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.
This circuit, from a 1973 documentation, can be made with BC549 transistors. The frequency depends on...
The transistors in this 80's documentation project may be BC548, the most modern equivalents of...
This circuit was described in an article in the February 1993 Radio Electronics magazine. The...