Overview

The Capacitive Soil Moisture Sensor (v1.2 / v2.0) is an analog soil moisture probe designed to overcome the rapid electrode oxidation and electrolysis corrosion that plague traditional resistive soil sensors (like the YL-69 / FC-28).

Instead of passing direct electric current through the soil, the module forms a coplanar capacitive element using insulated copper traces embedded within the fiberglass PCB probe. An onboard 555 timer IC (typically the low-power CMOS TL555I or NE555) generates a high-frequency square wave (~1.5 MHz). As soil moisture increases, the dielectric constant ($\epsilon_r$) of the surrounding soil rises significantly (water $\approx 80$ vs dry soil $\approx 3-5$), changing the capacitance and producing an analog output voltage that decreases inversely with moisture levels.

Quick reference

Terminals

Electrical specifications

Operating principle & calibration

1. Inverse Voltage Relationship: Unlike resistive sensors, higher moisture results in LOWER output voltage. In air (0% moisture), the output is highest ($\approx 3.0\text{ V}$ at 3.3V supply); in pure water (100% saturation), the output is lowest ($\approx 1.2\text{ V}$).
2. Two-Point Calibration: Calibration requires recording two ADC values for each individual sensor:
   • $ADC_{air}$: Sensor held in free air (0% moisture baseline).
   • $ADC_{water}$: Sensor submerged up to the white line in a cup of water (100% moisture baseline).

$\text{Moisture \%} = \frac{ADC_{air} - ADC_{raw}}{ADC_{air} - ADC_{water}} \times 100\%$

Wiring

Example

const int sensorPin = A0;

// Replace these values with your 2-point calibration readings
const int AirValue = 620;   // ADC reading in dry air (3.3V reference)
const int WaterValue = 310; // ADC reading in water cup

void setup() {
  Serial.begin(9600);
}

void loop() {
  int rawADC = analogRead(sensorPin);
  
  // Constrain ADC reading within calibrated bounds
  int constrainedADC = constrain(rawADC, WaterValue, AirValue);
  
  // Map inverse voltage to 0-100% moisture percentage
  int moisturePercent = map(constrainedADC, AirValue, WaterValue, 0, 100);

  Serial.print("Raw ADC: ");
  Serial.print(rawADC);
  Serial.print(" | Moisture: ");
  Serial.print(moisturePercent);
  Serial.println("%");

  delay(2000);
}

Common mistakes

• Assuming 0 V is dry and 5 V is wet: Reversing the polarity interpretation leads to inverted readings (reporting 100% moisture in dry soil).
• Powering from 5 V when reading with a 3.3 V ADC (ESP32 / STM32): Supplying 5 V to $V_{CC}$ causes the $V_{dry}$ output to reach up to ~4.2 V, exceeding the 3.3 V maximum input limit of ESP32 GPIO pins. Always power from 3.3 V when using 3.3 V microcontrollers.
• Submerging the top electronics: Water contacting the 555 timer IC and surface-mount components alters the oscillator frequency or causes corrosion.
• Clone resistor errors: Some cheap v1.2 clones ship with an incorrect voltage regulator or missing zero-ohm resistor on the output buffer line. If output voltage stays constant regardless of moisture, inspect the R4/R5 surface-mount components near the connector.

Notes

• v1.2 vs v2.0: Version 2.0 adds a reverse polarity diode protection and a 3.3V voltage regulator (such as XC6206) to ensure consistent readings even when supply voltage fluctuates between 3.3 V and 5 V.