Sender

#include <ArduinoMqttClient.h>
#include <WiFiNINA.h>
#include "arduino_secrets.h"
#define echoPin 2
#define trigPin 4
#define led A5

long duration;
int distance;

//////please enter your sensitive data in the Secret tab/arduino_secrets.h
char ssid[] = SECRET_SSID;        // your network SSID (name)
char pass[] = SECRET_PASS;    // your network password (use for WPA, or use as key for WEP)

WiFiClient wifiClient;
MqttClient mqttClient(wifiClient);

const char broker[] = "broker.hivemq.com";
int        port     = 1883;
const char topic[]  = "arduino/simple/wave";


//set interval for sending messages (milliseconds)

const long interval = 8000;
unsigned long previousMillis = 0;

int count = 0;

void setup() {
  pinMode(trigPin,OUTPUT);
  pinMode(echoPin,INPUT);
  pinMode(led ,OUTPUT);
  //Initialize serial and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  // attempt to connect to Wifi network:
  Serial.print("Attempting to connect to WPA SSID: ");
  Serial.println(ssid);
  while (WiFi.begin(ssid, pass) != WL_CONNECTED) {
    // failed, retry
    Serial.print(".");
    delay(500);
  }

  Serial.println("You're connected to the network");
  Serial.println();

  Serial.print("Attempting to connect to the MQTT broker: ");
  Serial.println(broker);

  if (!mqttClient.connect(broker, port)) {
    Serial.print("MQTT connection failed! Error code = ");
    Serial.println(mqttClient.connectError());

    while (1);
  }

  Serial.println("You're connected to the MQTT broker!");
  Serial.println();
}

void loop() {
  digitalWrite(trigPin,LOW);
  delay(20);
  digitalWrite(trigPin, HIGH);
  delay(10);
  digitalWrite(trigPin, LOW);
  
  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;
  // call poll() regularly to allow the library to send MQTT keep alive which
  // avoids being disconnected by the broker
  mqttClient.poll();

  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    // save the last time a message was sent
    previousMillis = currentMillis;

    if(distance <= 250)
    {
      Serial.print("Sending message to topic: ");
    Serial.println(topic);
    Serial.print("Hello World! ");
    Serial.print(distance);
    Serial.println(" cm");
    digitalWrite (led, HIGH);
  delay(100);
    digitalWrite (led, LOW); 
  //delay(300);
  digitalWrite (led, HIGH);
  delay(100);
    digitalWrite (led, LOW); 
    digitalWrite (led, HIGH);
  delay(100);
    digitalWrite (led, LOW); 

    // send message, the Print interface can be used to set the message contents
    mqttClient.beginMessage(topic);
    mqttClient.print("Hello World!: ");
    mqttClient.print(distance);
    mqttClient.println(" cm");
    mqttClient.endMessage();
      
    }
    Serial.println();
  }
}