Combustible gas alarm circuit

The combustible gas alarm circuit is shown in the figure. The circuit consists of a gas sensor, a multivibrator and an audio output circuit
. The multivibrator is composed of
two NAND gates lu inside the NAND gate integrated circuit, IC2 and external RC
components. The audio output circuit is composed of resistor
Rs, audio amplifier tube VT and speaker BL.
    (2) Working principle When there is no polluting
gas in the room or the concentration of combustible gas is within the allowable range
(lower than the limit value), the resistance between terminals a and b of the gas sensor
is larger, and terminal b (pin ① of IC1)
The output voltage is low, the multivibrator does not work, and
there is no sound from the loudspeaker BL.
    When combustible gas (coal gas or natural gas) leaks and the indoor combustible gas concentration exceeds the limit value,
when the output voltage at terminal b of the gas sensor is higher than the conversion voltage of IC1, the multivibrator works and the oscillation is output from the ⑦ pin of IC Signal. After the signal is amplified by VT, it drives the
speaker BL to sound an alarm. Adjust the resistance value of Rz so that the voltage between gas sensor c and d is 4.5V.