Circuit: Plant Humidity Monitor
February 11, 2026
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Houseplants rarely die suddenly. They decline slowly, often because of inconsistent watering. If you’ve experienced this problem, check out this clever plant humidity monitor circuit that Elektor presented in the summer of 1993.
At the heart of the design was a CD4047 oscillator (IC1) generating complementary outputs (Q and Q̅) at about 58 Hz. These signals formed a symmetrical AC drive for the soil electrodes. The resulting voltage at the wiper of preset P1 reflected the soil resistance and therefore its moisture content. A fixed 2.5-V reference — derived from the symmetric electrode current via R2 and R3 — was compared to this measurement using a TLC271 comparator (IC2). Because the electrode polarity alternates, a 4066 analog switch (IC3) swapped the comparator inputs in sync with the oscillator, ensuring accurate threshold detection on both half-cycles.
The output stage provided immediate visual feedback. A green LED indicated adequate moisture, while a red LED illuminated when the soil resistance exceeded the preset threshold (up to about 1.82 kΩ, depending on adjustment). Interestingly, the soil’s capacitance could have caused both LEDs to glow faintly at the transition point, offering a useful “borderline” indication. The electrodes were best made from carbon rods salvaged from old batteries to prevent corrosion, and the circuit ran from a regulated 5-V supply (via a simple 7805 regulator), allowing multiple sensors to share one power source if needed.
Editor's Note: This article originally appeared in a 1993 edition of Elektor. Given the project’s age, some components might not be available. Still, we think the design will inspire you to tackle a project of your own.
A Practical Circuit
The circuit, designed by J. Ruiters, measured the resistance between two electrodes inserted in the soil. As soil dries, its resistance increases. Rather than using DC (which would cause electrolysis and corrode the electrodes), Ruiters’s design drove the probes with a low-frequency alternating signal, keeping the measurement stable and extending electrode life.At the heart of the design was a CD4047 oscillator (IC1) generating complementary outputs (Q and Q̅) at about 58 Hz. These signals formed a symmetrical AC drive for the soil electrodes. The resulting voltage at the wiper of preset P1 reflected the soil resistance and therefore its moisture content. A fixed 2.5-V reference — derived from the symmetric electrode current via R2 and R3 — was compared to this measurement using a TLC271 comparator (IC2). Because the electrode polarity alternates, a 4066 analog switch (IC3) swapped the comparator inputs in sync with the oscillator, ensuring accurate threshold detection on both half-cycles.
The output stage provided immediate visual feedback. A green LED indicated adequate moisture, while a red LED illuminated when the soil resistance exceeded the preset threshold (up to about 1.82 kΩ, depending on adjustment). Interestingly, the soil’s capacitance could have caused both LEDs to glow faintly at the transition point, offering a useful “borderline” indication. The electrodes were best made from carbon rods salvaged from old batteries to prevent corrosion, and the circuit ran from a regulated 5-V supply (via a simple 7805 regulator), allowing multiple sensors to share one power source if needed.
The Plant Humidity Monitor Project
The original article, “Plant Humidity Monitor,” appeared in Elektor July/August 1993. Check out the article.Editor's Note: This article originally appeared in a 1993 edition of Elektor. Given the project’s age, some components might not be available. Still, we think the design will inspire you to tackle a project of your own.
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