The MQ-2 is a Metal Oxide Semiconductor (MOS) gas sensor designed to detect combustible gases like LPG, methane, propane, butane, hydrogen, and smoke in concentrations from 200 ppm to 10,000 ppm. It consists of a Tin Dioxide (SnO₂) sensing layer on an alumina ceramic tube, internally heated to 200°C–300°C by a nickel-chromium coil. In clean air, adsorbed oxygen traps electrons on the SnO₂ surface, keeping resistance high; when a target gas is present, it reacts with the oxygen, releasing electrons and lowering the resistance. This change is converted by the breakout board into a 0V–5V analog output (AO) and a digital output (DO) via an LM393 comparator, with a potentiometer for adjustable sensitivity—making it a cheap, simple, and popular choice for gas leakage and smoke alarm projects.