int high_threshhold = 20;
int low_threshhold = 5;
int debouncing = 500;
int nrOfSensors = 13;

boolean trigger[13];
unsigned long timeStamp[13];

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  
  for (int x = 0; x < nrOfSensors; x++) { //we repeat the block below, x increases every time
    int sensorValue = analogRead(A0 + x);
    if (sensorValue >= high_threshhold) trigger[x] = false;
    if ((sensorValue < low_threshhold) && (trigger[x] == false)) {
      if ((millis() - timeStamp[x]) > debouncing) {
        Serial.write (x);
        trigger[x] = true;
        timeStamp[x] = millis();
      }
    }
  }
}