Construction Diary -- Part 2, Spinning the Nipkow Disc

[Andrew Davie]

I should place a photograph too, but I am not impressed of what my camera makes of it. Too automatic.

Well, I'm impressed... very very nice! This gives me something to aim for Your Nipkow disc is also extraordinarilly accurate, I think... is this a club disc?

Cheers
A

[DrZarkov]

Klaas, may I put your screenshot a) on my website (www.retrocom.de), which is currently in construction, and b) on Wikipedia? There are several articles about mechanical TV, Nipkow, Baird and even NBTV, but no pictures.

[Viewmaster]

I should place a photograph too, but I am not impressed of what my camera makes of it. Too automatic.

In the white (yellow) circle you can see the uneven ilumination from the different LEDs. I have just a single plastic diffuser sheet, which still shows circles for each LED.

That's a very nice disc you have there, as Andrew remarked. Both radial hole seperation and spiral is perfect.
It also gives me something to aim for too.
Again, could I also ask you, is this a home made disc, a club disc or factory made?
My positioning jig is nearly finished for the holes in my own first disc so my fingers..... and toes..... are crossed now!
Albert.

[Andrew Davie]

Well, I managed to blow up another IR LED. Just playing around with powering things differently -- I was seeing how it all powered up from the 12V shared with my NBTV circuits. Unfortunately I goofed and hooked things up to the 21V rail. D'oh! Fortunately, it was only the IR LED that failed, and past experience taught me to buy these in bulk, so I had a couple handy.

It was up and running again pretty quickly, and this time I powered the motor from the 21V (or so), replaced the 10K pot with a 50K pot... and now the synching is much nicer. I think the higher voltage gives the motor extra torque, so it 'locks' to the synch much more strongly.

Now that I'm getting a good locked frame, I'm really starting to notice the deficiencies in the holes on my Nipkow disc. Yes, it's a good disc as mentioned before... but mine pales compared to Klaas's! I'm half thinking about taking to mine with a circular file and widening -- ever so slightly -- some of those holes, so that they narrow those black bands.

[Klaas Robers]

The disc that I made the photo of is not a Darvic club disc, but an aluminium disc, punched by Mart Schouten. Mart is a ham friend of me, making NBTV before I did. He made these discs with the bicycle chain aroud a wooden disc as an angular divider. The holes are square and the diameter of the disc 30cm.

My camera made an over exposure I think. This is because the (white) circle is more yellow than the greyer parts, which are more orange. The red channel was overexposed and clipping I fear. I don't know how to prevent this.

The picture may be used as long as my name is mentioned.

[Klaas Robers]

Yes, indeed Andrew, I see that you have the Peter Smith circuit. For reasons that are not clear (to me) this circuit prefers to lock to frame one line wrong. I see the last vertical line of the characters KAREN, which should be at the first (most right) line not there, but completely left. As I read from Garth Porter, this can also be the case in the opposite direction, so the most left (last) line can be shown as the first line.

This is best seen at the Grant Dixon test pattern, where left and right should be two lines of grey scale. If misalligned the grey scales have uneven widths left and right.

Please try once to get the correct position by hand, check if the LED extinguishes and switch the sync in. It should synchronise in this postition, but I am not sure, as I don't have this circuit operative.

If this works we can see if we can make an automatic switch that works on the LED and disconnects the "jumper" if the LED burns.

[Roland]

I might even have a play driving everything off the same 12V supply, as suppled by the NBTV boards. I know, bad idea. I'll have a play, anyway, and report the results. My gut feeling is that it's going to work A-OK. I kind of like the 'elegance' of only having one rectifier/supply.

On this subject - I converted my Nipkow disk monitor last night to a single 12v supply. However it was not a success and the picture gained a nasty horizontal banding - even with no signal applied. I tried a few quick fixes with capacitors - but in the end had to revert back to the 2 PSU solution.

I'm pretty sure what I'm seeing is the square wave driving the motor getting mixed up with the video signal. The motor control on this monitor is PWM - but the next one I'm building will probably have a variable voltage regulator speed control and this may work better off a single supply.



Roland.

[Klaas Robers]

Roland,
I am not surprised. The motor current is quite large and it is difficult to keep it out of the video signal.
I suggest you to use different stabilisers (e.g. 7812) for the different circuits. They can then still use the same unstabilized voltage. In my PCBs for the LED-driver I used a stabiliser on the driver board, which is ment to power only the LED-driver and the sync separator. For other circuits another stabiliser should be used.
On the other hand, there is nothing wrong on using separate power supplies and more (small) transformers. Also in the professional world: some CD-players use completely separated power supplies for the digital part, for the analogue part and for the servo (motors) part. And this isn't done because it is cheaper....

