const int sensorPin = A0;
const int green = 9;
const int yellow = 10;
const int red = 11;

void setup() {
  pinMode(green, OUTPUT);
  pinMode(yellow, OUTPUT);
  pinMode(red, OUTPUT);
  Serial.begin(9600);
}
void loop() {
  int val = analogRead(sensorPin);
  if (val < 100) {  
    digitalWrite(red, HIGH);
    digitalWrite(yellow, LOW);
    digitalWrite(green, LOW);
  } else if (val < 500) { 
    digitalWrite(red, LOW);
    digitalWrite(yellow, HIGH);
    digitalWrite(green, LOW);
  } 
  else {                
    digitalWrite(red, LOW);
    digitalWrite(yellow, LOW);
    digitalWrite(green, HIGH);
  }
  delay(100); 
  }