Arduino IDE Coding

#include <Servo.h>

// Pin definitions
int TRIGGER_PIN = 7;
int ECHO_PIN = 6;
int SERVO_PIN = 9;

// Distance and servo angles
int MAX_DISTANCE = 20;
int OPEN_ANGLE = 90;
int CLOSE_ANGLE = 0;

// Create servo object
Servo servo;

// Variables
long duration;
int distance;

void setup() {

  // Set pin modes
  pinMode(TRIGGER_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);

  // Connect servo motor
  servo.attach(SERVO_PIN);
}

void loop() {

  // Send ultrasonic signal
  digitalWrite(TRIGGER_PIN, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIGGER_PIN, HIGH);
  delayMicroseconds(10);

  digitalWrite(TRIGGER_PIN, LOW);

  // Read echo time
  duration = pulseIn(ECHO_PIN, HIGH);

  // Calculate distance
  distance = (duration / 2) / 29.1;

  // Check object distance
  if (distance <= MAX_DISTANCE) {

    // Open lid
    servo.write(OPEN_ANGLE);
    delay(1000);

  } else {

    // Close lid
    servo.write(CLOSE_ANGLE);
  }

  delay(500);
}