Proximity probes? Or accelerometers?

I understand about the analog circuitry and actually plan on making a five
or six channel filter at each of the possible resonance points relative to
the fundamental ... and then strobing the filters to light a "normal",
"low", "high" lamp for each channel.

Jim




As is tradition, we tend to get off topic ;-)
I think you can accomplish this using the methods discussed.
Mount 2 proximity probes 90 degrees apart.
Calibrate the readings for normal low vibrations (be sure to account for
any nonlinearities in the probe).
Design the circuit to trip the buzzer/lamp when the vibration exceeds the
normal level.
You may need some analog circuitry to help (gain, etc).
But you don't need to get much fancier than that.

Adam