The speed indication of this circuit is made on an analog instrument. The circuit is used to count pulses (rpm) and was found in April 1976 Radio Electronics magazine.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...