A Rain Sensor That Won't Rot in a Month: Powered From a GPIO Only While Measuring

About the project

A rain sensor is a board with two interleaved combs of tracks plus an LM393 comparator module. Water bridges the tracks, the resistance drops, and the output voltage changes. The problem is that a constant current through a wet board means electrolysis: the copper dissolves, and after a few weeks outdoors the tracks are simply gone. The fix is easy: power the sensor only for the moment of a measurement.

What you need

• ESP32 DevKit
• An FC-37 / YL-83 rain sensor (the plate plus a module with AO and DO)
• Wires; for outdoors, a sealed box for the module

Wiring

The module draws 5–8 mA, which an ESP32 pin (up to 20 mA) supplies easily. For sensors that need more, use a transistor or a MOSFET.

Outputs

• AO: a dry plate gives almost VCC, and the more water, the lower it goes. That tells drizzle from a downpour.
• DO: LOW when wet (the level is past the threshold set by the trimmer).

Code

// A rain sensor that lasts for years: power it from a GPIO only while measuring
const int RAIN_POWER = 26;   // the module's VCC
const int RAIN_AO = 34;
const int RAIN_DO = 27;      // LOW = wet (threshold set by the trimmer)

void setup() {
  Serial.begin(115200);
  pinMode(RAIN_POWER, OUTPUT);
  digitalWrite(RAIN_POWER, LOW);
  pinMode(RAIN_DO, INPUT);
}

void loop() {
  digitalWrite(RAIN_POWER, HIGH);
  delay(10);                         // let the comparator settle
  int raw = analogRead(RAIN_AO);
  bool wet = digitalRead(RAIN_DO) == LOW;
  digitalWrite(RAIN_POWER, LOW);     // no current through the plate between readings
  int water = map(raw, 4095, 0, 0, 100);
  Serial.printf("Water %3d %%  %s\n", water, wet ? "RAIN" : "dry");
  delay(2000);
}

In the simulator change "Water on the plate". Try it the other way round too: connect the module's VCC to 3V3 and leave the plate wet — after half a minute the simulator reminds you about corrosion.

Good to know

• The plate goes outside, the module into a box. The comparator fears moisture more than the tracks do. The cable between them can be 5 m long.
• Tilt it 30–45°. Water pools on a flat plate, and the "rain" goes on for an hour after it has stopped.
• Alternating current protects even better: some projects swap the polarity with two GPIOs. For most uses short power pulses are enough.
• The same circuit suits resistive soil moisture probes — they have the same corrosion problem.