Capacitive Soil Moisture Sensor V1.2

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.

PACKAGE INCLUDES:

• Capacitive Soil Moisture Sensor Module
• 3-wire interface cable, 20cm long

KEY FEATURES OF CAPACITIVE SOIL MOISTURE SENSOR MODULE:

• Analog output of moisture content
• More corrosion resistant than resistive type of sensor
• 3.3 or 5V operation.  Low power so may be driven from digital pin on MCU

This soil moisture sensor module uses capacitance rather than resistance to determine the water content of soil.  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.

Theory of Operation

The module uses a NE555 CMOS timer.

A peak voltage detector converts the waveform from the NE555 into a DC voltage that can be read by the ADC input of a microcontroller.

When the probe is exposed to moisture, it affects the capacitance of the circuit which in turn affects the peak amplitude of the signal and therefore the DC voltage output that is being monitored by the MCU.  Higher moisture = lower DC voltage output.

If you want to do a deep dive into the theory of how this thing works including schematics, there is an interesting though somewhat technical read where researchers reverse engineered how modules of this type work.

Basic Usage

The probe is inserted into the soil to be monitored.  The horizontal line shows the maximum depth that the probe should be inserted.  The green area shown to the right is the recommended depth to place the probe.  It is important that the electronics at the top of the probe are not subject to water or water splashing to avoid damage.

The analog output of the probe is monitored by the analog input on an MCU like an Arduino.

The measurement range between wet soil and dry soil is determined experimentally by measuring the soil under different conditions and the software configured to detect when the soil is too dry, too wet or just right.

Module Connections

There is a 3-pin JST PH2.0 type connector on the probe.  One end of the supplied cable plugs into the connector and the other end is a standard Dupont style 3-pin female connector.  The cable is color coded with black for ground, red for VCC and yellow for AOUT

1×3 Connector

• GND = Ground, must be common with the MCU
• VCC = 3.3V – 5.5VDC.  May be powered from a digital output pin on a MCU
• AOUT = Analog output usually connected to an analog input on a MCU

TECHNICAL SPECIFICATIONS