Construction Diary -- Part 2, Spinning the Nipkow Disc

Original build of a televisor by a complete novice.

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Construction Diary -- Part 2, Spinning the Nipkow Disc

Postby Andrew Davie » Sat Apr 07, 2007 11:14 pm

I started tonight thinking about how electric motors work, and how to hook up my Nipkow disk. Fortunately, just about all the work has been done for me already.

Firstly, I noted that the sample motor control circuit (see http://www.taswegian.com/NBTV/forum/viewtopic.php?t=122 ) has 12V powering the motor. I figured I'd give the two motors I have a go (these were kindly supplied by Peter Yanczer with the Nipkow disk I bought from him).

While I was rather tentative at first (no explosion?...! great) after a while it became obvious that electric motors were a no-brainer. I plugged it in to the 12V supply. It spun. After I'd had a bit of a play trying to stop the motor with my fingers (testing the torque), I figured I might as well hook up the Nipkow disk to the hub thingy, and then hook up the disk to the motor via a rubber belt (all held in my hands).

It's a bit tricky juggling, but I can report a few observations...

1) Holding a spinning nipkow disk, an electric motor, and keeping a rubber pully/belt aligned and tensioned between the two is very difficult.
2) Rapidly spinning Nipkow disks are dangerous!!!!

I have no idea how rapidly the disk was spinning in the end, but I did get a good feel for what sort of tension was required in the belt to give good 'traction', and got a fair idea of where, relatively, the motor would need to be mounted.

At this point I realised that both my motors spin clockwise when looking down on them from 'above'. This is unfortunate, because I want my Nipkow disk to spin anticlockwise (scanning from bottom right, UP). I also want my motor and hub to be BEHIND relative to the viewer. This means that the body of the motor needs to be between the spindle and the hub, and there's not really a lot of room there. Enough, but it's a bit of a spanner in the works.

Attached is a picture showing how the motor will be mounted, also showing the rubber belt and the hub attached to the Nipkow disk.

All in all, it seems *really* simple to me, and in theory I could build up a bit of a rig to hold it all stable, and in almost no time at all actually be trying to view a picture on my "monitor".
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motor1.jpg
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Postby Klaas Robers » Sat Apr 07, 2007 11:35 pm

Andrew,

try to exchange the wires of the motor. Then it will spin the other direction.

Other solution is to cross the driving belt. This will invert the rotation direction of the hub only. This is not a useless solution. The Telehor disc monitor that could be used for clock-wise scanning (German system) and anti clockwise scanning (Baird system) used this for reversing the disc direction.

So ...... enough solutions.
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Postby Andrew Davie » Sun Apr 08, 2007 6:28 pm

Klaas Robers wrote:Andrew,

try to exchange the wires of the motor. Then it will spin the other direction.

Other solution is to cross the driving belt. This will invert the rotation direction of the hub only. This is not a useless solution. The Telehor disc monitor that could be used for clock-wise scanning (German system) and anti clockwise scanning (Baird system) used this for reversing the disc direction.

So ...... enough solutions.


Both interesting solutions. I thought, though, I read a caution against spinning the motor in the opposite direction -- excessive wear and tear? One wonders why the terminals are polarity marked, if it makes no difference.
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Postby Andrew Davie » Sun Apr 08, 2007 6:31 pm

Today I went out and bought some simple wood cuts to build a framework on which I can suspend my Nipkow disk. I don't think this will form the basis of my final monitor, but it will definitely be good enough to get to the point where it is working.

I put all the components together to give a good idea of what it all looks like when it is working. I like it a lot... only thing missing is one of those old-style radio horn things, which I'd like to put on the left, behind the Nipkow disk -- and secrete a speaker inside.
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mockup1.jpg
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mockup2.jpg
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mockup3.jpg
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Postby Andrew Davie » Sun Apr 08, 2007 10:22 pm

Amazing, but it's .... actually working!
Here is my first screenshot, with one of the test patterns. There are obviously a few problems (contrast is low, image is not starting at correct scanline (ie: left/right separated sometimes).... but ... wow.
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working.jpg
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Postby gary » Sun Apr 08, 2007 10:36 pm

Andrew,
Congratulations! that's a pretty good first effort! I like the woodwork
and the large magnifying glass.

Please try and get a snapshot of one of the 'human' bits of video later on
on the CD.
gary
 

first attempts

Postby Klaas Robers » Mon Apr 09, 2007 4:07 am

Congrats Andrew!!

