Thank for yours many reply.

I have test this circuit for read the "W" output alternator:



    input  >------ R 2,2K-------------*-------------- R 10K ------    >  to
pin RB6 ( input  pic 16F84 )
                                             I
                                         I
                                         I
                                  Zener  5,1v
                                         I
                                         I
                                         I
                                     0v

Results:
Truck at   600T/mn  pulse = 2,2 ms  F= 227 Hz
Truck at   700T/mn  pulse = 2,0 ms  F= 250 Hz
Truck at 2550T/mn   pulse = 0,7 ms  F= 715 Hz


My problem :
Some parasites on the "W" output alternator degrade the signal end the result
is wrong.

How it's possible to perfect the electronique circuit ?

Thank for your help

Jacky

Weslay@aol.com