Have you ever put a 'scope on a knock sensor?
Small signals indeed. You'll probably need to amplify them quite a bit, filter out the noise, determine what is steady state for your particular application (then have to redetermine what is steady state over a wide range of temperature's, fuels, driving conditions, etc.etc.), and I could go on and on...
Vehicle's have high speed DSP's in them for a reason, one of them to determine if the engine is knocking or not. A PIC generally can't handle this sort of job (read that as needing some sort of high speed FFT) without external help (fairly tight bandpass filters, amplifiers, etc). Knock sensors are generally sensitive to the 2-3khz range, which is what abnormal combustion resulting in knocking GENERALLY sounds like, but during normal combustion, these sensors are also putting out some voltage. It also depends on how tightly it's put in. Too tight and it's too sensitive, too loose, not sensitive enough. The torque value on the knock sensor for my Nissan is 18ft-lbs...not much at all. And the service manual specifically states 18ft-lbs in big bold lettering with a warning talking about how doing it by feel won't be nearly good enough for the ECM.
Not saying it can't be done...Anything's possible. But, without a 'scope, and/or good baseline data vs. good knocking data, in the first place, I don't see anything useful coming out of this, especially if you're messing with a computerized engine. How would a person get it to knock? On the old engines, just turn the distributor a bit, disconnect the EGR valve, create a vacuum leak to lean it out and power brake it, something like that...but today, short of forcing an ECM into a 'limp home' mode, I don't see how you can get it to knock reliably to get a decent reading on it. Not to mention that today's engines run on the ragged edge of knocking as it is... A little bit of knock is ok, a very little bit. In fact, most engines will knock just a tiny bit under part-throttle acceleration. And, one of the more important things...if you are retro-fitting an engine that didn't come with a knock sensor and were just going to plug one into a freeze plug hole or onto a bolt head...blocks and knock sensors are designed to work together. You can't just go slapping a knock sensor into any old hole and expect it to go 'BAM, it's knocking'.
At any rate, what's the end application? Maybe it can work...anything's possible...
Also, for the sake of ease, I'd lose the 7-segment LED and get a cheap 8 character LCD. A lot easier to work with in PBP (built-in commands), fewer wires/connections, less processing power, about the same price if you get a couple on eBay, and you can display more info.
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