HW-038 Water Level Sensor with an ESP32 DevKit V1

// HW-038 Water Level Sensor on ESP32 DevKit V1: wiring and a free simulation at https://www.tinkered.ai/components/water-level-sensor-hw-038/esp32
const int POWER_PIN = 13;    // HW-038 +: powered only to read (the traces corrode under constant DC)
const int SIGNAL_PIN = 33;   // HW-038 S
const int DRY_BELOW_MV = 320;  // under this the traces are dry

// S in millivolts at 0, 25, 50, 75 and 100 % of the sensing length. These starting points are
// a 5 V tap-water calibration scaled to the 3.3 V that + gets here: S depends on the water and
// on the sensor, so record your own at known levels and replace them.
const int CAL_MV[5] = { 1080, 1570, 1820, 1970, 2070 };

int readMilliVolts() {
  digitalWrite(POWER_PIN, HIGH);
  delay(10);                                    // let S settle
  int mv = analogReadMilliVolts(SIGNAL_PIN);    // calibrated: the ESP32's raw 12-bit reading is not linear
  digitalWrite(POWER_PIN, LOW);
  return mv;
}

int levelPercent(int mv) {
  if (mv < DRY_BELOW_MV) return 0;
  if (mv <= CAL_MV[0]) return 1;
  for (int i = 1; i < 5; i++) {
    if (mv <= CAL_MV[i]) return 25 * (i - 1) + (long)(mv - CAL_MV[i - 1]) * 25 / (CAL_MV[i] - CAL_MV[i - 1]);
  }
  return 100;
}

void setup() {
  Serial.begin(9600);
  pinMode(POWER_PIN, OUTPUT);
  digitalWrite(POWER_PIN, LOW);
  analogSetPinAttenuation(SIGNAL_PIN, ADC_11db);   // 11 dB, the widest range: Espressif rates it accurate to about 2.45 V
  Serial.println("HW-038 ready");
}

void loop() {
  int mv = readMilliVolts();
  Serial.print("S ");
  Serial.print(mv);
  Serial.print(" mV  level ");
  Serial.print(levelPercent(mv));
  Serial.println(" %");
  delay(1000);
}