SHUATISHUATI 9PCS Gas Detection Sensor Module, MQ-2 MQ-3 MQ-4 MQ-5 MQ-6 MQ-7 MQ-8 MQ-9 MQ-135Smoke Sensor Module, Gas Sensor Module, Gas Sensor Blocks for Arduino

first off, several reviews complain that the sensors are unlabeled. that is false, though it's easy to overlook the labels, which are printed on the rings which attach the meshes to the PCBs. The MQ-3 is the sensor with the orange ring, and is unlabeled. The large silver ring is the MQ-7, which *is* labeled.All sensors have four pins for 5V, ground, analog out and digital out.It's easy to sample all nine sensors at once using e.g. an Arduino Mega 2560. You'll want to ensure you have nine input pins available. Each sensor can consume several hundred milliamps (they have heaters built into them), so you'll most likely want an external source of 5V power. Remember to tie your grounds together.