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--- In Electronics_101@..., "lcdpublishing"
<lcdpublishing@...> wrote:

The voltage shown on the "Scope" is not actually accurate. It takes
a bit of messing around to calibrate it based on the volumn control
which is set in Windows (task bar), working from a known osccilating
voltage etc. etc. etc.

So I just roughed it in so that what is 5 volts on the circuit board
would show up as 5 volts on the screen - it is just a rough setting.

Do you think that a probe of the type that Virtins sells would
correct this display?

Chris

Nothing in the world is going to make a square wave show up square on
a capacitively coupled sound card. And the capacitors are on the
sound card, not in the probe. Still, the probe seems useful... But,
not at 250 Hz!

Demo 5 appears to show a square wave but mentions that the waveform
top is not flat due to the AC coupling of the sound card. I wonder
how they got it flat to start with. You CAN NOT have DC levels on an
AC coupled circuit. But then, we don't have the circuit for your
sound card.

And then they printed the traces in red to guarantee that a high
percentage of adult males can't see them. Including me...

Now, if one were to do a little software magic, the signal you posted
could be changed mathematically to display something else. Maybe the
display is more a function of magic that an actual representation of
the signal after the differentiator.

Or maybe there are sound cards that are better at this than others.

Richard

Use a couple of zeners in series, taking virtual ground from where the two connect. Then true ground is negative with respect to your new virtual ground. Be careful to set up the series current limiting resistor to just power the gizmo so as not to burn up unnecessary battery power. This scheme is used in many well logging tool designs, and power consumption is a big issue in those designs as well.

Hi
Just the way we have PC based scopes are there any PC based curve tracers
available that one may purchase for there work ?

I have been unable to find something I would like. Anny suggessions will be
welcome.
Gaurav

The voltage shown on the "Scope" is not actually accurate. It takes
a bit of messing around to calibrate it based on the volumn control
which is set in Windows (task bar), working from a known osccilating
voltage etc. etc. etc.

So I just roughed it in so that what is 5 volts on the circuit board
would show up as 5 volts on the screen - it is just a rough setting.

Do you think that a probe of the type that Virtins sells would
correct this display?

Chris



--- In Electronics_101@..., "rtstofer" <rstofer@...>
wrote:

--- In Electronics_101@..., "lcdpublishing"
<lcdpublishing@> wrote:

I just upload a JPG image from my scope thing. It is located in

FILES-CHRIS_LCD and it is called ON-OFF.JPG

What you are seeing is measured at the output of an opto

isolator

(MT6) and the signal I was expecting to see was a square wave

form. I

am toggling the output at a constant frequency. Obviously what

you

will see in the image is NOT a square wave but I do seem to

think it

has to do with my probe (which is a piece of wire and some

diodes).

I somewhat recognize this shape, only very extreme, when my real

scope

was working and I was adjusting and calibrating the probe.

Can someone tell me what you think and what I am seeing here?

Thanks

Chris

We kicked around the idea that both inputs to the sound card are
coupled through capacitors to block the DC path. This circuit is

also

called a differentiator. For AC signals, it doesn't matter (above
some frequency) but for DC, it tries to differentiate the edges

which

results in an impulse.

The large vertical spike above 0V occurs on the rising edge of the
square wave. Once the edge is gone, the voltage stabilizes at the
high level but the derivative of a constant is 0 which is the
horizontal line where the trace falls back. However, at some

point,

the blocking capacitors (or some other component) start to charge

up

and that gives you the sloping line over to the right of the

vertical

spike. It is in the same direction: if the leading edge drives the
differentiator positive, the integrator (the sloping line) will

also

go positive. Given enough time, it might rise to the voltage

level of

the signal.

Similarly, on the falling edge, you get a negative spike

(derivative)

and eventually a negative slope from the integrator.

That's about it. The only reason the first spike looks strange is
that the capacitor was probably fully charged and the spike didn't
have enough energy to quite drain it.

With a 250 Hz signal, I wouldn't spend a lot of time worrying about
the length of the ground connection. In fact, at that speed, I

would

use Romex - #12 solid would be good. It just doesn't matter.

