#define TRIG 13
#define ECHO 12
#define BUZZER 5

void setup() {
  // put your setup code here, to run once:
 pinMode(TRIG,OUTPUT);
 pinMode(ECHO,INPUT);
 pinMode(BUZZER,OUTPUT);
 Serial.begin(9600);  
}

void loop() {
  // put your main code here, to run repeatedly:
 digitalWrite(TRIG,LOW);
 delayMicroseconds(5);
 digitalWrite(TRIG,HIGH);
 delayMicroseconds(10);
 digitalWrite(TRIG,LOW);
 long time = pulseIn(ECHO,HIGH);
 float distance_cm =(time/2) *(0.0343);
 Serial.print("Distance_cm :");
 Serial.println(distance_cm );
 if(distance_cm<20)
 {
  digitalWrite(BUZZER,HIGH);
  delay(1000);
  digitalWrite(BUZZER,LOW);
  delay(1000);
 }
 else
 {
  digitalWrite(BUZZER,LOW);
 }
 else if
 {
  digitalWrite(BUZZER,HIGH);
 }
 
 }