This is a smoke/gas detection circuit. The sensor is the GH-312 sensor and it is used to detect : smoke、liquefied gas、butane and propane、Methane、alcohol、hydrogen、etc. Also has a 16F84A microcontroller and a 8X2 LCD. The circuit is powered by a 9V battery but, since it’s used a 7805 regulator, a 12V battery can also be used. Attached is a picture of the circuit initializing. The LCD displays the circuit name “Gas Detector” and it’s version. After initialization the circuit will enter a normal state where it detects no gas. The display shows “Sensing No Gas”.
PIC16F84A Gas Detector using GH-312 sensor - [Link]