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Hello. I am new here and am a hobbiest/tinkerer who has no formal electronics training but who is self tought on an as-needs basis = I have little knowledge :(

I am currently working on a data logging project using an Arduino. The specific sub-project I currently require help with is taking a sine wave signal from an induction sensor and converting it to something that the Arduino can use for engine RPM calculations. (I know a hall effect sensor is much better for this but I have the induction sensor already installed and need to use that).

The induction sensor is outputing a sine wave that has a peak to peak voltage that varies with engine RPM. At engine idle it can read as low as around 8v P2P but at redline may read around 100v P2P.

For my final RPM calculation I will not reference the quantum of the sensor voltage output and also need to restrict it to something that the Schmitt Trigger can handle. I therefore propose to use a pair of Zener Diodes with bias voltages to clip it to +/- 4.7v (or thereabouts).

This clipped voltage then gets processed by an Op-Amp that unlimately generates a square wave output of +5v/0v that the Arduino can use.

After many (and I mean maaaany hours of fudging around) I have come up with this;

This is a public project so I am not sure if I should be pasting a URL to it or this screen image?

I have run a simulation using a sine wave P2P of 100v on a 1Hz cycle (didn't go faster as it gets hard to see the resultant graphs properly). It appears to work..........

I have a couple of questions however;

1. Can somebody with electronics knowledge (as opposed to my guesstimation!) look over the circuit and comment?

2. As I need the final output to be +5v or 0v I had to connect the Vcc +/- backwards. The simulation allowed me to do this but can it be done in reality?

3. The trigger points that my resistors produce do not match a Schmitt Trigger calculator that I found. These resistor values should give an output high voltage at 0v and a low value at -0.833v but the simulation graphs don't appear to agree with that. The sine wave signal may or may not be clean so a little hysterisis (correct term?) will be a good thing. In an ideal world I would like the output to set high at a sine wave input of 0v then set low at something like -1v. I have not been able to achieve this....sighhhhhh

Your help is greatfully appreciated!