Sump alarm on an Arduino Uno

// Sump alarm: a float switch in the sump, a buzzer and a red LED.
//
//   pin 2    the float switch (its other wire to GND)
//   pin 8    the buzzer (its other leg to GND)
//   pin 13   the red LED, through 220 ohms
//
// The float switch is a reed in the stem and a magnet in the float. As it
// ships it is normally open: with the water low the float hangs down and the
// reed is open, so pin 2, pulled up by the chip, reads HIGH. When the water
// lifts the float the reed closes and pin 2 reads LOW. Turn the float over on
// its stem and it works the other way around; flip HIGH and LOW below to suit.
//
// Water near the switching level slops about, so a real float chatters. The
// sketch only believes a change that has held for half a second, and once
// the alarm is sounding it stays on until the water has been back down for
// two seconds.
//
// The float is a signal contact, 10 W and half an amp at most: it tells the
// Arduino, and the Arduino drives the buzzer. A pump would go on a relay.

const int FLOAT_SWITCH = 2;
const int BUZZER = 8;
const int LED = 13;

bool alarm = false;
bool lastRaw = false;
unsigned long changedAt = 0;
unsigned long lastReport = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: sump alarm");
  pinMode(FLOAT_SWITCH, INPUT_PULLUP);
  pinMode(BUZZER, OUTPUT);
  pinMode(LED, OUTPUT);
}

void loop() {
  // LOW means the float is lifted: the water is up.
  bool high = digitalRead(FLOAT_SWITCH) == LOW;
  if (high != lastRaw) {
    lastRaw = high;
    changedAt = millis();
  }
  unsigned long held = millis() - changedAt;
  if (!alarm && high && held >= 500) {
    alarm = true;
    Serial.println("ALARM: water high");
  }
  if (alarm && !high && held >= 2000) {
    alarm = false;
    Serial.println("OK: water back down");
  }

  digitalWrite(LED, alarm ? HIGH : LOW);
  if (alarm) {
    // A two-tone siren, a quarter of a second each.
    tone(BUZZER, (millis() / 250) % 2 ? 1800 : 1200);
  } else {
    noTone(BUZZER);
  }

  if (millis() - lastReport >= 1000) {
    lastReport = millis();
    Serial.print("float ");
    Serial.print(high ? "up" : "down");
    Serial.print(", alarm ");
    Serial.println(alarm ? 1 : 0);
  }
}