KY-026 Flame Sensor with an Arduino Uno

// KY-026 Flame Sensor on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/flame-sensor-ky-026/arduino-uno
const int AO_PIN = A0;     // KY-026 AO: near 1023 in the dark, falls with infrared
const int DO_PIN = 2;      // KY-026 DO: HIGH while it sees a flame
const int BUZZER_PIN = 8;  // an active buzzer: sounds while driven HIGH

int rest = 1023;           // AO with no flame, measured at start-up
unsigned long lastPrint = 0;

int readAo() {
  long sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(AO_PIN);
  return sum / 16;         // a flame flickers: average it
}

void setup() {
  Serial.begin(9600);
  pinMode(DO_PIN, INPUT);  // the module has its own pull-up
  pinMode(BUZZER_PIN, OUTPUT);
  rest = readAo();
  Serial.println("KY-026 ready");
}

void loop() {
  int ao = readAo();
  bool flame = digitalRead(DO_PIN) == HIGH;
  // how much infrared, as a percent of the way from the rest level down to 0
  int level = rest > 0 ? constrain(100L * (rest - ao) / rest, 0, 100) : 0;

  digitalWrite(BUZZER_PIN, flame ? HIGH : LOW);

  if (millis() - lastPrint >= 500) {
    lastPrint = millis();
    Serial.print("AO ");
    Serial.print(ao);
    Serial.print("  IR ");
    Serial.print(level);
    Serial.print(" %  ");
    Serial.println(flame ? "FLAME" : "clear");
  }
}