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 Forum index » DIY Hardware and Software » Les Hall's Projects including eChucK
Super Cheap Computer Test Equipment
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Inventor
Stream Operator


Joined: Oct 13, 2007
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PostPosted: Sat Apr 21, 2012 10:48 am    Post subject: Super Cheap Computer Test Equipment
Subject description: Build an electro-music lab gear setup with $20 and one evening of your time...
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OK, let's talk scopes. I'm faced with creating or buying some kind of scope-like device for the purpose of testing my electro-music projects and I've looked at several options which are as follows:

1. Pay $400+ for a decent 4-channel scope. This is overkill for our purposes, though if you're doing anything involving sensitive or fast signals it would be necessary. Way out of my budget at this time.

2. Pay $100+ for a decent 1-channel scope. Great option because later when you get a multichannel scope you can use this one in a project to display cool waveforms or dedicate it to your audio stream in some way. Still too pricey for me this month and I need to look at two channels together anyway.

3. Get a great deal on a old used klunker. This is the best option for most of us, but I'm sick of used equipment and *really* tired of lugging around heavy stuff.

4. Build something that works for you using cheap parts that you may already have. My chosen option.

So let's explore option 4 in my next post in this silly thread.

Les

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Last edited by Inventor on Sat Apr 21, 2012 11:50 am; edited 1 time in total
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Inventor
Stream Operator


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PostPosted: Sat Apr 21, 2012 11:12 am    Post subject: Reply with quote  Mark this post and the followings unread

I've chosen to use my computer as the oscilloscope. This means using the soundcard as the ADC and choosing an appropriate software application for the display and controls, then making some hardware probes yourself. Since I plan to purchase a decent $400+ scope later this year when it's economically convenient, I only need something to measure signals temporarily. To be practical it must be cheap compared to a $50 garage sale special.

The best way to keep costs low is to use free software and minimal hardware, preferably something that you already have in hand or can purchase really at a very low price. Since I'm starting out from scratch rebuilding my lab that I gave up to move to Texas, I have to purchase the items required.

For software I came up with a simple, easy, and unorthodox solution: Audacity! Yes, our favorite open source recording and editing software package for audio can be a scope of sorts believe it or not. All you have to do is know how to use it right which I'll describe in the next post to this thread.

For hardware I'm going with a breadboard based system becuase that's what I do typically when I create electronics, I prototype on a breadboard. The test circuit will be on a breadboard and the scope probe electronics will be on a separate breadboard. Clumsy? not really, you'll see why in my post after next where I describe the details of the probe electronics.

So that'll be the scope: Audacity on a Mac mini with a temporary reusable breadboard probe system. Details to follow.

Les

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Stream Operator


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PostPosted: Sat Apr 21, 2012 11:24 am    Post subject: Reply with quote  Mark this post and the followings unread

To use Audacity as a software oscilloscope, I'll first get the latest version of Audacity which currently is Audacity 2.0, recently released and greatly improved as I've been told by darkvortex. Then I'll do the following:

1. Adjust the audio source to the line-in jack. Note that you can also switch it to your microphone for a quick non-probe view of the ultimate output signal (in the case that you've got audio in the air).

2. Press the record button. The two waveforms (left and right) become immediately viewable on Audacity's scrolling window.

3. Scale the time index by zooming in and out with the magnifying glass buttons on the toolbar. The refresh rate is slow and the signal is noticably drawn, but you can see it pretty well.

4. Scale the voltage on the vertical axis by hovering the mouse over the vertical scale to the left of the waveform and right or left clicking. each click zooms in or out, and where you click sets the vertical bias.

5. View any signal's history by zooming way out, pausing the recording, and zooming back in on some desired region.

Note also that the sound card is AC coupled, so only the AC coupling mode of the oscilloscope is available here. This is fine for most purposes and you can probe the signals with a meter to get the DC value. More on this later in the next post which is a detailed description of the probe electronics.