An AC coupled scope is useless for DC circuits such as logic. All

you

can tell is about where the edge occurred and which direction is

was

going. You can't tell anything about the voltage because the
differentiator could rise to infinity if the circuit didn't limit

it

and the rise time approached 0.

In theory, with the diodes in the circuit, you can only get about

1.4V

positive or negative before the diodes clip. Yet, you are getting
over 4V of signal. Perhaps the diodes are slow but my money is on

the

differentiator.

Richard

seeing how you are ac coupled, dc blocked, it lookes to me like trigger pulses on and off.

Don

--- In Electronics_101@..., "lcdpublishing"
<lcdpublishing@...> wrote:

I just upload a JPG image from my scope thing. It is located in

FILES-CHRIS_LCD and it is called ON-OFF.JPG

What you are seeing is measured at the output of an opto isolator
(MT6) and the signal I was expecting to see was a square wave form. I
am toggling the output at a constant frequency. Obviously what you
will see in the image is NOT a square wave but I do seem to think it
has to do with my probe (which is a piece of wire and some diodes).

I somewhat recognize this shape, only very extreme, when my real scope
was working and I was adjusting and calibrating the probe.

Can someone tell me what you think and what I am seeing here?

Thanks

Chris

We kicked around the idea that both inputs to the sound card are
coupled through capacitors to block the DC path. This circuit is also
called a differentiator. For AC signals, it doesn't matter (above
some frequency) but for DC, it tries to differentiate the edges which
results in an impulse.

The large vertical spike above 0V occurs on the rising edge of the
square wave. Once the edge is gone, the voltage stabilizes at the
high level but the derivative of a constant is 0 which is the
horizontal line where the trace falls back. However, at some point,
the blocking capacitors (or some other component) start to charge up
and that gives you the sloping line over to the right of the vertical
spike. It is in the same direction: if the leading edge drives the
differentiator positive, the integrator (the sloping line) will also
go positive. Given enough time, it might rise to the voltage level of
the signal.

Similarly, on the falling edge, you get a negative spike (derivative)
and eventually a negative slope from the integrator.

That's about it. The only reason the first spike looks strange is
that the capacitor was probably fully charged and the spike didn't
have enough energy to quite drain it.

With a 250 Hz signal, I wouldn't spend a lot of time worrying about
the length of the ground connection. In fact, at that speed, I would
use Romex - #12 solid would be good. It just doesn't matter.

An AC coupled scope is useless for DC circuits such as logic. All you
can tell is about where the edge occurred and which direction is was
going. You can't tell anything about the voltage because the
differentiator could rise to infinity if the circuit didn't limit it
and the rise time approached 0.

In theory, with the diodes in the circuit, you can only get about 1.4V
positive or negative before the diodes clip. Yet, you are getting
over 4V of signal. Perhaps the diodes are slow but my money is on the
differentiator.

Richard

First of all, thanks for everyone's quick responses. I thought using a voltage divider would take a toll on my 9V cell. Thanks for clearing that up. I don't really want to go the route of buying another chip...but I haven't put any thought into diodes. I like the zener idea with a virtual ground. I'll give that method a go.

Thanks again!
John

"AnaLog Services, Inc." <wireline@...> wrote: Some opamps work well from a single supply, and that is a preferable choice if you can get by with it. Or you can fool them with a resistor biasing scheme to work with a single supply (this is often done in the well logging tools I work on). Or you can create a virtual supply with zeners where the virtual ground is above true ground and can thus have a virtual bipolar supply. Or you can use a little switching supply that produces a bipolar output. There are any number of options, but look at an opamp like the CA3140 that likes single supply operation if your application allows same.

Roy,

Yes, there is a ground wire. Here is what I have for my 'Probe'
which is based on a schematic in the documentation for the Soundcard
scope.

I have the cord from a PC microphone that is about 7 feet long. I
cut the microphone off. Inside are 3 conductors, the braid which is
ground and two insulated conductors. At the cut-off end, I attached
it to a circuit board using this schematic....

