Demo project "Gas leak detector" Mq-2 + ESP8266 + SIM900;
Wifi modele detect gas leak using sensor and send data to SIM900, which send SMS to user.
To configurate phone number and sensor's threshold go to http://192.168.4.1 in a web browser.
Connect SIM900 to ESP8266 using pinout D3, D4. Mq-2 to ESP8266 - using pinout A0.