DESCRIPTION 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. 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 TL555I CMOS timer to create a 1.5MHz clock.  The TL555I is similar to the ubiquitous NE555 but is newer CMOS technology with a higher frequency capability and other improvements. A peak voltage detector converts the waveform from the TL555I 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. PACKAGE INCLUDES: 1x Capacitive Soil Moisture Sensor Module 1x 3-wire interface cable, 8″ long