MQ-7 sensor is suitable for detecting Carbon Monoxide gas concentration in the air from 20 to 2000 ppm. MQ7 has an heater inside the module and an electro-chemical sensor, MQ7 requires 5V AC or DC. Two pins are for heater coil, and other four are for fetching signals. Output of MQ7 is analog. Higher the gas concentration, higher the output voltage at signal pins. It has lower conductivity in clean air. This gas sensor has a high sensitivity and fast response.