<a href="http://www.msdignition.com/1protool.htm#anchor168229">MSD knock alert box</a>
This is WAY too expensive for what it is. Is there any way to build one of these units and tap into the original knock sensor somehow? If not I think I'm going to end up getting a scanner sooner than I thought.

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, Jon (350 TBI!)
91 Red My website

 

This topic was discussed several months ago here, and the general consensus was that nothing was available (although you and I both know that is not true - you posted one source, and I remember seeing one in JEG's which I believe was the same MSD) and no one could come up with a circuit. One post had a link to a Gran National site - the following is the schematic from that site: www.gnttype.org/techarea/projects/knock/circuit.gif that showed using a 555 IC to alert you when the knock sensor was detecting knock.

I breadboarded that circuit and it wouldn't work. I tried modifying it and fiddeled with it for hours, and I still couldn't get it to reliably work.

I believe the the knock sensor is a pizoelectric device (not sure), so the output is not just "switched ground", but a varing (AC, like audio?) voltage. Pizoelectric elements generate a voltage when struck - that is the principle that spark ignitors (such as used to start a stove flame or propane torch) use - the harder you hit, or strike it, the higher voltage it produces (thousands of volts if you strike it hard enough).

That said, I think that the device you posted a link to (MSD) uses a microprocessor programmed to process the signal in a specific way. In other words, a fairly complex electronic device that a "home brewer" would have a hard time duplicating (where are you going to get the program? writing the program yourself is a task outside the scope of most of us).

So far, I have yet to see a detailed explanation of how the output of the knock sensor is electronically processed.

You might have to just bite the bullet on this one.

EDIT:
I just checked, it was JEG's where I saw the sensor and yes, it was the same MSD for $150.

[This message has been edited by Stuart Moss (edited May 27, 2001).]

 

Yes, knock sensors are piezoelectroc, and produce an AC signal which varies rapidly in amplitude and frequency. Knocking is indicated by a large high-frequency component. I don't want to believe that there's any DSP going on in the stock computer which is a relatively stupid device. I imagine there'd just be some sort of analog filter to discriminate the high frequency knock from the background noise.