SainSmart MQ138 Formaldehyde Aldehydes Gas Detection Sensor Module for Arduino With the signal output LED indication 2-way signal output (analog output, and TTL-level output) TTL output valid signal is low; (low output signal light, which can be accessed microcontroller IO port) Analog output increases with the concentration, the higher the voltage the higher the concentration Aldehydes, alcohols, ketones, aromatic compounds with high sensitivity