#include <Servo.h>

#define TRIG 7
#define ECHO 6
#define SERVO_PIN 9
#define BUZZER 4

Servo gate;

long getDistance() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  long duration = pulseIn(ECHO, HIGH);
  long distance = duration * 0.034 / 2;
  return distance;
}

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(BUZZER, OUTPUT);

  gate.attach(SERVO_PIN);
  gate.write(0);              // Gate initially closed
  digitalWrite(BUZZER, LOW);
  Serial.begin(9600);
}

void loop() {

  long d1 = getDistance();
  delay(50);
  long d2 = getDistance();
  delay(50);
  long d3 = getDistance();

  long distance = (d1 + d2 + d3) / 3;   // Noise filtering

  if(distance < 20) {         // Vehicle detected
     digitalWrite(BUZZER, HIGH);   // Buzzer ON
     gate.write(90);               // Open gate
     delay(3000);
     digitalWrite(BUZZER, LOW);    // Buzzer OFF
  }
  else {
     gate.write(0);                // Close gate
  }
}