These soil moisture sensor modules use capacitance rather than resistance to determine the water content of soil. This offers significant advantages over resistive measurement. These sensors have only one probe, no exposed metal to rust, and don’t harm your plants by introducing electricity into the soil. They output an analog signal that is proportional to how wet the soil is.
These sensors make use of a 555 timer IC and operate by measuring how quickly (or slowly) a capacitor charges through a resistor, but in these sensors the capacitor is not a literal component, but is formed by two PCB traces that are near one another. Their capacitance, and therefore their charging rate, changes in response to how much water is around them.
Analog output of moisture content
More corrosion resistant than resistive type of sensor
Operating Voltage: 3.3 to 5.5V
Operating Current: < 5mA
Output Voltage at 5V: 1.5V to 3V
Sensor Probe L x W (PCB): 98 x 23mm
Note that this sensor can only provide a qualitative measurement of soil moisture. As the soil gets wetter, the output value decreases, and as it gets drier, the output value increases. When powered at 5V, the output ranges from about 1.5V (for wet soil) to 3V (for dry soil)
Low power so may be driven from digital pin on MCU
The Capacitive Soil Moisture Sensor Module determines the amount of soil moisture by measuring changes in capacitance to determine the water content of soil.
This can be used in an automatic plant watering system or to signal an alert of some type when a plant needs watering.
The main down-side to the common fork type resistance sensor is that the probes inserted into the soil must be conductive bare metal and the small electrical current that flows between them results in corrosion of the probes through electrolysis over time.
The capacitive probe improves that situation because the sensor metal inserted into the soil can be covered in solder resist to minimize corrosion and electrical current is not flowing through the soil to induce electrolysis. The main weakness in the lifespan of the probe is the uncoated cut sides of the PCB which can absorb moisture over time as well as the exposed electronics at the top of the probe if they get splashed by water. The customer can add further protective coating such as clear fingernail polish or similar coating if desired without seriously affecting the performance of the probe.
