This gas sensor circuit was found in July 1976 Radio Electronics magazine. The TGS sensor was a common type at the time. Note the power supply to the circuit.
We find this circuit in an American 80's documentation. It makes use of IC sensor which is mounted...
This is a configuration obtained from an old manual, using an operational amplifier of the time....
In the figure we have the way to add hysteresis to an operational amplifier. The relationship...