This 1980s circuit was found in Motorola documentation. The circuit uses period components that admit of equivalents. Light feedback in a modern version can be done by optical fiber.
This is one of the possible configurations using the LM139, 239 or 339 voltage comparators. The...
Found in a 303 Circuits from the 90s, this circuit converts signals from the 10 kHz to 150 kHz...
This circuit is from a Radio Electronics issue of May 1972. It consists of a commercial heating...