buffering low variable ac source as input

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?

Maybe a LM741 as a comparator. I think the lowest voltage that it will see is 1.2 volts, so have it rigged to trigger at anything over 3 volts.

A mosfet with a low gate saturation, but would still handle 12 volts gate. I do not have a part number.

Not sure about the AC signal, in my experience a hall effect will produce a square wave.

hall effect may not be right then. i know its a magnetic sensor with 2 wires out. made a nice sine wave on scope. couldnt really read it while driving though. i could imagine the police report, distracted measuring signal on scope when he ran over the toyota.

thanks for the reply. found a better version of what i was trying to do. http://www.dainst.com/info/circuits/zero_crossing.html pretty close to my original plan.

well i ditched that idea as counting low freq pulses is far less accurate than timing the pulse. I went similar to your idea, actually using a 339 comparator. While the data sheet stated the differential input voltage can exceed supply voltage with no damage, it failed to state that over supply voltage it's output is VERY erratic. it worked well up to 240 hz then went wacko. i used two led's in parallel to the input as an ac clamp. the max output of the sensor doesnt even light the led at full brightness. this solved the higher freq but introduced erratic output when there was no input. i then used two 10k pullups to stop that, and it almost work, it's good at a stop or over 20 hz, but still flakey at low input voltages. ( less than .5v ) so i'm going to remove all that and supply the comparator with 13.8v. With the 339's open collector output i think this may work out. my backup is use opto's instead of leds to signal between stop or ok to read. who needs to read below 3mph anyways. lol thanks Mackrackit.

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lol thanks Mackrackit

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Any time, nice to know I actually helped!