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making an oscilloscope

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doubleM

New Member
im trying to make an oscilloscope out an old crt computer monitor. its just like using an old tv except i have to connect it through vga to my desktop bring up the screen. i cut the vertical deflector coil wires. when i turn it on i get the one line on the screen. i stuck a AA battery on it and the line moves up and down vertically...so i know that part works.
i tried connecting it to my small radio and the lines wont move. how can i get waves out of it? anyone ever tried this?
 

Papabravo

Well-Known Member
Since an oscilliscope and a monitor are constructed different principals I think it will be a cold day in hades before you can make this work.

In an oscilliscope there is a fast vertical amplifier that creates a deflection voltage on the vertical plates for each horizontal trace. In a monitor there is a fixed vertical deflection for each of the 525 horizontal lines. The time base in a monitor is fixed -- not adjustable. The odd and even lines are painted on alternate frames. The phosphor persistance is also different so the results will be unsatisfactory.

Monitors have no triggering circuits so even if you could paint a waveform it would not do you any good because on the next trace it will be in a different place and the screen will be full of traces in no time.

Then you'll want dual trace so you can tell when things happen with respect to other things.

If you want or need an oscilliscope then just get one.
 

dougy83

Well-Known Member
i tried connecting it to my small radio and the lines wont move. how can i get waves out of it? anyone ever tried this?
If the battery makes the line move, then it looks like you've got the right wires. I'm guessing you'll need to use a bit more current to drive this thing than your small radio can put out. Try connecting the output of a bigger amplifier to the coil.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,


Using the deflection coils to make the line or dot move around on screen
will certainly work, but the problem is with the inductance. There is just
too much inductance to get any real speed going there. The TV normally
operates at less then 20kHz, so you might get that if you are lucky and
do everything just right. You wont get anything like a real scope though,
which has bandwidth beyond a megahertz. Maybe you only want to look
at low frequency signals?

If you really want to use a TV as a scope, then your best bet is to design
a digital scope (fast ADC and memory) and use that and generate a composite
TV signal that shows that wave on the TV screen. It wont be that good
but it will be fast. Better yet, use a computer monitor as you'll get better
display resolution.
 

Hero999

Banned
The inductance can be overcome to some extend by controlling the current through the coils rather than the voltage across them. Some of the newer CRT monitors could work at much higher frequencies than 20kHz.

Unless this is purely for educational purposes it probably isn't worth it.
 

BrownOut

Banned
You might consider finding a junk oscilloscope and must salvaging the CRT from it. For all the effort you're going to put into this you might be happier with the results.

Who cares if it's worth it? We do these things to learn and amuse ourselves :)
 

MrAl

Well-Known Member
Most Helpful Member
The inductance can be overcome to some extend by controlling the current through the coils rather than the voltage across them. Some of the newer CRT monitors could work at much higher frequencies than 20kHz.

Unless this is purely for educational purposes it probably isn't worth it.

Hi Hero,


Yeah, but controlling the current means higher voltages with fast
rise time. Do you really think you can get a old CRT tv to reach
speeds of 1 MHz or faster? Imagine how high the voltage would
have to be.
 

Hero999

Banned
You should be able to get better high frequency response from the horizontal coil than the vertical.

A CRT oscilloscope is just a toy really.
 

mneary

New Member
You should be able to get better high frequency response from the horizontal coil than the vertical.
That makes sense.

However, I've actually made a stereo Lissajous with a television and found to my surprise that the coils were (nearly?) identical. They certainly optimized the horizontal deflection coil for bandwidth and deflection, but the vertical coil apparently didn't need to be different.
 

Hero999

Banned
You could be right, although I don't think you'd notice at audio frequencies.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You could be right, although I don't think you'd notice at audio frequencies.
The horizontal and vertical scan coils are completely different, wildly different impedances (which is how you find out which is which), and optimised for vastly different frequencies (50/60Hz for frame, and 16/15KHz for line).

100Hz sets have different coils again, as do computer monitors, for the even more different frequencies.
 

xtrmi

New Member
I was considering the same kind of project and thought of using a fast ADC and memory. Has anybody tried this?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I was considering the same kind of project and thought of using a fast ADC and memory. Has anybody tried this?
Try google - there are endless such projects out there, generally you need programmable hardware to do it fast enough.

But as you're building all the expensive and difficult parts of a scope, why not just buy one and save money?.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
u can also make oscilloscope from sound card.. there alot of projects for pc oscilloscope based on sound card or parallel port ,out there ,,,
Yes you can, but it's only an incredibly low specification - nothing like a real scope, and no where near as good as the 'toys' mentioned earlier.
 
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