Found in Popular Electronics from the 90s, this circuit is based on a PLL567 and employs high-end or ultrasonic speakers depending on the frequency of oscillations.
This circuit is from 2001 documentation, serving to multiplex stereo signals for FM transmitters. The...
This circuit reverses the signal if both positive and negative. We found it in an American...
Found in a publication by Gernsback from 1960, this circuit makes use of germanium transistors of the time....