#define echo_pin 4
#define trig_pin 2
#define buzzer_pin 6

int distance;
long duration;

void setup() {
  // put your setup code here, to run once:
pinMode(echo_pin, INPUT);
pinMode(trig_pin, OUTPUT);
pinMode(buzzer_pin, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(trig_pin, LOW);
delayMicroseconds(10);

digitalWrite(trig_pin, HIGH);
delayMicroseconds(10);

digitalWrite(trig_pin, LOW);

duration = pulseIn (echo_pin, HIGH);

distance = (duration/2) / 29.1;

if (distance < 100) 
{
  digitalWrite(buzzer_pin, HIGH);
  tone(buzzer_pin, 400);
  delay(500);
  tone(buzzer_pin, 300);
  delay(500);
}
else 
{
  digitalWrite(buzzer_pin, LOW);
  noTone(buzzer_pin);
}

}