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)
This little gadget encountered a long lasting success amongst electronics enthusiasts since its first appearance on this website in 1999. Nevertheless, in the correspondence exchanged during all these years with many amateurs, some suggestions and also criticism prompted me to revise thoroughly the circuit, making some improvements requiring the addition of four resistors, two capacitors and one transistor.
This resulted in a more stable and easy to setup device, featuring a more visible flashing indicator with no resort to ultra bright LED devices.
Extensive tests were also carried out with different flower-pots and probes. Although, as can be easily imagined, differences from various pots and probe types proved to be exceedingly high, typical resistance values across two 60mm long probes driven fully into the pot's ground about 50mm apart measured around 500 to 1000 Ohm with a high water content and about 3000 - 5000 Ohm when the ground was dry.
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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