This shield circuit can be used in several applications that involve a feedback signal. The circuit is from American documentation from the 80s and the transistors are according to the loads driven.
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...