soundcard as oscilloscope

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Abstract Dissonance wrote:

This is your sine wave...

I have a variac hooked up to a breadboard and basicaly just running into a
voltage divider and from that into my sound card through its line in jack.

And this is your sine wave on "Analog Hole" DRM

http://www.geocities.com/abstract_dissonance/osc.JPG

(well, not really, though soon that might be another potential problem)

 

[Abstract Dissonance]

Yeah, me, too! We can't just keep calling you "Mr. Dissonance". ;-)

oh, I didn't even see that line in his original message ;/

Anyways,
Its Jon.. but I prefer Mr. Dissonance J/K

It sounds like you're confusing tight control of the _frequency_ with
control of the _waveform_.

I guess. But I figured it must be a sin wave since, say, if they used a
square wave then it would screw up everything? (i.e. things like
transformers would behave differently and stuff?).

I guess I just though it must be a sine wave for some reason... I suppose
cause all the examples I've seen show a pure sine wave and never the real
thing or mention that its distorted.

The frequency, the power company can and does control.

The waveform, however, once the power leaves the power station, there
isn't a thing they can do about the waveform, other than install those
big power factor correction capacitors that you sometimes see on line
poles.


You're seeing pretty much exactly what you should be seeing with those
settings - the resolution of the monitor comes into play and could be
aliasing with your sample rate - there are just too many variables in
a row to expect any kind of precision from a setup like yours.

But I _do_ concur - DON'T put a variac output directly to a sound card
input. Theoretically, the neutral of the mains is at ground, but that's
not guaranteed, and you could set up a ground loop that, while not
necessarily lethal, could screw up your measurements and/or damage your
equipment.

I think the nuetral is about 1 V above ground or so. I'm not to worried
about scewing up my sound card all that much to tell you the truth... its a
POS anyways... I'm more worried about screwing up my computer but I feel
save that the sound card would prevent that(atleast I hope).

Get some isolation in there.

Well, I don't have an isolation transformer so I don't know how to do
this... I brought a lot of stuff but not nearly enough it seems. Just
trying to get into some projects(such as building a PS with some cheap
transformers I brought) and would like to see the output(so atleast I have
some idea whats going on).... I don't like poking around with a DMM to much.

I'd think a cool project now would be to build an oscillator of some
kind and look at its output. A mic into the sound card could also be
fun, for a few minutes. ;-) (cf. Microsoft Sound Recorder)

yeah, thats why I wanted some oscilloscope. I suppose I don't need anything
fancy to do that(hence using the sound card and/or TV). I just started
yesterday on this stuff so I still need more time to play

Have fun!
Rich

Thanks again,
Jon

 

[Abstract Dissonance]

Could be many things:

(1) Using a Variac for a signal source is very hazardous!! Most
variacs are autotransformers, so there's no isolation from the power
line. You're probably lucky yours is wired correctly, with the neutral
wire passing through as the "low" or chassis ground side. Otherwise
there would be a big ZAP if the hot wire got hooked up to your sound
input, the ground side. Been there, done that.

Well, I wired the wall outlet myself so I know its hooked up right

Are you saying that the Variac's chassis is wired to neutral? (shouldn't it
be wired to ground? and ground is almost always hooked up right?)

The AC line can have all kinds of distortion. In our neighborhood some
transformer is saturating, as the waveform is mildly clipped. So sez
my brother that works for the power company.

ok... I guess I thought I was living in a perfect world or something.... I
didn't realize it was such a mess.

So you'd better find a better and safer signal source!

(2) As others have noted, sound card inputs, especially the microphone
inputs, have a bit of DC bias voltage coming out, to activate electret
microphones. Thats going to feed back and fight whatever circuit
you're probing. Better put a 0.47uf 600 volt capacitor in series to
block the DC (in both directions).

I tried that.. I didn't change anything(that I could tell).

(3) Another probloem-- the sound card mike input, the ground side, is
often tied directly to the computer's chassis ground. Computers usually
have a lot of crud superimposed on the ground bus.. That's going to
create a ground-loop with your source and add a bunch of stray
gibberish in there.

it could be that.

So at best yu're going to end up with a pretty cruddy scope. Barely
good for audio.
Hope you don't intend to service anything better than a boombox with
this setup!

Well, I was just doing it for fun... to see if it worked. I suppose it did
but I thought there was something wrong with my circuit... aslong as its not
me then I'm fine Now I can move on to other things.

Thanks,
Jon

 

[Abstract Dissonance]

You really can't make much of a scope out of a TV. I tried. The main
problems are:

(1) The horizontal sweep has to always run at 15.75KHz, as the CRT
high voltage is derived from that. No way around this.

I don't understand this. Why do I have to run the deflection coils from the
circuitry of the TV? Couldn't I just tap into the plug and use my own
circuits to generate the current needed for deflection?

