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 is from American documentation from the 70s. Transistors are not common today, but...
Found in old documentation, this simple test checks both the polarity and the existence of voltage...
This circuit is from a 1987 American publication called Electronics Handbook. The circuit can be...