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Plant Humidity Monitor Schematic Circuit Diagram

Monitoring Plant Soil Humidity with a Simple Circuit

This compact circuit serves as a soil humidity monitor for plant care and proves particularly beneficial for those who occasionally forget to water their plants, leaving them parched on a windowsill.

Creating a Plant Humidity Sensor

The plant humidity sensor is constructed using cost-effective components and delivers dependable feedback when the soil is too dry. Circuit IC1 is configured as an oscillator, generating two complementary switching signals, Q and Q, with a frequency of approximately 58 Hz. These signals ensure that Q and Q\ are never simultaneously active, forming an alternating current source that effectively prevents electrolysis on electrodes A and B.

Utilizing the Wiper of P1 for Humidity Sensing

The voltage at the wiper of P1 is contingent on the electrode resistance (RA-B) and, by extension, the soil’s humidity level. IC2 compares this voltage with a reference, indicating whether the soil humidity has dropped below a predefined threshold, signifying that it’s time to water the plant. The reference is derived from the symmetrical electrode current and is drawn from the junction R2-R3, maintaining a consistent 2.5 V potential concerning the ground.

Comparator Operation and Potential Measurement

Despite the alternating electrode current, the comparator’s operation is straightforward. It performs a direct current (d.c.) voltage comparison, effectively gauging the potential difference. Assuming Q=0 and Q/=+5 V, the wiper of P1 reaches a potential U1 concerning the ground.

Plant humidity monitor Schematic diagram

Swapping Input Signals for Comparator Operation

In this configuration, the wiper’s position is determined by 5-U1 when the electrode current is reversed (Q=+5 V; Q=0 V). If, in the initial scenario (Q=0 V and Q=+5 V), the wiper potential is higher than the reference, it automatically becomes lower than the reference in the alternate situation. To ensure the comparator triggers in both cases, the input signals are interchanged using IC1’s Q and Q\ outputs. This reversal is executed through the four electronic switches within IC3.

Indicating Soil Dryness with LEDs

The red LED, D2, illuminates when the soil is excessively dry, indicating that U1 surpasses 2.5 V. This condition corresponds to a soil (inter-electrode) resistance of 0-1.82 kΩ, depending on the setting of preset P1. As P1 is adjusted closer to the electrode connection, a higher soil resistance (indicating drier soil) is required for the green LED to turn off and the red LED to activate.

Useful Indication of Soil Moisture Levels

Remarkably, the soil’s capacitance might cause both the red and green LEDs to illuminate simultaneously, indicating a state of being ‘between wet and dry.’ This simultaneous lighting provides a useful indication of the soil’s moisture level falling within that intermediate range.

Plant humidity monitor Schematic diagram

Creating Effective Plant Soil Electrodes

For optimal results, the electrodes should be crafted using carbon rods obtained from used batteries. This cost-effective solution not only saves money but also prevents corrosion. These electrodes are inserted into the plant soil approximately 4 cm apart. The precise adjustment of P1 will vary based on the specific plant being monitored and must be determined through practical testing. In most cases, setting P1 to mid-travel position yields satisfactory outcomes.

Stable Power Supply for the Sensor

The sensor demands a consistent 5 V power supply. To fulfill this requirement, power is sourced from an affordable mains adaptor. The direct output voltage from the adaptor is purified and stabilized using a 7805 regulator. Since the sensor consumes a mere 5 mA current, this power supply can be utilized to power multiple sensors simultaneously, making it a versatile and efficient solution.

Dual Functionality: Power Supply and Central Indicator

Aside from supplying power to the sensor units, the power supply serves as a central indicator. LED D1 illuminates when any connected sensor unit indicates ‘dry soil.’ If none of the sensors indicates ‘dry soil,’ but at least one falls between ‘dry and wet,’ LED D1 lights with reduced intensity. This occurs when the voltage applied to the LED control input ranges between 2 V and 3 V, signifying a ‘between dry and wet’ condition. All sensor outputs must be connected to the LED control input on the supply, establishing a wired-OR configuration for accurate monitoring.

Parts list  (Monitor circuit)

Resistors:
  • R1 = 100 kΩ
  • R2, R3 = 15 kΩ
  • R4 = 820 Ω
  • R5, R6 = 680 Ω
  • R7 = 22 Q
  • P1 = 11kΩ preset
Capacitors:
  • C1 = 39 nF
  • C2 = 100 nF
  • C3 = 10 μF, 16 V
Semiconductors:
  • D1 = LG3369EH . (3 mm, low current green)
  • D2 = LS3369EH (3 mm, low current red)
  • D3 = 1N4148
Integrated circuits:
  • IC1 = 4047
  • IC2 = TLC271
  • 1C3 = 4066
Miscellaneous:

Enclosure 65x50x30 mm, e.g., Bopla EG406
PCB Ref. 934031

Parts list: (Power supply circuit)

Resistors:
  • R1, R2 = 10 kΩ
  • R3 = 220 Ω
Capacitors:
  • C1 = 100 μF, 25 V. radial
  • C2, C3 = 100 nF
  • C4 = 10 μF. 16 V. radial
Semiconductors:
  • D1 = 1N4001
  • D2 = LED. 5 mm, red
  • T1 = BC547B
Integrated circuits:
  • IC1 = 7805
Miscellaneous:

Enclosure 65x50x30 rum, e.g.. Bopla EG406
PCB Ref. 934032

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