Hello Arduino Community,
When it comes to measuring soil moisture professionally, the Irrometer-200SS is the go-to sensor for best accuracy. Now, there was always the challenge of rigging up a suitable circuit to read from this sensor, and the reference circuit by Irrometer is challenging.
I have tested and open-sourced this circuit that uses an LMC555 to read the resistance and convert it into frequency (read in Arduino using PulseIn). It is a low-cost circuit and reliable and well tested.
You can get more information, schematic, PCB, and code here: GitHub - Fasal-Tech/irrometer555-withMux: This repository contains the PCB design files and sample Arduino code to interface the popular Watermark Irrometer 200SS soil moisture sensor and read the equivalent soil tension in Frequency.