Household harmful gas alarm circuit

This kind of gas alarm is suitable for small coal mines and households. The circuit of the alarm is shown in the figure. It consists of a gas sensor and a potentiometer RP
to form a gas detection circuit, and a time base circuit 555 and its peripheral components form
a multivibrator. When there is no gas gas,
the conductivity between A and B of the gas sensor QM-N5 is very small. Since the output voltage of the sliding contact of the potentiometer RP
is less than 0.7V,
the ④ pin of the 555 integrated circuit is forcibly reset, and the oscillator In the inactive state, the alarm
does not make any sound. When there is gas in the surrounding air,
the conductivity between A and B increases rapidly, the ③ pin of the 555 integrated circuit becomes high
level, the oscillator circuit starts to vibrate, and the speaker emits an alarm sound to remind
people to take corresponding measures to prevent Prevent accidents from happening.
    In addition to alarming the presence of gas, the alarm can also alarm for smoke and other harmful gases. Adjusting RP can make the alarm
adapt to alarms under environmental conditions of different gases and concentrations.