Try to screen off the enormeous light from the LED's coming from the other side of the disc. Make a kind of funnel to see only the image part of the disc. Remember that you throw away 99,9% of the light, so you have tot do with just 0,1%. That is the dim picture that you look at. So no brightness in the whites.

In the times of J.L. Baird the amount of light from the Neon was much lower, I guess between 0,1% and 1% of what you have got now. The strang thing is that Nipkow Disc TV is only possible because of the latest developments on High Brightness LED's, Numerical Controlled Laser cutted discs, modern servo technology and Digitally Recorded CD-quality waveforms.

But enjoy the first results of all your work.
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Postby Andrew Davie » Mon Apr 09, 2007 5:42 pm

gary wrote:Please try and get a snapshot of one of the 'human' bits of video later on on the CD.


I find it VERY difficult to get pictures of any quality with my camera -- here are the best results so far. I stand back, with zoom on, and this is under room lighting conditions. The actual image looks much better than the blurry inserts shown, but you can get some idea from the in-situ test card shown.
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faces2.jpg
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Last edited by Andrew Davie on Tue Apr 10, 2007 12:46 am, edited 1 time in total.
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Postby Andrew Davie » Mon Apr 09, 2007 8:46 pm

I've been playing with everything, of course -- there are a few things that I've found....

#1 cuts from the edge of a spinning Nipkow disk... hurt.

Diffusing the light from the high-intensity LEDs has been problematic. I'd had 9 sheets of waxed, greaseproof paper in front of them for the image above... but I spotted some very plausible diffusing material whilst at the supermarket -- plastic lids for pet food cans.

These proved *awesome*, and it took three of these to correctly diffuse the light. Now the image quality has improved about fourfold. Much more contrast, surprisingly, although to my eye it does look a bit more flickery. I'm guessing it's a tradeoff -- flicker vs. definition. In the collage of images shown here, the one at top-right most closely represents what it looks like to the eye.

Another thing I've found -- placing a handkerchief along the left edge of the Nipkow disk, sort of stuffed down between the disk and the slot it sits in, is excellent for fine speed control. I just have to press very lightly on the handkerchief to slow the disk a tad -- its natural tendancy is to speed up (ever so slightly). I can hold an image stable (within a few millimeters) indefinitely using this technique.

I also hooked up my sound circuit, and -- hey, it's a fully functioning mechanical television! The brightness is pretty good -- I can comfortably watch it in a room lit with a 75 watt globe, and it's definitely viewable with a reading light shining directly at it.
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Postby DrZarkov » Mon Apr 09, 2007 9:50 pm

Congratulations! Great work!
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Postby Stephen » Mon Apr 09, 2007 10:50 pm

Very impressive, Andrew. Congratulations. Your work was well worth the effort.
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Postby Klaas Robers » Mon Apr 09, 2007 11:08 pm

Andrew, I used frosted plastic sheets. Be aware that one sheet will give an unevenly illuminated surface. This is what I have had until now. But use two sheets with some air space in between them. The distance makes a lot of difference. I should still try to find double sided etched glass. The thickness of the glass is important for the best diffusing action.

The problem of the diffuser is that you want to spoil light as little as possible, but you want also to diffuse it as much as possible. These actions are interacting and work against each other.

I did a few things to your photographs. You don't know it, but I can easily recognise the faces........
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faces3_154.jpg
Some more contrast.... some change in brighness..... some more gamma
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Postby Andrew Davie » Tue Apr 10, 2007 1:11 am

Klaas Robers wrote:I did a few things to your photographs. You don't know it, but I can easily recognise the faces........


I was sure that the faces on the CD would be well known to club members of long standing :) As to posting enhanced images, I have considered this (in fact, my earlier collage was enhanced) but I've decided to post unenhanced images, as it gives a better idea of the relative quality of each image.

Here's my go at enhancing one of the faces :)
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Postby Roland » Tue Apr 10, 2007 9:49 am

Andrew Davie wrote:I find it VERY difficult to get pictures of any quality with my camera -- here are the best results so far. I stand back, with zoom on, and this is under room lighting conditions. The actual image looks much better than the blurry inserts shown, but you can get some idea from the in-situ test card shown.


Very impressive pictures and inspires me even more to improve my system. I certainly agree with the problems of photographing the display. I've been working in semi darkness and I've found it virtually impossible to get any sort of focus lock from my digital camera. I know I can do better with a traditional film camera but there seems more future in improving my actual display so it is much brighter like yours.

:-)

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Postby Andrew Davie » Tue Apr 10, 2007 8:26 pm

Still trying to find out how to use my camera to take photos of my monitor's display in action...
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