// ---------- PIN DEFINITIONS ----------
const int pulseOutPin = 2;   // Pin to send pulse
const int pulseInPin = 3;    // Pin to receive pulse
const int buzzerPin = 4;     // Buzzer pin

// ---------- TIMING ----------
const unsigned long sendInterval = 5000;      // 5 seconds between pulses
const unsigned long receiveTimeout = 1000;    // 1 second to wait for response
const int pulseDuration = 50;                 // 50 ms pulse

const int buzzerToneFreq = 1000; // Max tone 1000 Hz
const int buzzerDuration = 500;  // 0.5 second buzzer duration

unsigned long lastSendTime = 0;

void setup() {
  pinMode(pulseOutPin, OUTPUT);
  pinMode(pulseInPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
  digitalWrite(pulseOutPin, LOW);
  Serial.begin(9600);
}

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

  // ---------- SEND PULSE EVERY 5 SECONDS ----------
  if (currentTime - lastSendTime >= sendInterval) {
    sendPulse();
    lastSendTime = currentTime;

    // ---------- WAIT FOR RESPONSE ----------
    bool pulseReceived = waitForPulse(receiveTimeout);

    if (!pulseReceived) {
      // ---------- BUZZER IF NO PULSE RECEIVED ----------
      tone(buzzerPin, buzzerToneFreq, buzzerDuration); // sound 1000 Hz
      Serial.println("No pulse received! Buzzer ON.");
    } else {
      Serial.println("Pulse received. All good.");
    }
  }
}

// ---------- FUNCTION TO SEND PULSE ----------
void sendPulse() {
  digitalWrite(pulseOutPin, HIGH);
  delay(pulseDuration);   // brief electrical pulse
  digitalWrite(pulseOutPin, LOW);
  Serial.println("Pulse sent.");
}

// ---------- FUNCTION TO WAIT FOR PULSE ----------
bool waitForPulse(unsigned long timeout) {
  unsigned long startTime = millis();
  while (millis() - startTime < timeout) {
    if (digitalRead(pulseInPin) == HIGH) {
      // pulse detected
      return true;
    }
  }
  return false; // no pulse received in timeframe
}