Look in that same FILES-CHRIS_LCD folder and view the
image "SortOfProbe.JPG" to see the schematic. I have it built up the
same exact way using perf board.

For the actual "Probe" I am using a 12" long piece of solid
conductor wire - then attached to that is alligator test lead
clipped onto the device I am measuring. With or without the
aligator ended test lead, the readings are the same on the scope.


As for the output of the opto, there is currently only a 10K pull-up
resistor there and I am attaching the "probe" to the side of the
resistor that the opto is connected to.

Crude - very crude, but free ;-)

Chris




--- In Electronics_101@..., "Roy J. Tellason"
<rtellason@...> wrote:

On Sunday 08 October 2006 02:45 pm, lcdpublishing wrote:

I just upload a JPG image from my scope thing. It is located in

FILES-CHRIS_LCD and it is called ON-OFF.JPG

What you are seeing is measured at the output of an opto isolator
(MT6) and the signal I was expecting to see was a square wave

form. I

am toggling the output at a constant frequency. Obviously what

you

will see in the image is NOT a square wave but I do seem to

think it

has to do with my probe (which is a piece of wire and some

diodes).

And a ground wire? That'll make a significant difference there.

In fact my

tek probes have both long and short ground wires for exactly that

reason.

I somewhat recognize this shape, only very extreme, when my real

scope

was working and I was adjusting and calibrating the probe.

Can someone tell me what you think and what I am seeing here?

That's some pretty extreme ringing, for one thing. How exactly

is the output

of that opto wired?

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that

can

be killed but can't be tamed. --Robert A. Heinlein, "The Puppet

Masters"

-
Information is more dangerous than cannon to a society ruled by

lies. --James

M Dakin

On Sun, 08 Oct 2006 21:06:56 +0200, Roy J. Tellason <rtellason@...> wrote:

And a ground wire? That'll make a significant difference there. In fact my
tek probes have both long and short ground wires for exactly that reason.

I bet you a soundcard the ground wire length is totally irrelevant there.


I don't even want to speculate about that squiggle ;-)
What is connected to the opamp output?
Is ground connected?
How looks the opto input? squarish or not?

ST

On Sunday 08 October 2006 02:45 pm, lcdpublishing wrote:

I just upload a JPG image from my scope thing. It is located in

FILES-CHRIS_LCD and it is called ON-OFF.JPG

What you are seeing is measured at the output of an opto isolator
(MT6) and the signal I was expecting to see was a square wave form. I
am toggling the output at a constant frequency. Obviously what you
will see in the image is NOT a square wave but I do seem to think it
has to do with my probe (which is a piece of wire and some diodes).

And a ground wire? That'll make a significant difference there. In fact my
tek probes have both long and short ground wires for exactly that reason.

I somewhat recognize this shape, only very extreme, when my real scope
was working and I was adjusting and calibrating the probe.

Can someone tell me what you think and what I am seeing here?

That's some pretty extreme ringing, for one thing. How exactly is the output
of that opto wired?

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin

Some opamps work well from a single supply, and that is a preferable choice if you can get by with it. Or you can fool them with a resistor biasing scheme to work with a single supply (this is often done in the well logging tools I work on). Or you can create a virtual supply with zeners where the virtual ground is above true ground and can thus have a virtual bipolar supply. Or you can use a little switching supply that produces a bipolar output. There are any number of options, but look at an opamp like the CA3140 that likes single supply operation if your application allows same.

On Sunday 08 October 2006 02:15 pm, John Baker wrote:

Hello all.

I've been experimenting with all sorts of opamp circuits. I was
wondering what would be the best way to get a + and - rail from a
battery. I was thinking that I could use a voltage divider with
resistors, but I don't think this method is very efficient.

Could someone point me towards some info about this matter?

You can do that, and I've seen a bunch of stuff that does exactly that. The
resistive divider can be fairly high values, like 10K for example, without
causing much in the way of problems. This isn't wasting much of your battery
power.

