Vehicle Detection Module

#include <Arduino.h>

// SmartJalan UNITEN — vehicle detection subsystem only.
// Data flow: approaching vehicle -> HC-SR04P -> distance measurement -> ESP32.

// Forward declarations
float measureDistanceCm();

constexpr uint8_t TRIG_PIN = 27;
constexpr uint8_t ECHO_PIN = 26;
constexpr uint8_t BUZZER_PIN = 4;  // Audio Warning buzzer signal.
constexpr float DANGER_ZONE_CM = 150.0f;
constexpr unsigned long SAMPLE_INTERVAL_MS = 200;
constexpr unsigned long ECHO_TIMEOUT_US = 30000;

bool lastDangerZoneState = false;
bool haveValidReading = false;
unsigned long lastSampleMs = 0;

float measureDistanceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  const unsigned long echoDurationUs = pulseIn(ECHO_PIN, HIGH, ECHO_TIMEOUT_US);
  if (echoDurationUs == 0) {
    return -1.0f;
  }

  // Sound travels to the vehicle and back, so divide by two.
  return (echoDurationUs * 0.0343f) / 2.0f;
}

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  digitalWrite(TRIG_PIN, LOW);
  digitalWrite(BUZZER_PIN, LOW);
  Serial.println("SmartJalan vehicle detection ready");
}

void loop() {
  const unsigned long now = millis();
  if (now - lastSampleMs < SAMPLE_INTERVAL_MS) {
    return;
  }
  lastSampleMs = now;

  const float distanceCm = measureDistanceCm();
  if (distanceCm < 0.0f) {
    digitalWrite(BUZZER_PIN, LOW);
    if (haveValidReading) {
      Serial.println("Distance Measurement: no echo received — Audio Warning off");
      haveValidReading = false;
    }
    return;
  }

  const bool inDangerZone = distanceCm <= DANGER_ZONE_CM;
  if (!haveValidReading || inDangerZone != lastDangerZoneState) {
    Serial.print("Distance Measurement: ");
    Serial.print(distanceCm, 1);
    Serial.print(" cm — ");
    Serial.println(inDangerZone ? "VEHICLE IN DANGER ZONE" : "vehicle outside danger zone");
  }

  // Vehicle detected → distance measured → threshold checked → Audio Warning.
  digitalWrite(BUZZER_PIN, inDangerZone ? HIGH : LOW);

  lastDangerZoneState = inDangerZone;
  haveValidReading = true;
}