Knock amplifier

[John Bennett]

Firstly, I'd like to say that I'm not parading this as one of my inventions, hell, I got the idea off one of JCD's posts, but today I built an amplifier circuit for a knock sensor.

Downloaded the datasheet for the LM386 audio amplifier. Added a 12V regulator (and a DC-DC 12V isolator) to the 200x gain circuit given (page 5, top right) , fed the output to a pair of headphones and grabbed a knock sensor from a Volvo 440 and a plug from a spare harness, then bolted the sensor to the cylinder head.

Rather impressive - it's crystal clear SR20DET sounds like a mad sewing machine You can even hear the VVT solenoid click in. No discernable sound of det so far. Bit more boost should change that

apologies for w*nk phone photos. Camera went missing when moving house . I can try and get a recording tomorrow ( if I can find something to make it with).

[jrtuning]

i would say top bodgeing but this is a bit better than that so top work

[John Bennett]

Average bodging I'd say . Think I wasted the whole of the working day on car stuff (my dashboard's up and running again now too ).

It's definitley worth people getting some bits and having a go. You can do the whole thing for considerably less than a fiver and knock sensors are easy enough to pilfer from scrappies. You can also save bits and time and forget about wiring the 'bypass' capacitor onto pin 7, and the 0.05uF cap & 10 Ohm resistor on pin 5.

[matt_j]

PML bennett good work well done If only you applied the same dedication to getting your bennett end away you would be living in the playboy mansion by now

[John Bennett]

PML bennett good work well done If only you applied the same dedication to getting your bennett end away you would be living in the playboy mansion by now

The knock sensor senses a distinct lack of knocking

[matt_j]

The knock sensor senses a distinct lack of knocking

what on EARTH are you attaching it to at the moment then Landlady said she would see you for a drink Get your concave forehead over here

[spesh]

Nice!
Would attaching it to the head be the best place to be able to hear det tho?

[John Bennett]

Nice!
Would attaching it to the head be the best place to be able to hear det tho?

Nope, probably the worst place I was in a hurry to try it, as it was getting dark.

To be honest, I expect you'd still hear it in a lot of cases - if you can hear it in the cabin when it's really bad, you'll certainly hear it from the head.

[spesh]

Just thinking that there are some 'spare' holes where the aircon pump would be on my 13 on under the pas pump, would this be a more suitable place? Gonna take a trip down maplins and get the stuff to make one of these - fantastic idea

[blue boy]

Excellent idea. My skills on this kind of thing are not up to scratch but if it really is possible for a dum arse liek me to build then i will give it a go.

[200krisx]

If only we could get plugs like the new BMW M5 use that are also used as knock sensors (I'm sure I read that somewhere )

I want to make one! Cant you make up a display unit e.g out of a cheapo AFR gauge?

[Squares]

The standard knock sensor is below the inlet manifold, pretty much attached to the middle of the block if it helps.

Good work John, could it in theory now be rigged up to some form of visual indicator?

[John Bennett]

Here's a rough PDF of the design I used (without the DC/DC converter as you probably don't need it). It's not exactly a circuit I've spent time altering, so suggestions for improvements are welcome (plus I don't muck around with audio amps much). It's for the 2-pin Bosch-type sensors that can be connected either way around.

Here's the link I looked at for an alternative design (I didn't use it as I didn't have the bits handy :
http://home.netcom.com/~bsundahl/kno...n.htm#Recorder

Playing with the 'frequency capacitor' value will affect what frequencies you can hear. The smaller you make it, the less bass noises you can hear as it filters them out.

I may get onto a visual indicator at some point. You'd want to filter the signal a bit more I'd imagine, or it'd be displaying all sorts of crap. . Suppose someone else could try it, if I don't get onto it...

[nene]

Dammit

Lum was asking me about this just before the scooby got nicked, but then it got nicked so we never got around to doing anything .

I was thinking of building a notch filter to just grab the characteristic frequencies of knock and feeding that to an audio output, where it could drive headphones and a VU meter. I don't know what the frequencies produced are, though. (yet)

One thing that does come to mind though is as I understand it the top of the engine is going to be really noisey anyway, with a lot of high frequency noises that could mask the sound of knock occuring. The human brain will be able to easily discern it, but when you're just looking at the amount of sound energy present in a frequency range you don't have that sort of ability. Maybe down by where the aircon pump attaches on the passenger side of the engine would be a good place?

[John Bennett]

Yeah, as I said to Spesh, I just stuck it on the head as it was getting dark. With the engine off you can hear the water bubbling inside and the slave cylinder moving when you push the clutch . I'll find somewhere lower on the block one evening this week.

I think the whole band-pass thing is a bit exaggerated for simply listening to knock (I can see the purpose when making a gauge though). At 6000rpm the engine just sounds like a quicker version of 1000 rpm and the volume difference isn't that huge, so I'm expecting when det strikes, it's going to overwhelm this noise.
Fry could make his SR20 det on demand (17psi) and it was extremely loud (from the cabin), so I'd be amazed if this wasn't deafening through this amp. There was actually a small amount of crackling I could hear when flooring it in 6th at 70mph, I must make a recording. I heard somewhere that a very small amount of det is normal, must go look it up a bit more....

There was a site telling you how to work out the fundamental knock frequency for engine somewhere, I'll have a look for that too.

[nene]

I'll leave this to you, for the time being

The car that really needs it (the B4) is in the bodyshop/garage after being recovered from stolen and my own car has .. well .. about 5 halves of an engine, and not a single one fully assembled at the moment

I do want to persue this on the B4 though. It already has a Bosch knock microphone attached for the `KnockLink ™`, though the means of detection it uses seems a little haphazard so he was questioning me about making one that's better and cheaper. It's already lost one engine due to the previous owner running it on 95RON and even with Tesco 99/Optimax/V-Power plus octane booster it's an experience we're anxious to avoid.

[spesh]

Hrm, Maplin have discontinued the LM386

[John Bennett]

Hrm, Maplin have discontinued the LM386

was that just typing 'LM386' into their search?


I searched for audio amplifier IC's and this was there
http://www.maplin.co.uk/Module.aspx?...%20IC&doy=14m9


Maplin are rather pricey, mind -it's around 30p in Farnell. (Not that that's a concern for cheap one-off circuit boards)

[spesh]

my searching skills are kinda poor lol Even been in maplins this morning for some other bits and asked them to look and they couldnt find it hehe

Cheers John, I'm gonna knock one up over the next week or two and have a tinker although my cars off the road and i dont fancy trying it on my derv fiesta run around cos that will break my eardrums with diesel knock lol

[David Reid]

John to help you if you dont already know.

I would expect knock to occur in the 10-20 Khz range.

What range are you using?