#define TURBIDITY_PIN 34

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
}

void loop() {

  int adc = analogRead(TURBIDITY_PIN);

  float voltage = adc * (3.3 / 4095.0);

  float ntu;

  if (voltage < 2.5)
    ntu = 3000;
  else
    ntu = -1120.4 * voltage * voltage +
          5742.3 * voltage -
          4352.9;

  Serial.print("Voltage: ");
  Serial.print(voltage, 3);

  Serial.print(" V   NTU: ");
  Serial.println(ntu);

  delay(1000);
}