Ultrasonic Distance Display

#include <Arduino.h>
#include <WiFi.h>
#include <PubSubClient.h>

// Replace these placeholders before deploying to your network.

// Forward declarations
float readDistanceCm(int trigPin, int echoPin);
void connectWiFi();
void connectMqtt();
void publishDistance(const char *zone, float distanceCm);

const char *WIFI_SSID = "YOUR_WIFI_NAME";
const char *WIFI_PASSWORD = "YOUR_WIFI_PASSWORD";
const char *MQTT_HOST = "192.168.1.100";  // Replace with broker IP address or hostname.
constexpr uint16_t MQTT_PORT = 1883;
const char *TOPIC_PREFIX = "parking/zone";

constexpr int LEFT_TRIG_PIN = 4;
constexpr int LEFT_ECHO_PIN = 1;
constexpr int RIGHT_TRIG_PIN = 5;
constexpr int RIGHT_ECHO_PIN = 2;
constexpr unsigned long PUBLISH_INTERVAL_MS = 2000UL;
constexpr unsigned long ECHO_TIMEOUT_US = 30000UL;

WiFiClient wifiClient;
PubSubClient mqttClient(wifiClient);
unsigned long lastPublishMs = 0;

float readDistanceCm(int trigPin, int echoPin) {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(3);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  const unsigned long durationUs = pulseIn(echoPin, HIGH, ECHO_TIMEOUT_US);
  if (durationUs == 0) return -1.0F;
  return (durationUs * 0.0343F) / 2.0F;
}

void connectWiFi() {
  if (WiFi.status() != WL_CONNECTED) {
    WiFi.mode(WIFI_STA);
    WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  }
}

void connectMqtt() {
  if (WiFi.status() != WL_CONNECTED || mqttClient.connected()) return;

  const String clientId = "parking-s3-" + String((uint32_t)(ESP.getEfuseMac() & 0xFFFFFFFF), HEX);
  mqttClient.connect(clientId.c_str());
}

void publishDistance(const char *zone, float distanceCm) {
  char topic[80];
  char payload[16];
  snprintf(topic, sizeof(topic), "%s/%s/distance_cm", TOPIC_PREFIX, zone);

  if (distanceCm < 0.0F) {
    snprintf(payload, sizeof(payload), "null");
  } else {
    snprintf(payload, sizeof(payload), "%.1f", distanceCm);
  }
  mqttClient.publish(topic, payload, true);
}

void setup() {
  Serial.begin(115200);
  pinMode(LEFT_TRIG_PIN, OUTPUT);
  pinMode(LEFT_ECHO_PIN, INPUT);
  pinMode(RIGHT_TRIG_PIN, OUTPUT);
  pinMode(RIGHT_ECHO_PIN, INPUT);
  digitalWrite(LEFT_TRIG_PIN, LOW);
  digitalWrite(RIGHT_TRIG_PIN, LOW);

  mqttClient.setServer(MQTT_HOST, MQTT_PORT);
  connectWiFi();
}

void loop() {
  connectWiFi();
  connectMqtt();
  mqttClient.loop();

  const unsigned long now = millis();
  if (mqttClient.connected() && now - lastPublishMs >= PUBLISH_INTERVAL_MS) {
    // Read sensors in sequence to reduce ultrasonic cross-talk.
    const float leftCm = readDistanceCm(LEFT_TRIG_PIN, LEFT_ECHO_PIN);
    delay(60);
    const float rightCm = readDistanceCm(RIGHT_TRIG_PIN, RIGHT_ECHO_PIN);

    publishDistance("left", leftCm);
    publishDistance("right", rightCm);
    lastPublishMs = now;
  }
}