Editing Alcohol Sensor Based on Gas-Sensitive Resistive Materials (section)

== Introduction ==

The MQ-3 is a low-cost metal-oxide semiconductor (MOS) gas sensor specifically designed for the detection of alcohol (ethanol) vapor in air. It is widely used in breathalyzers, vehicle safety systems, and environmental monitoring due to its high sensitivity to ethanol and relatively low sensitivity to interfering gases such as benzene and smoke.

The sensing material is tin dioxide (SnO<sub>2</sub>), an n-type metal-oxide semiconductor. Structurally, the sensor consists of a miniature Al<sub>2</sub>O<sub>3</sub> ceramic tube coated with a SnO<sub>2</sub> sensitive layer, gold electrodes, platinum lead wires, and a Ni–Cr alloy heater coil. These components are enclosed in a protective housing with a stainless-steel mesh.

The built-in heater maintains the sensing layer at an elevated operating temperature of approximately 250–400 °C, which is essential for activating surface reactions. The sensor operates at a supply voltage of 5 V with a heating power consumption below 750 mW.

In clean air, the SnO<sub>2</sub> surface exhibits high resistance. When exposed to ethanol vapor, surface reactions release electrons back to the conduction band, leading to a decrease in resistance. This resistance change is converted into a measurable voltage signal correlated with ethanol concentration.