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Please Help im simply after an approximate RPM from a car knock sensor with an arduino..
the knock gives a lot of 'noise' and difficult to just get approximate RPM from.... nearest 200 Rpm is fine! for what I need.
i am trying a comparitor going to a schmitt and then using arduino (black pill M4 Processor) pulse time frequency counting.
It works perfectly with a function oscillator..... very accurate to 1Hz..... but then on a knock sensor.... eeeek! too much noise.... any ideas? without going to a TPic8101 or similar dedicated chip. any ideas? please.

I guess I am just not getting it but it seems a bit impossible to read RPM based on knock sensor?! Also, WHY?!

I don't think that I can help you, but maybe your knock sensor is badly placed, if you put it on the cylinder 1, you have maybe less parasite noise that in the middle of the block.
DO you use a full band bosch sensor ? They are know to be the best.

I am using a lilknockmeter, and it detects RPM, I don't know how, but it use an atmega cpu, just like arduino. You can maybe take a look at how it does.

I assume that the knock sensor works as a kind of microphone and delivers an analog signal.
If so, amplifying, digitizing and then applying a FFT would give the fundamental frequency of that signal that, I think, should be related to the RPM you're looking for.