We found this frequency meter circuit in a 1974 documentation. The circuit is powered by 12 V and has 8 digits. Changes can be made using a prescaler to reach higher frequencies.
This 1990 circuit can also be used with fixed sound systems. What it does is trigger a set of LEDs...
This circuit tests the earth connection in computer outlets and other equipment that requires this...
We obtained this circuit in an English publication in 1983. Equivalent operational amplifiers can...