How to Build the Watch:

To begin building the watch, wire the ultrasonic sensor and the buzzer to the Arduino UNO according to the given schematic:

Finally, build a case for the electronics and attach it to the watch.

Code:

Declare that the Trigger pin is connected to pin number 13, Echo pin is connected to pin number 12 and buzzer is connected to pin number 9

int trigger_pin = 13;
int echo_pin = 12;
int buzzer_pin = 9;

In the Void Setup, begin the serial monitor and set up Trigger pin as output, echo pin as input and buzzer as output.

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

Write a pulse to the HC-SR04's Trigger Pin:

digitalWrite (trigger_pin, HIGH);   
delayMicroseconds (10);
digitalWrite (trigger_pin, LOW);    

Measure the response from the HC-SR04 Echo Pin and determine distance.

time = pulseIn (echo_pin, HIGH);    
distance = (time * 0.034) / 2;

If the object is lesser than 70 cm, print "object is near", ''Distance= " and the distance between the object and ultrasonic sensor in the serial monitor and set the buzzer to high.

if (distance <= 70) 
{ 
Serial.println (" object is near "); 
Serial.print (" Distance= "); 
Serial.println (distance); 
digitalWrite (buzzer_pin, HIGH); 
delay (1000);
}

Else print "Distance= " and print the distance and set the buzzer to low.

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

Result: