// adjust alarm distance here
const int ALARM_DIST = 100;
// pin name constants
const int ECHO_PIN = 9;
const int TRIG_PIN = 10;
const int GRN_LED_PIN = 11;
const int RED_LED_PIN = 12;

void setup() {
  Serial.begin(9600);
  pinMode(ECHO_PIN, INPUT);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(GRN_LED_PIN, OUTPUT);
  pinMode(RED_LED_PIN, OUTPUT);
}

void loop() {
  // create clean trigger pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  // wait for echo pulse
  unsigned long duration = pulseIn(ECHO_PIN, HIGH);
  // convert time to distance
  float distance = (duration / 29.1) / 2;  // /2, there and back...
  // print distance
  Serial.print("Distance: ");
  Serial.print(distance, 1);
  Serial.println(" cm");
  // set indicators
  if (distance <= ALARM_DIST) {
    digitalWrite(GRN_LED_PIN, LOW);
    digitalWrite(RED_LED_PIN, HIGH);
  } else {
    digitalWrite(GRN_LED_PIN, HIGH);
    digitalWrite(RED_LED_PIN, LOW);
  }
  // wait 0.1 seconds, then loop
  delay(100);
}