toxic gas alarm

1、Working principle: when the ambient air is normal, the corresponding resistance value of the gas sensitive element QM-2, the resistance in the R3, RW voltage to make the 555 set, 3 pin output high level, the alarm circuit is disconnected, no alarm. Voltage when the detection of gas sensor to gas, liquefied petroleum gas, gasoline, alcohol, smoke and other toxic gases, its resistance decreases, the resistor R3, RW partial pressure increases, thus the 555 reset, pin 3 output low level, making the relay and switch on the alarm circuit, sound and light alarm signal. 2、Description of each function module: ![enter image description here][1] 3、Components and PCB board map: ![enter image description here][2] 4、Physical map after welding: ![enter image description here][3] ![enter image description here][4] ![enter image description here][5] [1]: /editor/20160323/56f1eae0d244a.png [2]: /editor/20160323/56f1eb4166c6a.png [3]: /editor/20160503/5728403048db8.JPG [4]: /editor/20160503/572840355160b.JPG [5]: /editor/20160503/5728403928454.JPG