# Anyone made a sound card oscilloscope?

Discussion in 'General Electronics Discussion' started by Robert Hill, Jul 7, 2015.

1. ### Robert Hill

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Mar 5, 2015
Thanks for the replies. To start I think I was confusing the fact you can have a square wave which is positive and zero volts alternately and you can also have a square wave which is positive and negative volts alternatively, one DC the other AC!

I've just started playing with the sound card scope so this is helpful. I just plugged something easy (blinking LED from Arduino) in to see what I got.

Chris, thanks for the comparison it helps show what is going on.

So going off what Adam said am I right in thinking:

At 1HZ the input capacitor fills up almost instantly when the circuit enters its 'on' part of the cycle. This is because it is sized in order to handle current filling it for only milliseconds before everything changes direction. This sudden filling produces the spike in voltage but the voltage between the full capacitor and the circuit is equal so the spike falls back to 0 volts because the capacitor is now blocking the signal so there is no voltage or change of voltage for the scope to pick up. Then when the circuit enters the 'off' part of the cycle the capacitor instantly (or very quickly) empties itself creating a sharp voltage drop negatively away from the scope. This is picked up as the negative spike. When the capacitor reaches ground the voltage is no longer dropping so the scope trace returns to ground.

By contrast when the signal is at 1KHZ the capacitor never has time to fully fill before the voltage changes polarity. This means the signal is faithfully passed through the capacitor.

Is that right?

Adam, I'm guessing the large capacitor is to smooth the square wave a bit by having a more consistent filling and emptying at the same rate rather than the sudden on/off there is at present. This would then take longer to fill the capacitor at the input of the microphone and therefor clip the wave less?

I'll give it a go with a faster frequency and see what happens.

Thanks again!

2. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Adam, I wouldn't loose sleep over it. He who never errs most assuredly never commits to anything.

Chris

5,164
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Dec 18, 2013
Yep my old boss said the person that makes no mistakes is the person doing nothing. At the moment I am doing a PCB design for a vehicle TURBO cut out, replying to EP threads and replying to emails from a friend who needs some electronics help... so busy

4. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Robert, I've never seen it described quite like that but yeah you're spot on! Good job!

Chris

5. ### Robert Hill

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Mar 5, 2015
Nice one, I actually have been reading my electronics for dummies book (well before I moved onto the much more challenging are of electronics!).

It is nice to know that I am starting to be able to understand things and have the resources to understand other people's explanations of problems.

5,164
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Dec 18, 2013
Your doing a good job Robert, you'll be teaching me soon.

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Make a mental note of the day you found EP. Consider it to be most fortuitous. We have quite a cadre of very knowledgeable, helpful and friendly members. It's sad that you will never know our most shining star (Kris Blue) who was taken from us way to early in life. He had the patience of a Saint and loved tutoring aspiring electron heads ... like you!

Chris

Arouse1973 likes this.
8. ### Robert Hill

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Mar 5, 2015
Been experimenting with different frequencies.
This one is 1KHZ I believe, a fairly nice square wave.

The LED is going on and off every 300 miliseconds. It looks like the input is catching more of the wave before the capacitor fills up and spikes.

This one is lLED on and off every 100 milliseconds

On and off every 10 milliseconds

It's cool to see how the signal getting through changes at different frequencies.

5,164
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Dec 18, 2013
Thats great, feels good to know its what you have built.

10. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
I'm glad you're enjoying your journey but I'm clueless as to what ILED is.

Chris

11. ### Robert Hill

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Mar 5, 2015
In Steve's book he gives a circuit for calibrating the sound card osciloscope which includes a voltage setter as well as a multivibrator. I've built it on a breadboard and got the following on the osciloscope:

Not a very square, square wave, what do you think? is this the kind of wave you would expect from an astable multivibrator?

I'm hoping to build and solder up the circuit this weekend.

12. ### Steve Garratt

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Jul 22, 2014
Hi Robert,

Sorry for not noticing your posts sooner. The site has stopped sending me notifications for some reason. Anyway this isn't what I would expect to see from the output of the calibrator. It should be much squarer in shape and the frequency should be approximately 1kHz. The waveform above looks like 4kHz and it isn't very square. I think that you have some debugging to do unless you have done it already.

Steve

13. ### Robert Hill

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Mar 5, 2015
Hi Steve, thanks for getting back on this post.

I've got it all successfully working now.
There were a few components which seemed to have died and I replaced as welll as a copper track I hadn't cut. I believe what I was picking up in the screenshot in my previous post is the ripple in the power supply I was using. I added a jack socket to the circuit so I could power the calibrator without need for a 9v battery.

This works fine for calibrating but distorts the output from the signal generator as perhaps you would expect. Still nice to have options.

I'll try to put up some picks of the finished articles and hope to do a little review of your book for the book reviews section of this site.

Arouse1973 likes this.
14. ### Steve Garratt

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Jul 22, 2014
Hi Robert,

I'm very please that you got it all working ok.

You can stare at the board for ages and still miss errors. I know this only too well. After a time you will develop a method that works best for you. I now tend to spend a long time going over the tracks almost hole by hole looking for bad or non existent soldering, uncut tracks or tracks cut in the wrong place, bad component placement etc. Then I make a point of examining every millimeter of the gaps between tracks. I've been caught out by shorts across tracks too often.

Your component failures may have been consequential damage from an incorrect circuit before you found the errors.

I'm not sure why the provision of external power would distort the output of the signal generator. Do you have any ideas why that might be?

I would love to see some pictures and a review would be great. Any chance of you posting it to the Amazon book page as well as here?

Steve

15. ### Robert Hill

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Mar 5, 2015
Hi Steve,

I think that the power supply was not a very high quality one ~£10 from maplin. Therefore I think although it rectified the AC and reduced the voltage I believe it did not remove all of the AC qualities which introduces 'noise' into the voltage which supplies the amplifier part of the signal generator.

The other alternative is that because none of the circuit is shielded the power supply created some interference.
Either of these sound possible?

I'll certainly try to get a review on the amazon books bit

16. ### Steve Garratt

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Jul 22, 2014
Hi Robert,

Did you try it with a better power supply or battery and did that fix the problem?

Steve

17. ### Robert Hill

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Mar 5, 2015
Hi Steve,
Here's a screen shot of the pickup with the osciloscope and generator plugged into the mic and headphones and power connected but the signal generator turned OFF

As you can see despite the osciloscope not being connected to anything it still picks this interference up.

Now a shot of a 1khz square wave generated from the program, outputted to the generator and fed back though the osciloscope circuit to the program. The generator is running off the power supply:

And the same with battery:

Now note the second green signal (not connected second channel -
With the battery there is a very tiny ripple:

And with the power supply:

The green signal 'warps' the display of the red as the program tries to fit both on correctly.

Interesting ey?