#define trigPin 10
#define echoPin 11
#define buzzer 12
void setup()
{
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
pinMode(buzzer,OUTPUT);
}
void loop()
{
long duration, distance;
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
distance = (duration/2) / 29.1;
if (distance < 100)     // This is where checking the distance you can change the value
{
 // When the the distance below 100cm
digitalWrite(buzzer,HIGH);
} else
{
// when greater than 100cm
digitalWrite(buzzer,LOW);
} delay(10);
}