We found this circuit in an article in Radio Electronics magazine from November 1971. The circuit has several sensor inputs and is based on programmable unijunction transistors or PUTs.
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...