Six MQ gas sensors on A0 to A5, on an Arduino Uno

// Six MQ gas sensors on A0 to A5.
//
//   MQ-135 AO -> A0   MQ-7 AO -> A1   MQ-4 AO -> A2
//   MQ-5 AO -> A3     MQ-6 AO -> A4   MQ-9 AO -> A5
//   every VCC -> 5V   every GND -> GND
//
// Each one is a heated element whose resistance falls in the gas it is made
// for, over a load resistor on its board, so more gas reads higher. They read
// high at first, while the heaters warm up: wait for the numbers to settle.
//
// Six heaters draw about 900 mA, more than a USB port gives: a real build
// powers them from a 5 V supply of their own, its ground joined to the Uno's.
//
// Drag any sensor's gas level up.

const char *names[] = {"MQ-135", "MQ-7", "MQ-4", "MQ-5", "MQ-6", "MQ-9"};
const int pins[] = {A0, A1, A2, A3, A4, A5};

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: six MQ gas sensors on A0 to A5");
}

void loop() {
  for (int i = 0; i < 6; i++) {
    Serial.print(names[i]);
    Serial.print(' ');
    Serial.print(analogRead(pins[i]));
    Serial.print(i < 5 ? "  " : "\n");
  }
  delay(1000);
}