Water level sensor on an Arduino Uno

// Water level: the sensor's traces on A0, and the depth in words.
//
//   sensor S -> A0    sensor + -> 5V    sensor - -> GND
//
// The sensor is a row of bare copper traces, every other one tied to power
// and the rest to a transistor's input. Water bridges the traces, and the
// deeper it goes, the more of them it bridges and the higher S reads. It is
// a rough gauge, not a ruler: it tells dry from a splash from half from full.
//
// On a real one, power it from a pin and switch it on only to read, because
// the traces corrode while current flows through water.

const int SENSOR = A0;

const char *levelOf(int reading) {
  if (reading < 100) return "dry";
  if (reading < 400) return "low";
  if (reading < 600) return "half";
  return "full";
}

const char *lastLevel = "";

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: water level sensor");
}

void loop() {
  int reading = analogRead(SENSOR);
  const char *level = levelOf(reading);
  if (level != lastLevel) {
    lastLevel = level;
    Serial.print("reading ");
    Serial.print(reading);
    Serial.print(": ");
    Serial.println(level);
  }
  delay(200);
}