Plant Watering Watcher

A flashing LED signals the necessity to water a plant  

Very low current consumption - 3V powered circuit  
Circuit diagram:

Parts:

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)

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.

Notes:

• 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.

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