Radar de marcha atrás con Arduino

#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

int buzzer_pin = 8; 
int cm = 0; 

long readUltrasonicDistance(int triggerPin, int echoPin) {
    pinMode(triggerPin, OUTPUT);  
    digitalWrite(triggerPin, LOW);
    delayMicroseconds(2);
    digitalWrite(triggerPin, HIGH);
    delayMicroseconds(10);
    digitalWrite(triggerPin, LOW);
    pinMode(echoPin, INPUT);
    return pulseIn(echoPin, HIGH);
}


void setup(){
    lcd.begin(16, 2); 
    pinMode(buzzer_pin, OUTPUT); 
}

void loop(){
    lcd.print("Distance: ");
    lcd.print(cm);
    lcd.print(" cm");
    delay(10);

    cm = 0.01723 * readUltrasonicDistance(7, 7); 
  	delay(100); 

    if (cm < 15){ 
      	tone(buzzer_pin, 1000, 1000);
    }

    lcd.clear(); 
}