A Novel Low-Cost Conductivity Based Soil Moisture Sensor

Water management for irrigation purposes is especially decisive in places prone to droughts because soil moisture sensors are economically unattainable for farmers. The sustainable usage of water should not be restricted by the elevated price of the system. In this paper, we present a low-cost sensor for the monitoring of soil moisture, which can be part of a smart irrigation system. The sensor is composed of two coils, one is powered with alternate current and the other one is used to measure the induced voltage. It is based on conductivity and uses the method of mutual inductance. We study five prototypes, which have different numbers of turns in each coil. We compare them in order to determine the best model. The best sensor is the one that consists of one coil with 40 turns (which is powered) and one with 100 turns (which is induced). The best frequency is 260 kHz, the coil is induced with 10 peak to peak voltage and the induced voltage, which is measured with an oscilloscope, changes with the soil moisture. At this frequency, the sensor presents the biggest difference in volts. The differences are 1.2 V between 0 and 6% of water volume in the soil; 0.8 between 6 and 8% water volume in the soil; and 1.6 V between 8 and 17% of water volume in soil. Considering these differences, we can safely formulate an equation to extract the soil moisture values with high accuracy.