Les

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Stream Operator


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PostPosted: Sat Apr 21, 2012 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Now to the probes:

I will be measuring CMOS audio signals, so i need a high impedance buffer on the scope probes. The easiest way to do that is with the venerable voltage follower, so sploit! - onto the board goes a dual opamp (I use the LM358N dual opamp now, from www.sparkfun.com, as my general purpose opamp because it has a 20mA output current so I can drive most things without concern for current limitations and also it has other good characteristics.

IF for some reason you want to have an A+B or A-B option, then you can add another opamp with either a summing amp or a difference amp or both wired up. I don't need these so I'll just stick with the opamps.

The opamps of course need power and ground and two signals from the breadboarded circuit under test, so we add 22 or 24 gauge solid wire for those four connections. I will use longer wire and coil it up around a screwdriver shaft or other thin tube to make the two signal probes and shorter non-coiled wires for power (red wire insulation) and ground (black wire insulation). The signal probe wires will also have two unique colors of insulation to tell them apart. Add a 2.2k resistor and 10uF or larger capacitor to each output with the minus side of the capacitor at the output of the probe.

For connections, I'll use a single audio jack with left and right channels wired and a single audio cable. Total parts cost: about 10 dollars, and all parts are reusable! Smile

If you have more than two channels in your sound card, add that many probe channels naturally.

Les

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PostPosted: Sat Apr 21, 2012 11:47 am    Post subject: Reply with quote  Mark this post and the followings unread

That should do it for the scope unless I've missed something, I'll be building it when I get my next parts order in. Later I plan to do a poor man's signal generator and logic analyzer, oh and I forgot to mention that we can use the FFT feature of Audacity to get our spectrum of any selected audio signal duration.

That's all for now, the nurse is here to clean my leg wound and put medicine on it, so I've got to go, plus I'm getting tired of writing. Your comments and questions are welcome as always of course!

Les

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elmegil



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PostPosted: Sat Apr 21, 2012 9:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

I paid just shy of $200 for a DSO QUAD which is TINY (smaller than my cellphone) and seems to do a decent job; it has 4 inputs, 2 analog and 2 digital, and for my purposes so far it's been fine. If you don't like the supplied probes you can get adapters to BNC connectors for $3 apiece and then use any BNC probe you like. It does have an impedance problem which means it can't get anywhere near the advertised bandwidth (something like 72MHz?) but for audio, that's generally not necessary. And I think that's true even with third party probes, because it's an internal setting.

The docs are...online and somewhat spotty, but not terribly hard to learn from.

Not saying it's any better than your notes, but an option you left out.
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PostPosted: Sat Apr 21, 2012 9:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

While I'm up and feeling like typing, I'll prevent some boredom by telling you about the next piece of test equipment, our two channel function generator. For this we need an audio signal source such as ChucK, Audacity, or iTunes. Really any software that can produce audio on the line-out jack will work.

For hardware we will take the AC signal, bias it up with a simple RCRR network, and add a voltage follower yet again. It will fit on our probe breadboard and use up one more 1/8" (3.5mm) audio cable and jack.

Then we can simply generate audio that goes out the line-out jack, our cable takes it to the jack, the bias network adds DC, and the voltage follower presents it to the circuit with a low impedance output and a 20mA current drive thanks to our use of an LM358N opamp.

We already have power and ground on the board, so we simply add two more springy wires with two more colors (if we have two more colors, I do just barely) or if not then the same colors as used on the probes.

That's all we need to generate two signals, or more if you have a sound card with more than two outputs.

Les

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PostPosted: Sat Apr 21, 2012 9:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil, that sounds like a good option for me as well, can you post a link to a web page describing the scope?

Les

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elmegil



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PostPosted: Sat Apr 21, 2012 10:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is the designer's description of the model I got:

http://www.seeedstudio.com/depot/dso-quad-aluminium-alloy-black-p-1034.html?cPath=174

It's open source so I think that there are actually a few different manufacturers, I got mine from EBay for $185, free shipping. I still haven't figured out how to make it give me a running display instead of triggering and then being able to scroll back and forth, but I have been able to observe my waveforms sufficiently to correlate the clock and output for the MFOS precision CV calibrator circuit to be sure I was getting the steps I wanted, given that I made some notable changes to the circuit. And honestly I haven't spent a lot of time looking for the instructions for the running display. My meter's response time is slow enough that I have some issues confirming the steps if running at a significant speed.

