Square waves and optocouplers, any tutorial?

Using Arduino Project Guidance
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Hello everybody. nitro-ale here from italy with rusty english, so I apologize in advance.

A friend of mine asks me help for an automotive project. The goal is to read a signal from the car to detect the engine rpm.
So we are working with a square wave and this square wave is our input and since this input is 12V I want to split up the problem in two subproject.

  1. READING A SQUARE WAVE
    I have no experience with this. From my research I think I should use pulse(). So first question: where can I find a walktrough tutorial to set up a circuit to simulate a square wave and reading it?

  2. ISOLATING THE MICRO
    Same here, I think we can use optocoupler to isolate the arduino from the 12 V input signal. I am not used to this kind of work, is there any good tutorial that can help to understand how to use an octocoupler for isolating an input signal?

Thanks in advance, any suggestion is appreciated

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