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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).

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    \$\begingroup\$ You forgot to ask a question. Where are you stuck? Note: KHz is kelvin-hertz. You mean 'kHz'. What do you mean by 'pulses out of an AC signal'? There is an edit link below your post ... \$\endgroup\$ – Transistor Sep 8 at 9:20
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    \$\begingroup\$ A little better. What is the source of the pulses? Are you trying to measure current or PWM frequency, etc. What sort of motors? DC (fed by AC supplied controller), AC induction, stepper, brushless, ...? Can you see why the question isn't answerable yet? Add some explanation and/or some schematics. \$\endgroup\$ – Transistor Sep 8 at 9:34
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    \$\begingroup\$ @Transistor I added the question and fixed "kHz". I have some ideas on how to do it but I don't want to pollute the question with possibly wrong stuff. Something about getting the half wave first, then current limiting, then a Zener diode to cut the output voltage to a level. I'm quite sure I'd design a one time magic smoke emitter for the microcontroller that is supposed to count the pulses, so I thought it's better to get some opinion here :) \$\endgroup\$ – Kiwi Sep 8 at 9:37
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    \$\begingroup\$ @Transistor for some reason I wrote motor instead of alternator and that probably made the whole question much less understandable. Should be better now. \$\endgroup\$ – Kiwi Sep 8 at 9:42
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I want to build a device that measures frequency of possibly different electrical alternators

You could use a multimeter on "Frequency" mode. Otherwise, a very simple solution to generate pulses from your AC:

enter image description here

R2 limits input current. It should be as high as possible, but not too high, to provide enough base current at the lowest input AC voltage. D1 protects the transistor base against negative voltages. And... that's about it.

Now something more "elaborate":

enter image description here

R3/R5 bias the 74HC14 schmitt-trigger logic gate input at midsupply. C3 AC-couples the input. It forms a highpass with R3//R5, so it should be large enough to pass the lowest frequency of interest. 100nF cuts off at 0.3 Hz.

R2/D2 clip the input voltage to keep it safe for the logic gate. R6 is optional, it ensures that when clipping, most of the current will go through the zener instead of the ESD diodes.

That's about it... Instead of a logic gate, a microcontroller input can be used but for low frequency, low slew rate signals, a schmitt trigger is recommended. Or you can use the analog comparator in your micro, if it has one.

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not knowing the amount of radio-frequency interference on your signal, lets try to handle that.

this circuit has two input low-pass RC filters, with nominal 10usecond Tau or 16KHz F3dB.

The two stages will provide a gain of about 5,000 thus 1 millivolt input should cause the output to change from 0 to VDD.

To learn how this works, and to diagnose any design errors I provided, simulate this.

schematic

simulate this circuit – Schematic created using CircuitLab

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