(2) You have to keep the deflection yoke hooked up to the 15.75KHz
sweep circuit, otherwise the high voltage won't work. So if you're
going to use a different signal to sweep, you have to take off the
original yoke and slip on a different one.

?

On the TV I have there are 4 Hot wires going to the CRT. One is to the top
of the CRT and is a really thick wire that comes from this strange looking
capacitor like box. Another hot and orange, that is smaller, runs to the TV
tube from the same box. I Think they control the focus and strength because
they are near the controls "Focus" and "Screen" and I played around with
them and they change the focus and something else(its not really strength in
the same sense but it makes the beam brighter(along with other things)..

Then there are some "bus" like wires that go into the TV tube each having a
hot and 2 and 3 other wires(there are 2 of these busses). I suppose they
control the modulation of the signal and stuff to the TV tube to generate
the picture. I took both these out and there was no picture(no blue dot)...
I'm not sure why though.

The coils themself are attached to some jack by themselfs. I removed them
cut the wires and hooked them directly to my variac to generate some
pictures. I should be able to control the coils in any way I want that is
independent of the TV circuitry?

(3) The vertical deflection circuit is not linear, and is frequency
limited to under 600 Hz or so. No way around this.

How bout rewiring? its not linear simply because of the size of the
deflection? If the deflection is small then it should be ok?

(4) You have to build a whole new horizontal sweep circuit. And the
horizontal yoke is tuned to resonate at the retrace frequency, so it
doesnt work very well at other frequencies.

rewire?

You can make a cool looking display, but it's nowhere near even a $30
scope.

30$ scope? Where??? I'd drop 30$ in a heart beat for one(aslong as it
works).

Have fun and be careful!

Thanks,
Jon

 

[Abstract Dissonance]

In your typical TV from the past half-century, you're right. But the
very cheap $20 TV's at drugstores will let you unhook the horizontal
coil and the flyback will continue to make HV.


Again with the little cheap B/W TV's it's not quite as bad as a typical
20-year old console set, but you're right, you cannot push the
deflection coils much past what they were designed for.

What about a 5 yr old TV? have they changed in 20-30 years?

One can have a lot of fun with a "cool looking display" . But you're
right, expecting to turn a TV into a scope with bandwidth out of the
low audio range, and expecting it to be calibrated too, is way too much
to expect from a couple dollars of effort.

And the $20 one might spend on the B/W TV at a drugstore could've ben
spent at a hamfest to get a couple old Heath/Eico/etc. scopes. (Mr.
Abstract Dissonance might have fun fixing those up!)

Well, where and when are these hamfests? If they got scopes this cheap then
I'm there... I read on another site that they are a great place to find
drill bits for PCB boards too.

Tim.

Thanks,
Jon

 

[Abstract Dissonance]

I don't know where you're at, but here in the US 20- or 30-year old
scopes are readily available in that price range or a lot less.

AC-coupled Eicos and Heathkits with a bandwidth of a few hundred kHz to
a few MHz would be overpriced at $20. More typically they're free.

$100 is the ballpark for a mostly working Tek 465, which is a really
good scope (100MHz, dual channel, etc.)

I'd be lying to say we're at the peak of the glut of test equipment
(that seems to have happened maybe 5 or 6 years ago, and as Don would
tell us most of the tech schools dumped all their stuff before that)
but there's a lot of it out there at bargain prices.

Tim.

WHERE WHERE WHERE?

When I did a small search on oscilloscopes I couldn't find anything under
500$... those that where didn't work and looked very very cheap.

and what does "mostly working" mean?

Thanks,
Jon

 

[Paul E. Schoen]

I think the nuetral is about 1 V above ground or so. I'm not to worried
about scewing up my sound card all that much to tell you the truth... its a
POS anyways... I'm more worried about screwing up my computer but I feel
save that the sound card would prevent that(atleast I hope).


Well, I don't have an isolation transformer so I don't know how to do
this... I brought a lot of stuff but not nearly enough it seems. Just
trying to get into some projects(such as building a PS with some cheap
transformers I brought) and would like to see the output(so atleast I have
some idea whats going on).... I don't like poking around with a DMM to much.

You would probably see about the same waveform if you used a $5000
oscilloscope single-ended and tied the ground to neutral. The 1 volt or so
from neutral to ground has a lot of power behind it and is mostly noise. You
can see the noise by connecting your sound card input from neutral to
ground. This signal will be added to the true waveform. A simple cheap
isolation transformer would be a Wall-Wort with AC output, or break open a
DC type and remove the rectifiers. However, power transformers usually run
near saturation, so you will see the clipping at close to rated voltage, but
if you run the variac down the waveform should clear up.

If you have stereo input on the sound card you could try making a
differential input oscilloscope, and display the difference in the two
signals. You would need to make sure the external voltage dividers have
exactly the same ratio, but that should work fine.

