This 1975 tachometer for cars with platinum can serve as an idea for new projects. The components are from the era and a modern equivalent operational amplifier can be used.
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...