int s0 = A0;
int s1 = A1; 
int s2 = A2;
int s3 = A3;
int sig = 2; 
int led1 = 4;
int led2 = 5; 
int led3 = 6;

void setup() {
  // put your setup code here, to run once:
  pinMode(s0, OUTPUT); 
  pinMode(s1, OUTPUT); 
  pinMode(s2, OUTPUT); 
  pinMode(s3, OUTPUT); 
  pinMode(sig, INPUT);

  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);
  digitalWrite(led1, LOW);
  digitalWrite(led2, LOW);
  digitalWrite(led3, LOW);

  Serial.begin(9600);
}

void loop() {
  
// 0000  
  digitalWrite(s0, LOW);   
  digitalWrite(s1, LOW);     
  digitalWrite(s2, LOW);     
  digitalWrite(s3, LOW);  
  if(digitalRead(sig) == LOW) {
    digitalWrite(led1, HIGH);    
    delay(500);   
  }
  else {
    digitalWrite(led1, LOW); 
  }

// 0001  
  digitalWrite(s0, HIGH);   
  digitalWrite(s1, LOW);     
  digitalWrite(s2, LOW);     
  digitalWrite(s3, LOW);  
  if(digitalRead(sig) == LOW) {
    digitalWrite(led2, HIGH); 
    delay(500);
  }
  else {
    digitalWrite(led2, LOW); 
  }

// 0010  
  digitalWrite(s0, LOW);   
  digitalWrite(s1, HIGH);     
  digitalWrite(s2, LOW);     
  digitalWrite(s3, LOW);  
  if(digitalRead(sig) == LOW) {
    digitalWrite(led3, HIGH);
    delay(500);
  }
  else {
    digitalWrite(led3, LOW); 
  }

  }