This circuit was originally recommended to give the call signal on intercoms. However, it can be used with other applications. The circuit is from 1978 documentation. The transistor may be the BC548.
This circuit was found in a 1960s book of projects by Gernsbak. The circuit uses germanium...
This circuit was originally designed for testing crystals used in color televisions, but it works...
The circuit of a 1968 publication can be implemented with more modern transistors such as BC548 for...