Simple gas smoke alarm circuit principle

Crystal oscillator

Industrial Router Crystal 3.2*2.5mm 3225 26M (26.000MHZ) 12PF 10PPM 20PPM 30PPM

Photocoupler

As shown in the figure, the simple gas smoke alarm circuit. The alarm is composed of a step-down rectification and voltage stabilization circuit, a gas sensing element, a triggering, an alarm sound circuit and the like. The step-down rectification and voltage stabilization circuit consists of a transformer B, a bridge rectifier circuit, and an integrated regulated power supply 7805. The triggering and alarm sound circuit is composed of a controllable multivibrator (555, R2, W2, C4) and a Speaker Y. The semiconductor gas sensor adopts QM-25 type or MQ211 type, which is suitable for alarms of gas, natural gas, gasoline, alcohol, ether and various kinds of smoke. The heating voltage is required to be stable, so the voltage of the filament is regulated by 7805, and It is required to warm up for 3 minutes.
When the gas sensor QM is exposed to flammable gas or smoke, the resistance between the A and B poles is lowered, so that the voltage divider of the resistor and W1 rises, and the potential of the 5554 pin rises. When the potential of the 4 pin rises above 1V. When 555 starts to vibrate, its oscillation frequency is f=1.44/(R2+2Rw2)C4. The frequency corresponding to the parameter shown in the figure is about 0.6~8kHz. Adjust the potentiometer W2 so that its frequency is 1.5kHz. The output signal of the corresponding pin 3 of the corresponding 555 pushes the speaker Y to sound an alarm.
Under normal circumstances, the resistance between the AB poles of the gas sensor is large, the voltage divider of the resistor and W1 is reduced, and the 5554 pin is at a low level, the 555 is stopped, and the speaker Y does not work.