dougie2 wrote: And re. your discovered Filter error.... there isn't any.
dfabrio has himself mentioned that some people have had no success with the active filter, and you will see also that poliux could not get any signals through his active filter either. The reason for this is because the active filter produces far too much attentuation, because the amplifiers aren't really doing anything, and so it's working as a (very poor) passive filter. With the modification I have done, the amplifiers are now working properly and it's an active filter again.
In the original schematic, both operational amplifiers have their non-inverting (+) inputs tied directly to 0V. For this reason, the amplifiers won't really be doing anything, and this means that the AC signal is really just passing through the passive components around the amplifiers. To work properly, the amplifiers must have a virtual ground instead. These are the most basic operational amplifier principles.
You can download PSpice student version for free, and I can also send you my design files if you like. You can simulate both the original circuit, and then the modified circuit afterwards; you'll then see how the filter works properly.
To be truthful, even in the original circuit, the bandpass filter does still work (giving a broad bandpass region roughly around 6kHz), but with a LOT of attenuation (about -40dB). This is because the amplifiers aren't working as they should, and so the filter is working almost purely as a passive filter. In fact, U1B is doing absolutely nothing at all. If you were to somehow disconnect only U1B, you'll see that it makes no difference
So, the truth is that the original circuit will probably work okay if your knock sensor produces a large enough signal. This is why it works for you. But, for other people, such as myself and poliux, the filter is far too 'deaf' because it attenuates too much. With the correction to the circuit, both amplifiers (U1A and U1B) and working, and the filter is working as a proper *active* filter again; this means that the attenuation at 6kHz is -3dB rather than -40dB and therefore the active filter should work for everyone.
Aside from that, the 'two' errors that I introduced from the original schematic: I corrected one error after you pointed it out (misplaced output connection, which was purely a markup error); what this second error is I don't know.
The original author, various other people, and myself have identified that the active filter has a problem and does not work for some people, and I have presented here a way to solve it. I enjoy working on problems with people and totally welcome any suggestions anyone has or criticism of ideas I have put forward; indeed, you pointed out some small errors in my diagram for which I am grateful. If somebody gives a firm technical argument against what I have done then brilliant - I'll learn from it. But I would be really grateful if you would not discredit what I have done, or suggest I have spoiled the thread, just because of an unrelated schematic mark-up error that I have since corrected.