int trigger_pin = 2;
int echo_pin = 3;
int led_alert_close_distance = 13;
int led_safe_distance = 12;
long duration, cm;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(trigger_pin, OUTPUT);
  pinMode(echo_pin, INPUT);
  pinMode(led_alert_close_distance, INPUT);
  pinMode(led_safe_distance, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(trigger_pin, LOW);
  delayMicroseconds(5);
  digitalWrite(trigger_pin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigger_pin, LOW);
  pinMode(echo_pin, INPUT);
  duration = pulseIn(echo_pin, HIGH);
  cm = (duration/2) / 29.1;
  Serial.println(cm);
  if (cm < 80) {
    digitalWrite(led_alert_close_distance, HIGH);
    digitalWrite(led_safe_distance, LOW);
  } else {
    digitalWrite(led_safe_distance, HIGH);
    digitalWrite(led_alert_close_distance, LOW);
  }
  delay(250);
}