Why Capacitive Over Resistive?
Older resistive soil sensors corrode within weeks because they pass current through the soil. Capacitive sensors measure the dielectric constant of the soil instead — no exposed metal contacts in the soil, so they last years. The module outputs an analog voltage between 0 V (submerged) and ~3 V (bone dry) which maps neatly to Arduino's 10-bit ADC.
The relay acts as a high-current switch to run the 5V pump from a separate power rail, so the Arduino never sources pump current directly.
⚠️ Warning
Power the submersible pump from a dedicated 5V 1A power supply — NOT from the Arduino's 5V pin, which is limited to 500 mA from USB. Sharing power rails will brown-out the board and cause erratic ADC readings.
Components
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Wiring
Soil sensor: VCC → 3.3V, GND → GND, AOUT → A0.
(Using 3.3V prevents the ADC from saturating near 0 when submerged)
Relay module: VCC → 5V (Arduino), GND → GND, IN → D7.
NO terminal of relay → pump positive; COM terminal → external 5V supply positive.
Pump negative → external 5V GND (share GND with Arduino).
OLED (I2C): VCC → 3.3V, GND → GND, SDA → A4, SCL → A5.
Live simulation
Steps
- 1Install libraries: Adafruit GFX and Adafruit SSD1306 via Library Manager
- 2Wire the sensor to 3.3V (not 5V) to prevent ADC saturation
- 3Power the pump from a separate 5V 1A supply through the relay
- 4Run the sketch and open Serial Monitor — note the raw ADC value in open air (AIR_VALUE) and when submerged (WATER_VALUE)
- 5Update both calibration constants in the sketch and re-upload
- 6Insert sensor into dry soil — pump should trigger below 40% moisture and stop at 70%
- 7Adjust DRY_THRESHOLD and WET_THRESHOLD to suit your plant species
