This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by BC548. The relay must have 100 mA of maximum current.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...