Parts:

R1___________470K1/4W Resistor 
R2_____________3K3  1/4W Resistor 
R3___________100K   1/2W Trimmer Cermet  
C1_____________1nF  63V Polyester CaPACitor 
C2____________47&mICro;F  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:

• A square wave is used to avoid probes' oxidization.
• Probes CAN be long nails, carbon rods obtained from dISAssembled exhausted 1.5V batteries, or even a couple of screwdrivers.
• The probes must be driven in the pot's ground a few inches apart.
• Due to 3V supply, the LED needs not a limiting resistor.
• Power consumption: LED off = 50µA; LED full on = 1mA.
• To switch-off the circuit, you can short the probes. In this case SW1 can be omitted.
• Using an high-efficiency LED, brightness variations are better emphasized. In this case a limiting resistor could be necessary.