Found in American documentation from the 90s, this circuit can measure frequencies on a linear scale from 0 to 1 kHz. The components can be changed to measure other ranges.
This transistorized circuit with germanium transistors was found in a publication from the 1960s....
This is an old circuit, from 1997, when cathode ray kinescopes were still used in televisions. This...
This circuit found in a documentation of the 90s, uses integrated common at the time, but...