How to? Soundcard as Oscilloscope (low voltage circuits)

[kalel]

I am sure that this was discussed before, but I'm not sure if it was discussed in this format.

Measuring a low voltage with a multimeter is generally simple and relatively safe.
Probing signals requires some advanced knowledge. How not to damage your soundcard is not something for beginners, I believe.

One thing that is taken care of is software. There is great free (non commercial use at least) software out there.

One would be this:
https://www.zeitnitz.eu/scope_en

Let's say we're not worried about having too many samples per second with our hardware.
What would be the simplest and least expensive way to check a low voltage signal (e.g. from a breadboard oscillator circuit, from an arduino output or such) with a soundcard?

I'm sure that it can get very complicated trying to isolate signals completely and prevent any possible damage. But that wouldn't be very beginner friendly.

One thing I have considered is this:

Forgetting about soundcards, using an Arduino or similar MCU to probe the signal

While possible, I'm not sure that an Arduino UNO has much processing power. I'm also not sure that a high quality piece of existing software like the example above for soundcards is already compatible with it (if not, writing your own software as a beginner will likely not produce similar results).

Also, there are similar issues about protecting the Arduino as there are with protecting the soundcard.

Using external USB soundcards

There are cheap soundcards from China (cheap is important, as an expensive soundcard won't cost much less than a cheap oscilloscope, so what would be the point then), with less bandwith, but without the noise that you hear with microphones on motherboard built-in sound-cards (you might have a better motherboard without this noise, but you probably also experienced it sometime, on some computer or laptop).

What I'm worried about with this method is that maybe using an external USB soundcard is not protecting your computer much more than using the internal one. Any high voltage transients going from the card to the computer could still damage it.

And going to the final question, is there a relatively simple circuit that could be used for this, or should it simply be forgotten about as an option? It is true that the simplest oscilloscopes are not that expensive at $16 (those kit DSO's) so perhaps these home made solutions are not viable. There are a ton of experienced people here who know what they're doing, so I'd like your opinion at least.

I do think that any low bandwidth solution can also be useful, for example when probing low frequency oscillator circuits.

P.S. I know that prior work is recommended in this forum, but unfortunately I admit I don't have any.

[rstofer]

Well, you hit the high points and have pretty much stated why I wouldn't even consider using a sound card.  Ground loops being one of the problems.  One side of the sound card input will be ground and certainly the USB connector is grounded.  One side of signal generators MAY be grounded and what about the power supplies?

I don't have any low dollar PCs left and I'm not going to risk the expensive ones I have.

In any event, I would consider something like the Digilent Analog Discovery 2 although it isn't much less expensive than a Rigol DS1054Z (by the time you get the BNC adapter board and a couple of probes).

One thought about probing:  Know what you are measuring!  Take a preliminary measurement with a DMM before connecting to some other instrument that may have range limitations.

[kalel]

Well, you hit the high points and have pretty much stated why I wouldn't even consider using a sound card.  Ground loops being one of the problems.  One side of the sound card input will be ground and certainly the USB connector is grounded.  One side of signal generators MAY be grounded and what about the power supplies?

I don't have any low dollar PCs left and I'm not going to risk the expensive ones I have.

In any event, I would consider something like the Digilent Analog Discovery 2 although it isn't much less expensive than a Rigol DS1054Z (by the time you get the BNC adapter board and a couple of probes).

One thought about probing:  Know what you are measuring!  Take a preliminary measurement with a DMM before connecting to some other instrument that may have range limitations.

Thanks, and yes... the risk (for any computer) makes it all less interesting. Perhaps if someone made such a device cheaply (some kind of soundcard guardian), but if someone did, it's probably more expensive than the cheapest DSO. And not much better anyway.

Surely you could protect the computer fully by using a cheap recorder device, and then analyzing the signal on your PC by transferring it from some SD card, and if the device breaks - it breaks, but is that way useful at all? Probably not.

And yes, USB oscilloscopes seem like a neat deal. Especially if space is limited. Not everyone needs a display. Still, considering the prices I might not be doing such probing yet - and I probably don't need to. Sure, it's interesting to experiment and learn something about signals, but I'm not building anything that really depends on making signal quality measurements.

Also, the tip about using a multi-meter first sounds wise. It might seem reasonable to do such a check anyway, but it could be easily overlooked.

[rstofer]

Most of my projects are powered by a 5V wallwart.  I like that!

I'm not going to be working on higher voltages as a matter of choice.  I MIGHT do a 24VDC motor control but that's about the max.  I just don't have any interest in high voltages - been there, done that at an industrial level.

I have had an oscilloscope of one kind or another for over 60 years.  I can't imagine not having one.  But if I were trying to get into this game for a low entry cost, I would be seriously looking at the Digilent Analog Discover 2 and probably with the Pro Kit addons.

http://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/
http://store.digilentinc.com/analog-discovery-2-pro-bundle/

This, and a decent DMM is about all it would take to do anything I need to do in vast majority of cases.  That's why I have the -1 and -2 versions as well as the Digital Discovery.

People tend to poo-poo the AD2 without understanding what it can do.  It is worth downloading the software and running it in demo mode just to get a feel for the capabilities.  It's an amazing tool.

The power supplies are definitely low current, even when the AD2 is powered by a wall wart.  I would soon add a decent power supply.  But I could do a lot of circuit design without having to do that.

For the truly paranoid, there are USB isolators to break up that ground loop through the PC.  I have a couple I got off of eBay but I haven't used them yet.

[george.b]

Forgetting about soundcards, using an Arduino or similar MCU to probe the signal

While possible, I'm not sure that an Arduino UNO has much processing power. I'm also not sure that a high quality piece of existing software like the example above for soundcards is already compatible with it (if not, writing your own software as a beginner will likely not produce similar results).

Also, there are similar issues about protecting the Arduino as there are with protecting the soundcard.

Well, maybe not an Arduino, but there are MCUs, such as the dsPIC30F3010, which are pretty accessible and can do 1Msps, way more than what a sound card can do (192ksps, tops, AFAIK).

[danadak]

Google "protect sound card inputs", lots of hits.


Regards, Dana.