F
Fred McKenzie
MS-
I once had an automotive tachometer that hooked between the engine's
points and ground, and was calibrated in RPM. I do not recall, but
there may have been separate scales for 6 and 8 cylinders.
I don't think it had any active components, and didn't require an
external power supply. It was measuring frequency by clipping,
differentiating the waveform and reading the integral of the pulse on a
sensitive meter movement. It was calibrated from a 60 Hz sine wave.
You may find something similar (or one with active circuits) if you
search for analog tachometer circuit on the web. If you have an old
automotive tachometer, just trace out its circuit.
Using it for higher audio frequencies should be a matter of calibration.
Fred
