int echoPin=2;
int trigPin =3;
int buzzer =8;
long duration;
int distance;
void setup()
 {
pinMode(trigPin,OUTPUT);
pinMode(echoPin,INPUT);
pinMode(buzzer,OUTPUT);
Serial.begin(9600);
 }
void loop()
 {
digitalWrite(trigPin,LOW);
delay(2);// or delayMicroseconds(2);

digitalWrite(trigPin,HIGH);
delay(10);// or delayMicroseconds(10);
digitalWrite(trigPin,LOW);
duration=pulseIn(echoPin,HIGH);
distance=(duration*0.034/2);
Serial.print("Distance : " );
Serial.print(distance);
 Serial.println(" cm ");
if(distance >20)
{
digitalWrite(buzzer, HIGH);
Serial.print("person can enter the room" );
}
else
{
digitalWrite(buzzer, LOW);
Serial.print("No object" );
}
 }