Rain sensor with Arduino Uno — rain levels without a corroded plate

A rain sensor is a plate of interleaved copper tracks plus a small LM393 comparator board. Water bridging the tracks lowers the resistance, so the analog output falls as the plate gets wetter. It is used for rain alarms, to close a window, or to skip a garden-watering cycle in the monsoon.

The Uno has one easy trick for this sensor. Instead of wiring VCC to 5V, wire it to D7 and turn that pin on only while reading. The plate then carries current for a few milliseconds every few seconds instead of all the time, and it lasts far longer. The board needs only a few mA, well inside what a Uno pin can give.

The sketch reads the analog value on A0 and the comparator output on D2 every 5 seconds, and grades the result from dry to heavy rain.

Wiring

Board quirks

• With constant DC on the plate, electrolysis eats the tracks in a few weeks of monsoon. Powering it from D7 for 20 ms every 5 s cuts that to almost nothing.
• The comparator board isn’t waterproof. Mount only the plate outside, tilted so water runs off, and keep the small board in a box indoors.
• AO is about 1000 when dry and drops towards 200–300 under heavy rain. The thresholds in the sketch are a starting point — check yours with a few drops of water.
• Mud and dust on the plate make it read “wet” for hours after the rain stops. Wipe it now and then.

Code

// Rain sensor (plate + LM393 comparator board) on an Arduino Uno R3.
// VCC → D7 (powered only while reading, so the plate doesn't corrode), GND → GND,
// AO → A0, DO → D2. AO falls as the plate gets wetter.
const uint8_t POWER_PIN = 7, AO_PIN = A0, DO_PIN = 2;

void setup() {
  pinMode(POWER_PIN, OUTPUT);
  digitalWrite(POWER_PIN, LOW);
  pinMode(DO_PIN, INPUT);
  Serial.begin(9600);
  Serial.println(F("Rain sensor - one reading every 5 s"));
}

void loop() {
  digitalWrite(POWER_PIN, HIGH);
  delay(20);  // let the comparator settle
  int wet = analogRead(AO_PIN);
  bool rainDO = digitalRead(DO_PIN) == LOW;  // DO goes LOW past the pot threshold
  digitalWrite(POWER_PIN, LOW);

  Serial.print(F("AO "));
  Serial.print(wet);
  Serial.print(F("  DO "));
  Serial.print(rainDO ? F("wet") : F("dry"));
  Serial.print(F("  -> "));
  if (wet > 900) Serial.println(F("Dry"));
  else if (wet > 600) Serial.println(F("Drizzle"));
  else if (wet > 350) Serial.println(F("Rain"));
  else Serial.println(F("Heavy rain"));
  delay(5000);
}

// Rain sensor (plate + LM393 comparator board) on an Arduino Uno R3.
// VCC → D7 (powered only while reading, so the plate doesn't corrode), GND → GND,
// AO → A0, DO → D2. AO falls as the plate gets wetter.
const uint8_t POWER_PIN = 7, AO_PIN = A0, DO_PIN = 2;

void setup() {
  pinMode(POWER_PIN, OUTPUT);
  digitalWrite(POWER_PIN, LOW);
  pinMode(DO_PIN, INPUT);
  Serial.begin(9600);
  Serial.println(F("Rain sensor - one reading every 5 s"));
}

void loop() {
  digitalWrite(POWER_PIN, HIGH);
  delay(20);  // let the comparator settle
  int wet = analogRead(AO_PIN);
  bool rainDO = digitalRead(DO_PIN) == LOW;  // DO goes LOW past the pot threshold
  digitalWrite(POWER_PIN, LOW);

  Serial.print(F("AO "));
  Serial.print(wet);
  Serial.print(F("  DO "));
  Serial.print(rainDO ? F("wet") : F("dry"));
  Serial.print(F("  -> "));
  if (wet > 900) Serial.println(F("Dry"));
  else if (wet > 600) Serial.println(F("Drizzle"));
  else if (wet > 350) Serial.println(F("Rain"));
  else Serial.println(F("Heavy rain"));
  delay(5000);
}

Expected serial output

Rain sensor - one reading every 5 s
AO 1012  DO dry  -> Dry
AO 720  DO wet  -> Drizzle
AO 310  DO wet  -> Heavy rain

Rain sensor - one reading every 5 s
AO 1012  DO dry  -> Dry
AO 720  DO wet  -> Drizzle
AO 310  DO wet  -> Heavy rain

Common errors