const uint8_t SPinB[] = {6, 5, 4, 3};    // S0 ... S3
const uint8_t SPinL[] = {11, 10, 9, 8};  // S0 ... S3

const int SIGB = 2; // SIG pin input for buttons
const int SIGL = 7; // SIG pin output for leds

int readMux(int channel) {
  for (int i = 0; i < 4; i++) 
    digitalWrite(SPinB[i], channel >> i & 1);

  delay(10);
  return digitalRead(SIGB);
}

void setup() {
  Serial.begin(115200);
  pinMode(SIGB, INPUT);
  pinMode(SIGL, OUTPUT);

  for (int i = 0; i < 4; i++) {
    pinMode(SPinB[i], OUTPUT);
    pinMode(SPinL[i], OUTPUT);
  }
}

void loop() {
  // Print it to the Serial Monitor
  for (int i = 0; i < 16; i++) 
    Serial.print(readMux(i));
  Serial.println();

  // Find the first pressed button
  bool found = false;
  int index = 0;
  for (int i = 0; i < 16 && !found; i++) {
    if(readMux(i) == LOW) {
      index = i;
      found = true;
    }
  }

  if(found) {
    // Found a pressed button ?
    // The diagram has the buttons in a different order, 
    // therefor it is 15-index
    writeMux(15-index);
    digitalWrite(SIGL, HIGH);
  } else {
    // Nothing found? Then turn off all leds
    // The Enable is not used, so just set the SIG to LOW.
    digitalWrite(SIGL, LOW);
  }
  delay(50);
}

void writeMux(int channel) {
  for (int i = 0; i < 4; i++) 
    digitalWrite(SPinL[i], channel >> i & 1);
}