buffering low variable ac source as input

[nomad]

hello, i have a variable ac signal from a hall effect? speed sensor i'd like to read pulses from. the voltage and frequency both change with speed. V can go as high as 12v. but is usually about 5 to 7v at normal speeds. i was going to use an npn transistor driven into saturation with a couple of resistors as basically a switch. with a 5v source via resistor at the collector and a 4.3ish v zener limiting base voltage and a small signal diode in series with the zener to stop flow on the neg part of the cycle. only interested in the pulses or frequency so distortion etc are not a concern. oh i'd like it to be able to switch at the lowest possible signal but be able to handle the higher and negative voltages. the output from the sensor will light an led starting about 80hz or 10 mph. want to get a response sooner is why i went away from opto isolator. any suggestions on a different route to think about or try?