Wiki site, which should lead to all the docs and firmware variations, etc is at http://www.seeedstudio.com/wiki/DSO_Quad

The seeed forums are where the impedance problem is discussed at some length.

The one I ordered came with a pair of digital style probes (basically just wires with useful connectors on the ends) and a 1X / 10X switchable probe.
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PostPosted: Sat Apr 21, 2012 10:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

While I'm at it, I may as well tell you about a little trick I created when I was a sophomore in college taking my first digital class and my first analog class. I was quite the go-getter back then and was pleased to have figured this one out. Today I still think it's a neat trick.

What you can do is turn an oscilloscope into a 16 channel logic analyzer. The trick works as follows.

You set up an 8:1 mux, either digital or analog, then you create a bias voltage that increases according to a 3 bit binary counter (or an 8 step decimal counter), then you simply add the mux output to the bias output with the mux output attenuated to slightly smaller than the steps of the bias network. This creates a stairstep ramp waveform with the steps at the voltage level of the mux output. In other words, you made an 8 step ramp signal where each step contains the multiplexed value. Why do all this (it's not that complicated really)?

Because of the second trick to this trick: an optical illusion. This works on CRT scopes and may work on other scopes too, I'm not sure. It may or may not work on our Audacity scope, I have not tried it but anyway this is the second trick: You run the clock of the counter and mux circuit fast (10x or so) compared to the rate of change of the signals you are viewing, then when you see the ramp on the scope you just crank up the time scale so that you see about 20 to 100 ramps on one screen. Your eye will see the flats and not the verticals, so it actually looks like that channel has 8 signals on it!

In summary, what we do is mux our signals onto a stepped ramp at a clock rate that is quick compared to the observed signals and view the resulting waveform at a slow timescale (appropriate for viewing the desired signals). Then the scope beam spends very little time on the transitions of the ramp (it it's crisp) and most of it's time on the flats and you see 8 signals.

Here are a few additional tricks: you can mess with the intensity and focus controls to enhance this effect, making the transitions truly invisible to the naked eye. You can use an analog mux instead of a digital mux and your get an 8 channel scope probe instead of an 8 channel logic analyzer. You can add circuitry to blank the signal every so many counts to help with triggering and get it rock solid, or you can create a separate trigger output for your scope's trigger input. Do this on both channels of your scope and you've got 16 channels of viewable information on your scope's screen. With 16 channels of analog, you can probe most of the meaningful signals on your project all at once and take a snapshot of it's operation for your records and for sharing here on the electro-music forum!

Total cost is just a few dollars for chips and passives added to the breadboard that you're already using for your scope and signal generator. It's a simple concept and it works very well. I'd love to see someone post photos of their attempt at this here in this thread.

That's all for now, happy measuring. I'm outta here.

Les

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PostPosted: Sat Apr 21, 2012 10:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the link and description, elmegil, it's good to know what the alternatives are. There are also a bunch of other products that are good solutions for hobbyists at various price levels including computer based and stand-alone types. Tell you what, why don't we open up this thread as a place for people to post links and images of their favorite hobbyist scope alternatives?

Les

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LFLab



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PostPosted: Sun Apr 22, 2012 2:08 am    Post subject: Reply with quote  Mark this post and the followings unread

I found my 4chan scope (Tektronix 100mhz) in a dumpster at work, figured i'd gut it for parts turned out to be fully functional. quite a lucky break.

But, have you looked at the small stuff from china? there's a diy kit, and a sort of smartphone dso, both are way below 100usd and tiny. I'll hunt for url's later.
Also, there's readymade software to turn your pc in an scope, freeware don't recall the name.
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PostPosted: Sun Apr 22, 2012 2:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Appreciate the tips, LFLab, looking forward to checking out your links!

Les

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LFLab



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PostPosted: Sun Apr 22, 2012 2:34 am    Post subject: Reply with quote  Mark this post and the followings unread

http://www.seeedstudio.com/depot/dso-nano-v2-p-681.html?cPath=174
http://www.seeedstudio.com/depot/digital-storage-oscilloscope-with-panels-p-514.html?cPath=174

http://www.seeedstudio.com/depot/dso-quad-4-channel-digital-storage-oscilloscope-p-736.html?cPath=174

Last one is more expensive, but beginning to look like serious equipment indeed.

