Development of economical microcontroller-based soil moisture sensor using time domain reflectometry

Time-domain reflectometry (TDR) is a proven and popular measurement principle for evaluating frequency-dependent electric and dielectric properties of various materials including soil. However, the high price for TDR measurement equipment and complex laboratory setups is often a limiting factor for cost-sensitive applications such as in farming. This paper reports on the development of a low cost soil moisture sensor based on TDR. Equivalent time sampling was utilized to extent measurement frequency range. The developed sensor system is based on ARM Cortex M-4 microcontroller which working on a 25-KHz signal and virtual sampling resolution within 5 ns. While implemented using relatively cheap microcontroller, special approaches are used to eliminate systematic error, so that the sensor system had good accuracy in measurement range 0-100% Rh, error 3,49% and measurement time around 2,5 minutes.