Need to convert 40+ vpp pulse into clean 5v signal

Projects General Guidance
1

I am working on building a tachometer for a small engine and I am at the point where I need to get an rpm signal from the engine. I would like to use a wire wrapped around the spark plug lead to get a capacitively coupled signal.

I have used my o-scope and measured a 40v p-p signal from a single turn of 18 gauge wire wrapped around the spark plug lead.

I need to know how to convert that pulse into an arduino-friendly 5v pulse using either ICs or discrete components.

2

wrap it 3 times around the wire and then 10 times through a ferrite loop.

that would be the primary half of an isolation transformer.
then you can wrap as many loops in the ferrite loop to get your voltage.

alternately, you may be able to light an LED? ie: optoisolator and use that.

3

Try a pair of [u]clamping diodes[/u] (the 2nd schematic on that page).

I'd recommend a hiigher resistor value (maybe in the range of 1K to 10k).