Rain sensor with NodeMCU ESP8266 — A0 and DO wiring, code

A rain sensor is a comb of copper tracks on a plate plus a small comparator board. Water bridges the tracks and the resistance falls, giving you an analog wetness reading on AO and a simple yes/no on DO.

On the NodeMCU, AO goes to A0 — the ESP8266’s only analog pin — and the board must run from 3V3 so the signal stays inside A0’s 0–3.3 V range. DO goes to any free digital pin; D5 is a safe one.

Wiring

Board quirks

• The ESP8266 has one analog input. The bare chip reads only 0–1 V, but the NodeMCU adds a 220 kΩ / 100 kΩ divider, so its A0 pin takes 0–3.3 V. That is why the module goes on 3V3, not VIN: at 5 V, a dry plate would push AO above the range.
• Because A0 is the only ADC, a second analog sensor (soil moisture, LDR) can’t share it. Use the rain board’s DO output for one of them, or add an ADS1115 over I2C.
• Don’t call analogRead(A0) in a tight loop: on the ESP8266 constant ADC reads upset the Wi-Fi radio. The sketch averages 8 spaced samples once a second.
• Reading goes down as the plate gets wet: about 1000 dry, a few hundred in steady rain. Every plate is different — note your own dry and wet values before setting thresholds.
• The plate corrodes quickly with DC on it, especially in a monsoon that lasts months. Mount it tilted so water runs off, and replace it each season (₹40–60).

Code

// Rain sensor (FC-37 / YL-83 plate + LM393 board) on a NodeMCU (ESP8266).
// VCC → 3V3, GND → GND, AO → A0, DO → D5 (GPIO14).
// The NodeMCU's own divider maps 0–3.3 V on A0 to 0–1023, so AO goes straight in.
const uint8_t RAIN_DO = D5;  // LOW when the plate is wetter than the trimmer setting

void setup() {
  pinMode(RAIN_DO, INPUT);
  Serial.begin(115200);
  Serial.println();
  Serial.println("Rain sensor: A0 = wetness, D5 = threshold output");
}

void loop() {
  // analogRead(A0) in a tight loop disturbs Wi-Fi on the ESP8266, so take a few
  // spaced samples and wait between readings.
  uint32_t sum = 0;
  for (int i = 0; i < 8; i++) {
    sum += analogRead(A0);
    delay(2);
  }
  int raw = sum / 8;                         // about 1023 = dry, lower = wetter
  int wet = constrain(map(raw, 1023, 0, 0, 100), 0, 100);  // rough 0–100 % wetness
  bool raining = digitalRead(RAIN_DO) == LOW;
  Serial.printf("A0 %4d  wetness %3d %%  DO %s\n", raw, wet, raining ? "RAIN" : "dry");
  delay(1000);
}

// Rain sensor (FC-37 / YL-83 plate + LM393 board) on a NodeMCU (ESP8266).
// VCC → 3V3, GND → GND, AO → A0, DO → D5 (GPIO14).
// The NodeMCU's own divider maps 0–3.3 V on A0 to 0–1023, so AO goes straight in.
const uint8_t RAIN_DO = D5;  // LOW when the plate is wetter than the trimmer setting

void setup() {
  pinMode(RAIN_DO, INPUT);
  Serial.begin(115200);
  Serial.println();
  Serial.println("Rain sensor: A0 = wetness, D5 = threshold output");
}

void loop() {
  // analogRead(A0) in a tight loop disturbs Wi-Fi on the ESP8266, so take a few
  // spaced samples and wait between readings.
  uint32_t sum = 0;
  for (int i = 0; i < 8; i++) {
    sum += analogRead(A0);
    delay(2);
  }
  int raw = sum / 8;                         // about 1023 = dry, lower = wetter
  int wet = constrain(map(raw, 1023, 0, 0, 100), 0, 100);  // rough 0–100 % wetness
  bool raining = digitalRead(RAIN_DO) == LOW;
  Serial.printf("A0 %4d  wetness %3d %%  DO %s\n", raw, wet, raining ? "RAIN" : "dry");
  delay(1000);
}

Expected serial output

Rain sensor: A0 = wetness, D5 = threshold output
A0 1012  wetness   1 %  DO dry
A0  684  wetness  33 %  DO dry
A0  402  wetness  60 %  DO RAIN

Rain sensor: A0 = wetness, D5 = threshold output
A0 1012  wetness   1 %  DO dry
A0  684  wetness  33 %  DO dry
A0  402  wetness  60 %  DO RAIN

Common errors