This circuit was suggested by Motorola in a 1969 publication. It makes use of components of the time, but both the FET and the bipolar may be general purpose current types.
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...