This circuit was obtained from a 1967 General Electric manual. The circuit uses components of the time, but equivalents can be used, for example, a BC547 for the transistor and a TIC106 for the SCR.
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...