This 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.
Improvements:
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.
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.
Circuit diagram:
Plant Watering Watcher Circuit Diagram
Parts:
R1 = 470K
R2 = 47K
R3 = 100K
R4 = 470K
R5 = 3.3K
R6 = 15K
R7 = 100R
C1 = 1nF-63V
C2 = 330nF-63V
C3 = 10µF-25V
C4 = 10µF-25V
D1 = 1N4148
D2 = 5mm. Red LED
Q1 = BC557 PNP Transistor
P1 = Probe (See Notes)
P2 = Probe (See Notes)
B1 = 3V Battery (2xAA, N or AAA 1.5V Cells in series)
IC1 = 4093 = Quad 2 input Schmitt NAND Gate IC
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.
