With this circuit it is possible to use a frequency meter as a tachometer. The circuit was found in American documentation from the 90s. The phototransistor can be of any type.
This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can...
This voice scrambler and descrambler circuit was found in a Radio Electronics from July 1963....
Using a 4017 as a base, this circuit from a 1990s Popular Electronics generates low-frequency...