int red = 12;
int green = 11;
#include "NewPing.h"
#define TRIGGER_PIN 9
#define ECHO_PIN 10
#define MAX_DISTANCE 400
NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE);
float duration, distance;
void setup() {
Serial.begin(9600);
pinMode(red, OUTPUT);
pinMode(green, OUTPUT);
}
void loop() {
distance = sonar.ping_cm();
if (distance < 20) 
    {
        digitalWrite(red, HIGH);
        digitalWrite(green, LOW);
    }
    else
    {
        digitalWrite(red, LOW);
        digitalWrite(green, HIGH);
    }
    delay(500);
}