This circuit is from a 1968 documentation using one of the first integrated circuits available at the time, the TAA370. The circuit is powered by 1.3 V using a 200 ohm handset.
The transistor may be the BC548 in this circuit for low frequency signals. We found it in an old...
This circuit was found in documentation from the 1980s. The circuit can be implemented with other...
Using a period sensor, this circuit was found in the February 1986 issue of Radio Electronics...