One of the gas sensitive alarm circuits

Gas-sensitive alarm is an alarm realized with gas-sensitive components . The gas sensor is a "gas-to-electricity" conversion element, which
is very sensitive to oxidizing or reducing gases and organic solvent vapors, and can be used to detect and alarm flammable gases and poisonous gases.
    There are two types of semiconductor gas sensors: N-type and P-type. When the N-type gas sensor encounters reducing gas, its resistance decreases;
when it encounters oxidizing gas, its resistance increases. The characteristics of P-type gas sensors are exactly opposite to those of N-type.
    Commonly used gas sensors include QM-N5, QM211, MQ-N, MQ11 and QM31, etc.
    The circuit is shown in the figure. It uses the 555 time base integrated circuit A (used as a multivibrator) and the 7805 three-terminal fixed
integrated voltage regulator.
    Adjusting the potentiometer RPi can change the sensitivity of the device; adjusting RP2 can change the tone of the alarm sound.