Circuit: 555-Based Automatic Watering System

If you’ve ever returned from holiday to find your houseplants either dried out or overwatered, you’ll appreciate this clever automatic watering design by Robert Edlinge. Originally published in Elektor July/August 2005, the circuit monitors soil moisture and waters plants only when necessary.

The Circuit

The design measures soil moisture using two electrodes placed in the soil. Wet soil conducts electricity better than dry soil, so the resistance between the probes rises as the soil dries out. When the moisture level drops below a set threshold, the circuit activates a small water pump.

Two 555 timer ICs form the heart of the design. IC1 generates a 500-Hz AC square wave for the probes. Using AC instead of DC helps reduce electrode corrosion and unwanted chemical reactions in the soil.
  The signal passing through the soil is amplified and rectified by a voltage doubler using diodes D2 and D3. When the soil is moist enough, transistors T2 and T3 conduct, disabling the second timer IC. As the soil dries, the signal level falls, T2 and T3 switch off, and IC2 activates the pump through transistor T1.

Importantly, the pump operates in timed bursts rather than continuously. This allows the soil time to absorb water and prevents flooding the roots. The watering interval is adjustable with potentiometer P2, while P1 sets the moisture threshold.

“The principle of the circuit is simple: first the soil dampness is measured by passing a signal through two electrodes placed in the soil,” Edlinge noted about the 555-based design. “The moisture content is inversely proportional to the measured resistance. When this measurement indicates it is too dry, the plants are given a predefined dose of water.”

Edlinge also included several practical safety features. Extra electrodes can detect overflow water in the plant saucer and shut the pump off, while a float switch in the reservoir protects the pump from running dry.

The probes can be made from copper or stainless-steel wire, and water is distributed through small plastic tubing with outlet holes near the plants. Despite being published two decades ago, the design remains a practical solution for reliable, worry-free plant care.

The 555-Based Watering System Project
The original article, “An Electronic Watering Can,” appeared in Elektor July/August 2005. Here is the article.

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Editor's Note: The article first appeared in a 2005 issue of Elektor. Due to the project’s age, some parts and components might not be available. Still, the project should inspire you to start a new design.