So I have decided to roll my own engine monitor. Mainly because this is experimental aviation and I like to experiment. My plan is to use an Arduino micro-controller as the brains and a monochrome LCD graphic display to present the information. Most of the sensors on the Rotax 912 are resistance sensors so I simply plan to read those in a voltage divider circuits using the analog inputs on the Arduino. What I need some help with is the RPM input. The Rotax 912 has a 1 pulse per rev probe that I think is a magnetic pickup. In any case, I found a document that shows what the output of the RPM probe is at min and max rpm with 2 different resistance loads attached as shown in the next picture.

rotax tachometer specs.PNG

Better or worse, I chose to go with the 100 ohm load which at 500 RPM has about a 2.7v peak voltage output and at 6000 rpm peaks about 8v. The low RPM peak isn't enough to trigger the Arduino interrupt and the high RPM peak is over the Arduino's 5v pin tolerance. The inputs have some internal protection so may be OK with the short duration exceedance but the low rpm being below what will trigger the interrupt means that it needs some conditioning. Now is where I confess that I am a rank amateur at electronics. I've done several smallish projects and approached this one the same way by researching and coming up with what I think is a solution. That is where I need some help. I would like to have someone take a look at the circuit below to see if I am on the right track and maybe offer some advice on improvements. Or, maybe I'm all wet and I'd like to know that too.

Tach input circuit.jpg

So let em explain how I think this circuit works. On the lower left is where the 2 wires for the rpm sensor would connect and that loop has a diode to clip the negative side of the pulse and the 100 ohm load. Then the voltage across the 100 ohm resistor is fed into one input of an LM393 comparator. The other input of the comparator is fed about 1v from a voltage divider circuit. Every time the pulses from the RPM sensor come there is a flip-flop of which input of the LM393 is seeing a higher voltage which in turn flip-flops the output of the LM393 on or off. Since the powe input into the LM393 is from the Arduino 5v rail the output of the LM393 will be 5v which goes through a load resistor to the pin. So what the Arduino should see is 0v to 5v spikes every time the rpm sensor fires.

I have asked the above question on several different electronics forums and haven't even gotten a response of any kind. Someone please help me even if it is only to say that I'm totally wrong.