Forum > Engine Management

an electronic det can that works ~~~~NSFW~~~~

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--- Quote from: Danbob on March 23, 2018, 02:38:41 AM ---Sorry for dredging up an old thread, but it's a useful one!

I'm currently building a similar device but using a different brand of automotive stethoscope.  I've two questions I'm going Nock or somebody who has also built one could answer.

1.  Did you find it best to use a broadband sensor like the Bosch one! Or would using a tuned resonant sensor designed for that engine bore help filter out background noise?

2.  What is the function of the 33ohm resistor? Where abouts in the circuit did you have to put it?
My PCB is a different layout so I cant follow the photo for reference.  Would different value resistors suit different engine bores? Or is it not relating to that?


--- End quote ---

I can't tell by looking at the back of that board, but I'm assuming its either being used as a voltage divider to get the sensors output in the correct range for the chip so it doesn't sound like noise or its being used to match the impedance of the sensor to what the chip is looking for.  You could probably replace it with a little 100Ohm pot so you can tweak it until it sounds right.

Pretty cringy reading my posts from 6 years ago lol.

And back from the dead!

I didn't know anyone still lived here.

Anyway, you are correct the added resister goes right across the biasing resister that is already there. You need to make sure that what you have is actually a charge amp, meant for piezo input, and not just a high impedance op amp. Usually this will be a jfet amp (TL072). There is usually a feedback cap, in the hf stethoscope, the resister goes directly across it. Also don't be a afraid to play around with it. You can't really blow up a change amp unless you smash the sensor with a hammer, or ground the output, or something stupid like that.

Does hf no longer sell this?


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