Audrino & Heart Rate Sensor 

int PulseSensorPurplePin = A0;    // Pulse Sensor connected to analog pin A0
int LED = LED_BUILTIN;            // On-board LED
int Threshold = 550;              // Threshold for detecting a heartbeat

int Signal;                       // Holds raw analog reading
bool PulseDetected = false;       // True when a beat is detected
unsigned long lastBeatTime = 0;   // Time (ms) of the last beat
float BPM = 0;                    // Calculated Beats Per Minute

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(115200);
}

void loop() {
  // Read the sensor
  Signal = analogRead(PulseSensorPurplePin);

  // Check if signal crosses the threshold upward (beat detected)
  if (Signal > Threshold && !PulseDetected) {
    PulseDetected = true;  // mark that we’re in a beat

    unsigned long currentTime = millis();
    unsigned long delta = currentTime - lastBeatTime;

    if (lastBeatTime > 0) {  // skip first beat (no interval yet)
      BPM = 60000.0 / delta;  // 60000 ms per minute
      Serial.print("BPM: ");
      Serial.println(BPM);
    }

    lastBeatTime = currentTime;
    digitalWrite(LED, HIGH);  // flash LED on beat
  }

  // When signal goes back below threshold, reset for next detection
  if (Signal < Threshold && PulseDetected) {
    PulseDetected = false;
    digitalWrite(LED, LOW);
  }

  // Small non-blocking pause (optional for stability)
  // Just ensures serial output isn’t too fast
  static unsigned long lastPrint = 0;
  if (millis() - lastPrint > 20) {
    //Serial.print("Signal: ");
    //Serial.println(Signal);
    lastPrint = millis();
  }
}