Rain Sensor: Resistive & Capacitive Water Detection

Working Principle

Rain sensors detect the presence and intensity of water on their sensing surface. Two main types are common:

Resistive rain sensor: A PCB with exposed interleaved copper traces. When dry, resistance is very high (open circuit). Water droplets bridge the traces, creating conductive paths that lower resistance proportionally to the wetted area. A comparator/amplifier module converts this to analog and digital outputs.

Capacitive rain sensor: Uses two conductor plates separated by a dielectric. Water on the surface changes the dielectric constant, altering the capacitance. Capacitive types are more durable (no exposed metal to corrode) but more expensive.

Electrical Characteristics

Parameter	Resistive Module (FC-37/YL-83)
Supply Voltage	3.3–5 V
Digital Output	HIGH (dry) / LOW (rain detected)
Analog Output	~1023 (dry) to ~0 (fully wet) on 10-bit ADC
Threshold Adjust	On-board potentiometer (sets digital trip point)
Sensing Area	~50 × 40 mm PCB
Response Time	Near instantaneous (water contact)
Comparator	LM393 dual comparator IC
Current Draw	~15 mA active

Interfacing with an MCU

The module has 4 pins: VCC, GND, DO (digital output), and AO (analog output). Connect AO to an ADC pin for rain intensity measurement, or use DO for simple rain/no-rain detection. Adjust the potentiometer to set the sensitivity threshold for the digital output.

Calibration

• Dry baseline: Record the analog value when the sensor is completely dry (should be near 1023/4095)
• Wet threshold: Apply water drops and set the potentiometer so DO triggers at your desired rain level
• Intensity mapping: Create a rainfall intensity scale from analog values: >900 = dry, 600–900 = light rain, 300–600 = moderate, <300 = heavy rain
• Heating: In cold climates, add a resistive heater pad beneath the sensor to prevent ice/frost triggering

Code Example

/*
 * Rain Sensor (FC-37/YL-83) — Arduino
 * Wiring: VCC → D7 (power control), GND, AO → A0, DO → D8
 */
#define RAIN_POWER 7   /* Control power to reduce corrosion */
#define RAIN_AO    A0
#define RAIN_DO    8

void setup() {
    Serial.begin(9600);
    pinMode(RAIN_POWER, OUTPUT);
    pinMode(RAIN_DO, INPUT);
    Serial.println("Rain Sensor Ready");
}

int readRain() {
    digitalWrite(RAIN_POWER, HIGH);
    delay(10);  /* Allow sensor stabilisation */
    int val = analogRead(RAIN_AO);
    digitalWrite(RAIN_POWER, LOW);
    return val;
}

void loop() {
    int raw = readRain();
    bool rain = !digitalRead(RAIN_DO);  /* DO LOW = rain */

    Serial.print("Analog: ");
    Serial.print(raw);
    Serial.print("  Rain: ");

    if (raw > 900)      Serial.println("DRY");
    else if (raw > 600)  Serial.println("LIGHT RAIN");
    else if (raw > 300)  Serial.println("MODERATE RAIN");
    else                  Serial.println("HEAVY RAIN");

    delay(30000);  /* Check every 30 seconds */
}

Real-World Applications

Limitations

• Corrosion: Resistive sensors corrode rapidly if continuously powered; limited outdoor lifespan (weeks to months).
• False positives: Fog, dew, spider webs, and insects can trigger false rain detection.
• No rainfall rate: Cannot measure actual mm/hour precipitation; only wet/dry intensity.
• Orientation sensitive: Must be mounted horizontally and exposed to open sky; enclosed installations don’t work.