This circuit recognizes a tone determined by the tuned input circuit (R3 / R4 / C2). The circuit was found in Radio Electronics from the 90s. The 4046 is not a common component today.
This space mark demodulator for FSK signals was found in an American telecommunication...
This one transistor stage for audio signals was found in an old publication of the year 1958. The...
This audio step operates directly with the 110 V mains voltage rectified and reduced. The output...