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 obtained in a 1979 publication. The operational amplifier is of the type with...
This circuit was obtained in a 90’s Popular Electronics. The frequency depends on C1, a component...
Because it uses TTL integrated circuits, this siren must be powered by 5 V or as indicated with D1 to...