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.
This circuit was found in documentation two years 80. Transistors can be modern equivalents. Or...
This circuit, very critical due to the frequency at which it operates, was found in an American...
This stereo circuit with the 2002 integrated circuit. The power is 6 W per channel with 16 V of...