I have a specialized product that is essentially a high-accuracy AC current
meter. You are welcome to download the demo and other information on my web
page www.Ortmaster.com. For background info see my main website.

I had considered using a sound card for my application, but I'm not sure if
the accuracy and sensitivity is good enough. I have a special PC board with
an instrumentation amp, PGA from 1x to 200x, and a 10 bit A/D on a PIC. It
communicates with the PC on a serial port at 57.6 kBaud for 2400 samples per
second. Lowest range is about 5 mV FS, resolution about 5 uV. The parts cost
is probably only about $20.

Good luck, and play it safe!

 

[Ancient_Hacker]

Are you saying that the Variac's chassis is wired to neutral? (shouldn't it
be wired to ground? and ground is almost always hooked up right?)

I meant that most variacs pass through one of the incoming wires
straight through-- if that happens to be the "hot" side of the line,
and you try hooking that to the ground side of your computer, major
kabang will follow. Trust me on this. I still have the charred PC
board to prove it.

 

[Ancient_Hacker]

The coils themself are attached to some jack by themselfs. I removed them
cut the wires and hooked them directly to my variac to generate some
pictures. I should be able to control the coils in any way I want that
is
independent of the TV circuitry?

Okay, your're lucky. The older TV's I tried this on did not work with
the yoke disconnected. They depended on the inductance of the yoke to
help boost the B+ voltage.

 

[James Douglas]

I'm trying to use my sound card as an oscilloscope but for some reason it
just won't work.

I have a variac hooked up to a breadboard and basicaly just running into a
voltage divider and from that into my sound card through its line in jack.

I can control the amplitude of the wave form pretty easy by controlling
either a variable resistor or the variac... the problem is that that the
wave form looks like crap and I can't rectify it in any way(I put a
rectifier as the first component).

Heres a shot of the oscilloscope:

http://www.geocities.com/abstract_dissonance/osc.JPG

It does the exact same thing when I put a rectifier in except if I but it
backwards then it does nothing.

I've used several resistor ratios from 10M's down to 50k but nothing seems
to work. I'm not sure if it has something to do with the sound card(it
should have no problem sampling 60hz mcuh better than it is?) or if its just
the wave form is crap cause of the variac or what ;/

Anyone have any ideas?

I was thinking about taking an old pc monitor I have and trying to hack an
oscilloscope from it(I don't need anything fancy, just need to displace the
waveform)... Is this to much work?

Thanks,
AD

You can buy a good scope for about $100 used........i have done quite a
few from ebay for tinkering

 

[Rich Grise]

I know, I had one of those synchronous types too, but that is soooo looong
ago....
BTW, what is the age limit for a whippersnapper?

Meindert

I guess as long as they're snapping whippers....
;-)

Cheers!
Rich

 

[Joseph2k]

For you young whippersnappers, that's been the case since before the
digital clock was even thought of - last millennium, analog clocks used a
synchronous motor, locked onto the line frequency; regulating the
frequency is not a new phenomenon.

Cheers!
Rich

Alas, Rich, i must disabuse you. They used induction motors with well
understood slip and appropriate gear chains. I know i took enough of them
apart as a kid. Motor shaft was typically 1750 RPM.

 

[Sjouke Burry]

Rich Grise wrote:



Alas, Rich, i must disabuse you. They used induction motors with well
understood slip and appropriate gear chains. I know i took enough of them
apart as a kid. Motor shaft was typically 1750 RPM.

Alas, Joseph, i must disabuse you. They used
synchronous motors with out slip but with
appropriate gear chains.
At the power station they just made sure that
at the end of the day all clocks again showed
the correct time.
The momentary frequency was not very stable.

 

[Tim Shoppa]

WHERE WHERE WHERE?

When I did a small search on oscilloscopes I couldn't find anything under
500$... those that where didn't work and looked very very cheap.

Just look around E-bay. Like I said, going price for a Tek 465 (a
really nice scope) is $100-$200. And the lesser Eicos and Heathkits go
for $20 (if they sell at all). These prices are more than what you
would typically pay at a hamfest or flea market.

and what does "mostly working" mean?

CRT may be dim, controls noisy, uncalibrated, not all switch positions
may be working. But "mostly working" in that you can still hook up a
signal and see it.

Tim.

 

[Al]

Just look around E-bay. Like I said, going price for a Tek 465 (a
really nice scope) is $100-$200. And the lesser Eicos and Heathkits go
for $20 (if they sell at all). These prices are more than what you
would typically pay at a hamfest or flea market.


CRT may be dim, controls noisy, uncalibrated, not all switch positions
may be working. But "mostly working" in that you can still hook up a
signal and see it.

Tim.

I got this one and it's fantastic.

http://www.surplusshed.com/pages/item/r1456.html

It comes with complete schematics and manuals.

Al