I want to build a device that measures frequency of possibly different electrical alternators, not having much control of their parameters. For this I need to get digital pulses out of the AC signal from the alternator.
The only information I have, is that voltage from the alternator can vary from few V to 100 V, depending on their rotational speed. Frequency under the kHz.
The power usage of such circuit should be the small (<0.1W).
Output should be a digital signal of a fixed voltage. It's totally acceptable (and actually useful) to ignore signals below or close the high level of the digital signal (say 3.3 V).
How to get such pulsed output from the AC input, in a safe and fairly efficient way?
To make the question more generally useful, feel free to add more cases (smaller/higher voltages, higher frequencies).