Clap switch on an Arduino Uno

// Clap switch: clap once and the lamp comes on, clap again and it goes off.
//
//   pin 2  the KY-038's D0      A0  its A0
//   pin 9  the lamp, through 220 ohm to GND
//   + -> 5 V, G -> GND
//
// The KY-038 has no amplifier. Its microphone's signal is a few millivolts on
// top of half the supply, and the comparator on the board lets D0 go high on
// every instant that signal pokes above the trimmer. So a clap is not one
// clean high on D0: it is a burst of dozens of pulses over a tenth of a
// second, and a sketch that toggles on every rising edge toggles dozens of
// times.
//
// # Latch the first edge, then stop listening
//
// The fix is the same one a pushbutton needs: take the first rising edge and
// ignore D0 for a while after it. 300 ms is longer than any clap rings and
// shorter than two claps in a row.
//
// # And what A0 says
//
// Once a second the sketch prints the lowest and highest A0 it saw. In a quiet
// room they are the same number, about 512. Drag the level slider up and even
// "very loud" moves them by only a few counts either side: that is what an
// unamplified microphone gives a 10-bit ADC. The clap button is the thing that
// reaches the trimmer.

const int SOUND_DO = 2;
const int SOUND_AO = A0;
const int LAMP = 9;

const unsigned long QUIET_MS = 300;

bool lampOn = false;
bool wasHigh = false;
unsigned long lastClap = 0;

int lowest = 1023;
int highest = 0;
unsigned long lastReport = 0;

void setup() {
  pinMode(SOUND_DO, INPUT);
  pinMode(LAMP, OUTPUT);
  digitalWrite(LAMP, LOW);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: clap switch");
  Serial.println("press clap on the sensor");
}

void loop() {
  unsigned long now = millis();

  bool high = digitalRead(SOUND_DO) == HIGH;
  if (high && !wasHigh && now - lastClap > QUIET_MS) {
    lastClap = now;
    lampOn = !lampOn;
    digitalWrite(LAMP, lampOn ? HIGH : LOW);
    Serial.println(lampOn ? "clap: lamp on" : "clap: lamp off");
  }
  wasHigh = high;

  int level = analogRead(SOUND_AO);
  if (level < lowest) {
    lowest = level;
  }
  if (level > highest) {
    highest = level;
  }
  if (now - lastReport >= 1000) {
    lastReport = now;
    Serial.print("A0 over the last second: ");
    Serial.print(lowest);
    Serial.print(" to ");
    Serial.println(highest);
    lowest = 1023;
    highest = 0;
  }
}