Last edited by LFLab on Sun Apr 22, 2012 2:39 am; edited 1 time in total
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PostPosted: Sun Apr 22, 2012 2:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Hmmm, that DSO nano V2 looks like a good choice as well. I'll have to see what pops up in this thread and do some product evaluation before my next scope purchase. How about that freebie scope software?

Les

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LFLab



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PostPosted: Sun Apr 22, 2012 2:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Found that one as well:
http://www.zeitnitz.de/Christian/scope_en
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PostPosted: Sun Apr 22, 2012 2:42 am    Post subject: Reply with quote  Mark this post and the followings unread

The $48 scope looks like a winner too, however one must program it and you probably can't do that with a Mac. Thumbs down for incomplete product offer.

Les

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PostPosted: Sun Apr 22, 2012 2:44 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
Found that one as well:
http://www.zeitnitz.de/Christian/scope_en


Beautiful! Now all the PC owners have a good solution! I'm on a Mac, but it would be worth getting a cheap notebook just to run that thing, lol.

Les

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LFLab



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PostPosted: Sun Apr 22, 2012 3:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Inventor wrote:
The $48 scope looks like a winner too, however one must program it and you probably can't do that with a Mac. Thumbs down for incomplete product offer.

Les


Hmmm., don't recall people telling having to program the thing, and I wonder why, since it is a readymade solution, you'd think they programmed the AVR already.
Good point mentioning you are on Mac, that does severely limit the choices.
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elmegil



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PostPosted: Sun Apr 22, 2012 4:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah, be warned, for the DSO Quad, firmware flashing can't be done with a Mac, because of MacOS' throwing all kinds of hidden files (.DS_Store) into the filesystem. That's probably true for the Nano as well, as they are related.

That said, I have a Mac, and I haven't seen a need to flash my Quad, and I have bootcamp loaded with Windows XP if I really decide I must.

Also, there's always VirtualBox (https://www.virtualbox.org/) which is a free Virtual Machine solution; for the firmware thing I doubt it would prevent the problem I'm talking about, but it might let you use that other scope.... I use VBox to run Quicken for windows because Quicken for Mac lags quite a lot in feature and function.
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PostPosted: Sun Apr 22, 2012 5:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Back to the original list, in the "approaching $400" range, this is the one I *really* wanted to get but simply couldn't budget for: http://www.bitscope.com/

It has MacOS support/beta software. It's still under $400, but looks very good.
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PostPosted: Sun Apr 22, 2012 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh, I see now. When I read about the programming I for some reason thought that the device was shipped blank and that you had to program it. That apparently is not the case, so I CAN use the $48 option. It's a good stepping stone choice because you can use it for a scope now and when you are ready to get a better scope then you can just embed this panel scope into the panel of a project you are working on. Looks like a good option and one I favor for next month's purchase(s).

Les

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wmonk



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PostPosted: Sun Apr 22, 2012 1:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

I like Xoscope, it uses the audio interface to get the signals. Linux/BSD only I think.
http://xoscope.sourceforge.net/

A simple Max AU scope/waveform viewer plugin is here http://bram.smartelectronix.com/plugins.php?id=4

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LFLab



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PostPosted: Mon Apr 23, 2012 10:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Came across this thing today:
http://www.gabotronics.com/product-info/the-xprotolab.htm
freaking awesome, a scope you can plug onto the breadboard!
not a lot of features unsurprisingly, but pretty handy. maybe put one in a panel in the modular.
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PostPosted: Mon Apr 23, 2012 1:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
Came across this thing today:
http://www.gabotronics.com/product-info/the-xprotolab.htm
freaking awesome, a scope you can plug onto the breadboard!
not a lot of features unsurprisingly, but pretty handy. maybe put one in a panel in the modular.


That thing looks truly amazing! Now I'm really happy that I shared the info in this thread, it was worth all that effort just to get that link. Thanks LFLab!!!

Les

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