I'd refer posters to the original thread....
The signal required comes from a speed sensor and is (probably) used to adjust timing, fuel injection etc so needs to follow the engine rpm else the emissions produced would 'fail' the vehicle anyway.
It's not an 'AC' signal either. It's DC (with an AC component) i.e. it's a squarewave.
From the information already given there is (I surmised) a supply voltage fed to the sensor and a signal coming back from it. What I don't know is the supply VOLTAGE (could be 5V could be 12V?) but the signal seems to come back from a hall effect device - make your own mind up as to what that consists of.
Any comparator with a 'set' input lower than the signal source maximum (but higher than it's minimum) will produce an output that is as big as the comparator supply voltage (give or take) but without knowing what the MAXIMUM is allowed to be we can't simply guess and use (say) a 12V output for the potential of ruining the follwing signal processing circuitry.
Of course, an op-amp will do the same but the circuitry is (ever so slightly) more complicated - more so if it requires a dual-rail supply or offset biasing - hence my suggestion to use a comparator.
View attachment 36208
Here, the potential divider R2/R3 set the switching level and the voltage at the slider of R1 is the signal being compared.
As far as the OP is concerned R2/R3 are chosen such that the voltage at pin 7 of the IC is just lower than the lowest voltage produced by the shaft sensor and R1 is adjusted until the output at pin 1 switches between the supply and zero.
In a PRACTICAL circuit R1 would be removed and the original sensor signal would go to pin 6 and pin 1 would then go to where ever the sensor signal was meant to go.
R2/R3 could be replaced by a potentiometer (100K) to allow for adjustment of 'sensitivity'.