Capacitive soil moisture sensor with Wemos D1 mini (A0)

A D1 mini with a capacitive soil sensor is the start of every plant-watering project. The sensor’s AOUT goes to A0, the D1 mini’s only analog pin, and it is powered from 3V3.

A0 is where the D1 mini differs from a bare ESP8266 module: an on-board divider lets it read about 0–3.2 V instead of 0–1 V, so the sensor connects directly. The sketch averages a few readings and turns them into a percentage after a two-point calibration.

Wiring

Board quirks

• The ESP8266 has one analog input. The bare chip reads 0–1 V; the D1 mini adds a 220 kΩ / 100 kΩ divider in front of it, so A0 on the header spans about 0–3.2 V → 0–1023. A second analog sensor needs an external ADC such as an ADS1115.
• Calibrate on your own sensor: note the raw value in dry air and in a glass of water (only up to the line on the board), and put them in DRY_RAW / WET_RAW. Higher raw = drier.
• Calling analogRead(A0) back-to-back in a tight loop on the ESP8266 is known to knock Wi-Fi off. The sketch spaces its 8 samples 10 ms apart, which is plenty.
• Many cheap “v1.2” boards are missing the regulator’s supporting parts or have the 555 timer swapped for a part that needs 5 V. If the reading barely changes between dry and wet at 3.3 V, the board is faulty — not your code.
• Never put the sensor above the line marked on the PCB, and seal the top edge with nail polish or heat-shrink before the monsoon — water on the electronics drifts the reading.

Code

// Capacitive soil moisture sensor (v1.2) on a Wemos D1 mini (ESP8266).
// VCC → 3V3, GND → GND, AOUT → A0. The D1 mini's A0 divider reads about 0-3.2 V as 0-1023.
// Calibrate: note the raw value in dry air and in a glass of water, and put them below.
const int DRY_RAW = 760;  // sensor in dry air (yours will differ)
const int WET_RAW = 380;  // sensor in water up to the line

int readAveraged() {
  long sum = 0;
  for (int i = 0; i < 8; i++) {
    sum += analogRead(A0);
    delay(10);  // don't hammer the ADC: back-to-back analogRead() calls can drop Wi-Fi
  }
  return sum / 8;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  Serial.println("Soil sensor on A0 - raw 0-1023 (about 0-3.2 V)");
}

void loop() {
  int raw = readAveraged();
  int pct = map(raw, DRY_RAW, WET_RAW, 0, 100);
  pct = constrain(pct, 0, 100);
  Serial.printf("Raw %d  (%.2f V)  Moisture %d %%\n", raw, raw * 3.2f / 1023.0f, pct);
  delay(2000);
}

// Capacitive soil moisture sensor (v1.2) on a Wemos D1 mini (ESP8266).
// VCC → 3V3, GND → GND, AOUT → A0. The D1 mini's A0 divider reads about 0-3.2 V as 0-1023.
// Calibrate: note the raw value in dry air and in a glass of water, and put them below.
const int DRY_RAW = 760;  // sensor in dry air (yours will differ)
const int WET_RAW = 380;  // sensor in water up to the line

int readAveraged() {
  long sum = 0;
  for (int i = 0; i < 8; i++) {
    sum += analogRead(A0);
    delay(10);  // don't hammer the ADC: back-to-back analogRead() calls can drop Wi-Fi
  }
  return sum / 8;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  Serial.println("Soil sensor on A0 - raw 0-1023 (about 0-3.2 V)");
}

void loop() {
  int raw = readAveraged();
  int pct = map(raw, DRY_RAW, WET_RAW, 0, 100);
  pct = constrain(pct, 0, 100);
  Serial.printf("Raw %d  (%.2f V)  Moisture %d %%\n", raw, raw * 3.2f / 1023.0f, pct);
  delay(2000);
}

Expected serial output

Soil sensor on A0 - raw 0-1023 (about 0-3.2 V)
Raw 742  (2.32 V)  Moisture 5 %
Raw 511  (1.60 V)  Moisture 66 %
Raw 498  (1.56 V)  Moisture 69 %

Soil sensor on A0 - raw 0-1023 (about 0-3.2 V)
Raw 742  (2.32 V)  Moisture 5 %
Raw 511  (1.60 V)  Moisture 66 %
Raw 498  (1.56 V)  Moisture 69 %

Common errors