Coal mine gas concentration exceeds standard alarm circuit

Circuit composition The gas over-limit alarm miner's lamp circuit is composed of a gas-sensitive detection circuit, a multivibrator , an audio output circuit and a lighting circuit. The gas-sensitive detection circuit is composed
of
    a gas-sensitive sensor and an electric... ．
Resistor R2, wind and components. The multivibrator is composed of two NAND gates Dl and D2 inside the
NAND gate integrated circuit IC,
resistor R4, winder and capacitor G.     The audio output circuit is composed of resistor, audio amplifier tube Vr and speaker BL.     The lighting circuit is composed of battery GB, lighting lamp EL and light switch S.     (2) Alarm principle: When the indoor combustible gas concentration is within the allowable range (lower than the limit value) , the gas sensor A and B are connected. The value is larger. The voltage of pin ① of lC is low, the multivibrator does not work, and there is no sound from the speaker BL.     When the gas in the mine exceeds the limit, the resistance between the A and B terminals of the gas sensor QM-N5 decreases, causing the voltage of pin ① of the IC to be higher than the conversion voltage of Dl in the IC. The multivibrator works and starts from the IC. The ④ pin outputs an oscillation signal. After the signal is amplified by VT, it drives the speaker BL to sound an alarm. Adjust the resistance value of R2 so that the voltage between gas sensor a and b is 4.5V.