eBay - MQ-2 Smoke and Gas Sensor Detector Module
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The MQ-2 smoke and gas sensor module is sensitive to smoke and to the following flammable gases: LPG, butane, propane, methane, alcohol and hydrogen. The sensor has analog and digital output.
This sensor is compatible with Arduino, Raspberry Pi and other development boards. Get started with the MQ-2 smoke and gas sensor with Arduino: Guide for MQ-2 Gas/Smoke Sensor with Arduino.