Code -

    int trigger_pin = 2;
    int echo_pin = 3;
    int buzzer_pin = 11; 
    int time;
    int distance; 

      void setup ( ) {  
      Serial.begin (9600);   
     pinMode (trigger_pin, OUTPUT);   
     pinMode (echo_pin, INPUT);      
     pinMode (buzzer_pin, OUTPUT);
     }

  void loop ( ) {
   digitalWrite (trigger_pin, HIGH);   
   delayMicroseconds (10);
   digitalWrite (trigger_pin, LOW);    
   time = pulseIn (echo_pin, HIGH);    
   distance = (time * 0.034) / 2;

   if (distance <= 15)     
   {      
  Serial.println (" object is near ");     
   Serial.print (" Distance= ");           
    Serial.println (distance);       
   digitalWrite (buzzer_pin, HIGH);   
   delay (1000);        }

 else {       
   Serial.println (" object is far ");        
   Serial.print (" Distance= ");                 
   Serial.println (distance);             
   digitalWrite (buzzer_pin, LOW);        
   delay (1000);        
 } 
    } 

    Secondly, we will make the circuit.

 1.Connections : Ultrasonic sensor 

•     Gnd to Gnd 
•     Vcc to 3.3 v
•     Echo to  digital pin 3
•     trig to  digital pin 2 

2. Connections :  Buzzer 

•     Negative to Gnd 
•     Positive to digital pin 11

 3.Connections : Battery 

•     Negative :  Gnd 
•     Positive  :  5v

 After completing the connections as per the circuit diagram attach the circuit to the band .