Sound card scope probe

Ask questions about DIY board assembly issues, fault finding and testing. (Covers all Megasquirt versions and board.)

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jsmcortina
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Sound card scope probe

Post by jsmcortina »

I find this such a useful and simple device that I felt the need to share it here. I'd urge all Megasquirt users to build one !
I use it with freely downloadable software "Audacity" from Sourceforge to record crank signals as a simple and highly portable alternative to a two channel scope.
soundcard scope-probe
soundcard scope-probe
scope-probe.png (4.92 KiB) Viewed 2402 times
laptop scope probe
laptop scope probe
100_1184.jpg (82.71 KiB) Viewed 2402 times
James
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a73camaro
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Re: Sound card scope probe

Post by a73camaro »

Since there can be up to 15 volts being measured on an automotive system, there should be a resistor divider attenuating the measured signal. I believe that sound card inputs are 200 millivolts peak. There are sound cards that have a capacitor on the input so DC or very low frequencies will not get measured.

I'm not trying to rain on your parade, just point out some limitations with the design.
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jsmcortina
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Re: Sound card scope probe

Post by jsmcortina »

Did you notice the 1N4001 limiting diodes?

It is "cheap and nasty" but it works and I have found it extremely useful.

James
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adamsys
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Re: Sound card scope probe

Post by adamsys »

Great timing! I just bought an external USB sound device to experiment with. Cheap ($25USD) and protects my laptop motherboard.
Did you notice the 1N4001 limiting diodes?

It is "cheap and nasty" but it works and I have found it extremely useful.
I did notice the diodes. They seem to be going in both direction, what is the theory behind that.

Here is a link to WinScope, software that uses the soundcard as a scope too. http://www.zen22142.zen.co.uk/Prac/winscope.htm"

They have info regarding the microphone and line-in voltages:

"The maximum input amplitude should never be higher than 10mV for a Mic input and 2Volt for the line input."

I assume the diode are a way to deal with this, no?
a73camaro
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Re: Sound card scope probe

Post by a73camaro »

The diodes are voltage limiting. Once voltage gets above ~0.7 volts, the diode conducts and limits voltage. The diode in the opposite direction is for negative voltages. So yeah, I guess I noticed them.
Last edited by a73camaro on Fri Dec 10, 2010 7:53 am, edited 1 time in total.
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hassmaschine
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Re: Sound card scope probe

Post by hassmaschine »

neat, I'll definitely have to build one of these. I probably already have all the parts..
a73camaro
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Re: Sound card scope probe

Post by a73camaro »

Here is a circuit that attenuates the input signal
Image

From here http://www.electronic-engineering.ch/ra ... nnect.html
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jsmcortina
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Re: Sound card scope probe

Post by jsmcortina »

As I said before, I've found the basic circuit I posted works for me. Try it.

James
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vlopsahl
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Re: Sound card scope probe

Post by vlopsahl »

I made one after the first circuit posted. Tested it on my Audi A4 1,8T with a scope software I found on the net, Sound card scope 1.32 from Zeitnitz. First I tested the 60-2 trigger. I got this shot of the missing teeth. With this type of scope, is it so that I do not get any useful information about voltage? The scope software also makes an excel-sheet, and I calculated the idle to be around 900 rpm to check that it worked. I also tested the hall-signal on the cam.

Then I tested the idle control, integrated with the throttle body. It seams to be running at 500 Hz. This is a challenging thing to drive and I believe nobody has successfully made it work with standalone. I also think it is some kind of stepper drive, or dc-motor drive with a fine scaled TPS that sends feedback to the OEM ECU. Any thoughts on this?

Thanks
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S.Bretz
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Re: Sound card scope probe

Post by S.Bretz »

So this thing is hooked up to power and ground and turns slight voltage variances into an audio signal that can be looked at with some free software?

If so this would come in handy with the car and with the cars I work on at work.


Do both the left and right channels go to the same test point? and just ground the ground wire?
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Re: Sound card scope probe

Post by jsmcortina »

Connect ground to ground
Left = signal 1 to be measured
Right = signal 2 to be measured (if required)

Low voltages only.

James
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ol boy
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Re: Sound card scope probe

Post by ol boy »

Could this be used as a new software based VR conditioning circuit? Just an idea seeing the posted screen shots.
Thanks Ryan
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ol boy
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Re: Sound card scope probe

Post by ol boy »

Okay here's what I got after building the above 2 channel scope.
201wheel.jpg
One question.. or two.., okay three.. Where is my missing tooth in this picture? I have 41 tooth events on the picture shown but I don't see the missing tooth. Wheel is a 20-1. Should I have reversed the two leads on the VR sensor and see if that gave a slightly different log? Can I also log the 5 volt TTL for the LS2 coils, or is that voltage too high?

Thanks Ryan
306 SBFord, Torquer II EFI intake, 60 lbs injectors, 8 LS2 coils, VS Racing 7668 turbo, 4R70W, MS3x fw1.4 w/built in trans controller.
ol boy
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Re: Sound card scope probe

Post by ol boy »

I downloaded one of the sound card scope software's listed above. Here is what I got. You can see the missing tooth good now. Did the math and got 750ish rpm. Pretty neat.
20-1wheel2.jpg
306 SBFord, Torquer II EFI intake, 60 lbs injectors, 8 LS2 coils, VS Racing 7668 turbo, 4R70W, MS3x fw1.4 w/built in trans controller.
sfbmx88
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Re: Sound card scope probe

Post by sfbmx88 »

So based on the first diagram, what is the MAXIMUM voltage that you can safely test with this setup? No more than 5 volts?
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Re: Sound card scope probe

Post by jsmcortina »

I've thrown more than 12V at mine, but the nature of the circuit means the input will be clipped. What voltage did you have in mind?

James
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dontz125
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Re: Sound card scope probe

Post by dontz125 »

The 1N4001s are rated for 50v. The load resistors are 1k, so 50V would give 50mA or 2.5W. If you're using little 1/4W resistors, they're the limiting factor - 15V through a 1k resistor is just under 1/4 W. While a VR circuit can push nearly 100V (IIRC), 1) it's AC, not DC, and 2) you aren't running this all day.

If you pop your 1/4W resistors, put in 1/2W! :lol:
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Rick Finsta
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Re: Sound card scope probe

Post by Rick Finsta »

James, I'm looking to use this to make composite logs with my MSI. I am running generic wheel with a dizzy, using a VR sensor and PIN36 with single coil control. Since PIN36 is a grounding signal, how can I hook this up so I can overlay both the VR and the IGN to see the relationship?
Rick Finsta
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Re: Sound card scope probe

Post by Rick Finsta »

Anyone? Just looking to be able to test a ground signal on one channel and positive signal on the other channel for an overlay.
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Re: Sound card scope probe

Post by jsmcortina »

Rick Finsta wrote:Anyone? Just looking to be able to test a ground signal on one channel and positive signal on the other channel for an overlay.
Don't hook up to pin36 as the coil -ve connects there and puts hundreds of volts of back-EMF when the coil turns off. Instead scope at the low voltage signal
that feeds the BIP373. e.g. "top of R26" or wherever you have it connected.

James
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