Found in an English publication from the 1980s, this circuit can be built with equivalent transistors such as the BC549. The signal must be applied to an external amplifier.
We found this circuit, which appears with variations on this site, in a European documentation from the...
This circuit that drives an electromechanical counter of the time was found in Radio Electronics magazine...
Found in a Hands On Electronics of the 80s, this circuit makes use of a gas sensor of the time....