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Old 04-05-2010, 07:53 PM   #1
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Default Knock sensor filtration idea

So there have been some discussions here on the benefit of using a bandpass filter or the like to help out with knock sensing. JasonC even posted a diagram of a filter circuit/amp in this thread.
https://www.miataturbo.net/forum/t31284/

For those who do not know, the idea is that you use a bandpass to filter out all of the other background noise and just focus in on the frequency range that knock tends to produce. Jason indicated 13 kHz is about right based on his recordings. I run my electric det can through my stereo and used the EQ (16 band IIRC) to EQ out everything but the upper frequency range and IMO it definitely helps with hearing the knock. If you have ever listened to the Miata engine with a det can you know it is a noisy little bitch.

I was looking for a cheap and easy way to build a bandpass circuit to process the signal before it goes into my Adaptronic. The Adaptronic has an analog input for knock (not a digital like the MS) with a engine RPM filter. It works but he log plots show it is a really noisy signal. More discussion on that here https://www.miataturbo.net/forum/t34218/
The hope is that by filtering out all of the other noise that I can do a better job of setting the ECU knock threshold that controls timing retard.

So, rather than build a bandpass circuit from scratch, I wondered if I could just buy a small, cheap bandpass filter/amp that would achieve the same thing. So I started digging and found this, a cheap ($28 shipped) 4-band parametric EQ.
VISONIK HALF DIN 4 BAND EQUALIZER 1/2 CAR PARAMETRIC EQ - eBay (item 200301586749 end time Apr-16-10 11:47:00 PDT)
This seems like a win, since it is cheap, already built and packaged, already set up for 12V DC power, small (1/2 DIN) and the top parametric EQ range adjusts to either side of 13 kHz. Granted this is clearly not an audiophile quality product, but I am not using it to listen to a SACD recording of symphony, just using it to filter a knock sensor. I have run a knock sensor straight into a line input and it works. Plus this unit has input gain adjustment.

It is so cheap and easy to implement I think I might just get one and try it out unless there is some obvious fail mode that I have overlooked. Thoughts?
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Old 04-06-2010, 12:03 AM   #2
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that thing is the size of a small book. jason's circuit is probably the size of a postage stamp. just get some big shrink tube and encapsulate it and it's easily hidden away in the wiring.

either way, you still need power to the circuit or EQ and a line in and out.

with the EQ you'll have to figure out what 13 khz is and make sure it's adjusted right.

in any case, places like advanced circuits can make small pcbs for a few bucks each (if you get at least 10) and then you can just have someone stuff them with components and be all done for less than the $28 for the thing above per board. plus we all win.
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Old 04-06-2010, 01:04 AM   #3
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Nice idea but that EQ appears to not be a full parametric (i.e. it has no 'Q' aka "sharpness" ****). So its 15 kHz **** has a very broad 'Q' and theoretically won't work as well as a filter with a sharper bandpass characteristic. Its basically a 4 band graphic EQ with maybe a parametric EQ for the sub output.

I would like samples of knock from other engines to see if they are all near 13 kHz.
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Old 04-06-2010, 12:34 PM   #4
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Good point about the Q. It probably has a pretty broad boost band since it only has four parametric EQ channels.

Good point Y8S but I am not set up to do the board layouts, nor am I an analog circuit designer, nor am I short on projects, so I'm not motivated to go the DIY route. That is why I am looking for a turn-key solution. For this one I'd rather pay an extra $10-$20 over what I would spend on parts (DIY) just to not have to mess with it.

What I need is a small, DC powered single channel parametric EQ with a Q adjustment. There are guitar foot pedal units that fit this description but the only ones I have found so far do not have a high enough frequency range.
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Old 04-06-2010, 12:38 PM   #5
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If this could wait til after finals I'd be more than enthused to work on it.

I would imagine Jeff could get to it first, and offer it as an upgrade.
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Old 04-06-2010, 01:02 PM   #6
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ExpressPCB.com can do cheap boards but there needs to be enough volume (like 20 ish?)
Minimum order might be one *panel* which may fit 20 boards.
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Old 04-06-2010, 01:07 PM   #7
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Hmm
http://expresspcb.com/ExpressPCBHtm/SpecsMiniboard.htm

The can do 3 pcx 3.8" x 2.5" boards for $51 + shipping sent out the next day.
Each one would probably fit 3 of the circuits, maybe more, so you get at least 9 for $51.
All we need now is someone to lay it out in their free software.
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Old 04-06-2010, 01:17 PM   #8
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https://www.barebonespcb.com/!BB1.asp

any qty of any size board bigger than 1.5x1.5 inch ends up pretty cheap too. i dont think you have to use their software either.

plus free popcorn or pizza or some crap.
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