How to transform an AC signal of wide voltage to pulses
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:



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":



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.