-pot is too dry: it dims gradually as the water‘s content in the pot grows, turning off when the optimum moisture‘s level is reached. This condition is obtained trimming R3.

　　Circuit operation:

　　IC1D forms a square wave oscillator with approx. 10/90 mark-space ratio. It feeds the output probe P1 and its signal, inverted by IC1A is compared with that picked-up by P2 in the NAND gates IC1B & IC1C in parallel, driving the LED. When a low resistance exists between the probes, due to an high water‘s content in the flower-pot, the LED is off, turning gradually on as the resistance between the probes increases.

　　Notes: