The MQ6 sensor has an ceramic crystal coated with a sensing material is a metal oxide, mostly SnO2. During power supply the heater gets heated up for faster chemical reaction. The oxygen gets adsorbed on the crystal surface with a negative charge when the metal oxide is heated at a particular temperature in air. The donor electrons on the crystal surface are then transferred to the adsorbed oxygen which results in leaving positive charges in the space charge layer. Thus the surface potential serves as a potential barrier against the electron flow.
In presence of the gases the electric current flows in the sensor through the reaction of SnO2 molecules. Adsorbed oxygen forms a potential barrier at the grain boundaries which prevents carriers to move freely. The electrical resistance of the sensor is recognized by this potential barrier. The surface density of the negatively charged oxygen decreases in the presence of deoxidizing gas which reduces the barrier height in the grain boundary. This reduced barrier height decreases the sensor resistance.
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This video shows the working and configuration of the project.