One thing you probably want to do is put a capacitor across each of the two
resistors, what value depending on how low a frequency you expect the
circuit to handle. A value of 10uF would probably cover most cases.

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin

Shawn,

Sounds as if you will need a tunable filter so you can select the
various frequency components.

73, ron, n9ee/r




--- In Electronics_101@..., "sstandfast"
<sstandfast@...> wrote:

Hey guys,

I've been reading the latest posts about various scopes and the

pro's

and con's of each and that got me thinking about a (hopefully)

small

project of a gain vs. frequency meter. The device would take an
input signal and display the relative db of each frequency present

in

the signal. Similar to the Forier Analysis in PSpice. The

reference

voltage should be variable and it doesn't have to have an exremely
wide bandwidth - audio range would be enough. A graphic LCD could
then be used to display the output.

My question is, what would be needed for a device like this. I

know

that an input buffer/isolation stage (unity-gain OP-AMP's) to

provide

a high-impedance input will be needed, and the magnitude

comparison

can be achieved with either a compariator or an op-amp. I can
program a PIC to display the output, but what would be needed in

the

middle? How would I seperate out each frequency? Also, would

this

device be considered a "Frequency Counter"?

Thanks for any tips,

Shawn

John,

If you are using AC then an op-amp can be run on a single supply.
You have to put a voltage divider on the non-inv (+) input and drive
the inv(-) side and AC couple the input and output with caps. If
you use this put a cap, 1uf, from the non-inv to ground to keep any
non-DC sig off this input.

You can do for non-inv amp with resistor and input driving the non-
inv input.

In both cases the no signal will have the divided voltage on the op-
amp output.

If you look'n for positive voltages only you can use an LM358. When
supplied by + voltage and ground it's output will operate from rail
to rail. Have to watch gain to keep in linear output range.

If you have to have +/- DC output then you need +/- supplies. There
are +V to -V converters.

73, ron, n9ee/r





--- In Electronics_101@..., "John Baker"
<johnbaker_erie_pa@...> wrote:

Hello all.

I've been experimenting with all sorts of opamp circuits. I was
wondering what would be the best way to get a + and - rail from a
battery. I was thinking that I could use a voltage divider with
resistors, but I don't think this method is very efficient.

Could someone point me towards some info about this matter?

Thanks!
John Baker

On Sun, 08 Oct 2006 20:15:00 +0200, John Baker <johnbaker_erie_pa@...> wrote:

Hello all.
I've been experimenting with all sorts of opamp circuits. I was
wondering what would be the best way to get a + and - rail from a
battery. I was thinking that I could use a voltage divider with
resistors, but I don't think this method is very efficient.
Could someone point me towards some info about this matter?
Thanks!
John Baker

You can add capacitors in parallel for better AC stability of the ground.
Actively driving the ground with an opamp is another way.

ST

I just upload a JPG image from my scope thing. It is located in

FILES-CHRIS_LCD and it is called ON-OFF.JPG

What you are seeing is measured at the output of an opto isolator
(MT6) and the signal I was expecting to see was a square wave form. I
am toggling the output at a constant frequency. Obviously what you
will see in the image is NOT a square wave but I do seem to think it
has to do with my probe (which is a piece of wire and some diodes).

I somewhat recognize this shape, only very extreme, when my real scope
was working and I was adjusting and calibrating the probe.

Can someone tell me what you think and what I am seeing here?

Thanks

Chris

Hello all.

I've been experimenting with all sorts of opamp circuits. I was
wondering what would be the best way to get a + and - rail from a
battery. I was thinking that I could use a voltage divider with
resistors, but I don't think this method is very efficient.

Could someone point me towards some info about this matter?

Thanks!
John Baker

On Sun, 08 Oct 2006 18:55:58 +0200, Roy J. Tellason <rtellason@...> wrote:

Why do you say that the sound card would have low input impedance?

assumption.

ST

On Sunday 08 October 2006 09:11 am, Stefan Trethan wrote:

With the probably low input impedance of the soundcard the 10:1 are no
good.

Why do you say that the sound card would have low input impedance?

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin