ZigBee Wireless Soil Moisture Sensor Design for Vineyard ...

2.3.5 Resistive Soil Moisture Sensor
The fundamental theory of the soil moisture sensor is to use a voltage divider
circuit to read the voltage drop across the soil sample and convert the voltage
readings to moisture value. When the soil moisture level increases the
impedance decreases [14]. The schematic is shown below Figure 15.
Figure 15 used an AC voltage to avoid the polarization of the soil sample. The
main advantage of this method is the electronics is really simple and the total
cost of the system is low. Therefore it is suitable for data loggers and wireless
sensor networks. The main disadvantage is the system has poor accuracy.
The final readings are affected by the soil mineralogy. Moreover system
overall response time is slow due to the granular matrix around the probe.
Figure 15: Impedance Base Moisture Sensor [14]
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