Code

//The ultrasound sensor library
#include <HC_SR04/HC_SR04.h>

//Creating variables
int cm = 0;
//assigning the pins 
int trigPin = D4;
int echoPin = D5;

//allocating the pins to the sensor 
HC_SR04 rangeFinder = HC_SR04(trigPin, echoPin);

void setup()
{

}

void loop()
{
    //measure the distance 
    cm = rangeFinder.getDistanceCM();
    
    //vriable that contains the size of the door
    int i = 158
    
    //if statements to count the distance decrease to know someone has entered 
    if (cm < 158) Particle.publish("Someone entered", "Alert");
   
    // Wait 1 seconds
    delay(1000);
    
    
}