#define TRIG_PIN  5
#define ECHO_PIN  4
#define TRIG2_PIN  7
#define ECHO2_PIN  6
#define BUZZER_PIN  2

int freq = 1800;
int distance;
int duration;

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(TRIG2_PIN, OUTPUT);
  pinMode(ECHO2_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
}

void loop() {
  distance = distanceCM(TRIG_PIN, ECHO_PIN);
  duration = distance * 5;
  
  if (distance < 100) {
    alarm(duration);
  }
}

void alarm(int time) {
  tone(BUZZER_PIN, freq);
  delay(time);
  noTone(BUZZER_PIN);
  delay(time);
}

 float distanceCM(int TRIG, int ECHO) {
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  float duration_us = pulseIn(ECHO, HIGH);
  float distance_cm = 0.0343 * duration_us / 2;
  return distance_cm;
}