HiLetgo&reg; MQ-135 MQ135 Air Quality Sensor Hazardous Gas Detection Module For Arduino AVR Main chip: LM393,MQ-135 gas sensor With high sensitivity and good selectivity for air quality, harmful gas detection. With the DO switch signal (TTL) output and AO analog signal output Useing of high-quality double-panel design, with power instructions and TTL signal output instructions TTL output valid signal is low. (When the output low signal light, can be directly connected to the microcontroller or relay module)