[Klaas Robers]

I found a set up lens of +2 that could be placed in front of my photocamera. So I made a new photo of the Grant Dixon testpattern.
On this photo you can see that the square holes have a little overlap. So it is sharp. Also the difference in horizontal and vertical sharpness is very well visible. Look at both lines of the T.

[Andrew Davie]

... and a few steps backwards.

After so much effort getting the motor control circuit to work, you'd think I'd have learned a lesson. Instead of buying duplicate components, and transferring the circuit to my 'single board', I instead drew a diagram how it was all connected, ripped it all off the prototype board, and built it up on my single board. And... it doesn't work.

I'm going to have to go through the whole debugging process again. I've started with the IR LED combination -- for some bizarre reason this shows 12V at the receiver 'output' (cathode), so I figure that's shorted. I don't understand the numbers once again... and I went right back to square 1 and built up the IR send/receive part on the proto board, just to reassure myself they were working. Well, they weren't. Having just one spare handy, I hooked that up, and it worked ever so briefly... then stopped.

I must be doing something wrong. In any case, I'm out of IR LEDs, so it's a trip to a different electronics store tomorrow to see if I can find any more.

One thing that scared me today when doing this was when measuring the voltage in a part of my circuit, it read 120V. This got me REALLY nervous for a while, carefully back-tracking in the circuits to find out how I'd managed to build a 10x voltage amplifier. The trick to this, I discovered, is the little switch on the 10x probe -- which had accidentally been knocked back to '1x'

However, I've really been rather careless with this moving of the motor control circuit to my single board. When I was reconnecting things, after having soldered the motor circuit components in, I placed the LED negative, instead of to the LED controlling pin, to ground. This meant that there was now 20+V going through the LED matrix, when it was designed with just balancing resistors (or, suitable for 12V). The magic smoke literally streamed out of my lovely now ex-array. One of the resistors was burned brown, and at lest 75% of the LEDs have clearly burned out. Damnation!

My original array still works, so I'll use that for the while -- and possibly order another batch of LEDs soonish, so that they arrive by the time I burn out this display. I know now that I should have placed a precautionary resistor in the array board that would cater for the off chance that I'd connected the array direct to 20V.

So, I've really stepped back a week or two as far as progress goes. What I think I might do is go and buy duplicate components for a motor array, build it up on the prototype board again, get that going, and then use the working one as a way to debug the problems in the one that's now sitting on my single board. THe real issue is, I don't know if components are dead or not, or if I've just made a mistake with the wiring. Really really annoying.

One GOOD thing about this process is that I've solved a bit of a mystery I had. Every now and then, when measuring the line pulse, instead of a clean 60ms 12V signal, I'd get one that was 12V, but superimposed over a sine wave. I couldn't figure for the life of me why this was happening -- but things seemed to be working OK, so I kind of pushed this one under the carpet.

Well, it showed up as part of debugging my motor circuit in the single-board configuration -- and I realised that my two separate power rectifing circuits (the 12V powering the motor circuit, and the 20V powering the LED array/motor) did not share a common ground. And when I was measuring the line pulse signal, occasionally I'd choose one of those ground 'lines', and occasionally the other. The problem of the AC interference totally disappeared when I connected the grounds of the rectifying circuits together. So, lesson learned.

Very very frustrating weekend, though!

[Andrew Davie]

I found a lens +2 that could placed in front of my photocamera. So I made a new photo of the Grant Dixon testpattern.
On this photo you can see that the square holes have a little overlap. Also the difference in horizontal and vertical sharpness is very well visible. Look at both lines of the T.

Very impressive indeed. I attach a side-by-side image, converted to greyscale so that the comparison in image quality is reasonable. I'd say that yours is 50% sharper than mine, and a whole lot more accurate.

[Steve Anderson]

Andrew,

Well, it showed up as part of debugging my motor circuit in the single-board configuration -- and I realised that my two separate power rectifing circuits (the 12V powering the motor circuit, and the 20V powering the LED array/motor) did not share a common ground.

I quote myself here from a while back....

So, all 0V/Earths/Grounds should be connected together via nice fat cable and as short as is practicable.

Your frustration, my exasperation is a result of someone not listening! Read each word, take in each concept, if still in doubt, ask. I'm not being critical, but as a lecturer (part time) I sometimes despare!

Steve A.

..but heck, you're doing a damn fine job!! 10/10. Wanna job as a lab assistant? Genuinely, I would seriously consider someone like you, even with no formal education in electronics. A concept I have being trying to get over to the locals here with little success, where paperwork is everything. Where enthusiasm and a will to learn is dismissed.

Sorry about the 'tone' of this post, yesterday was a bad day.

[Andrew Davie]

Andrew,

Well, it showed up as part of debugging my motor circuit in the single-board configuration -- and I realised that my two separate power rectifing circuits (the 12V powering the motor circuit, and the 20V powering the LED array/motor) did not share a common ground.

I quote myself here from a while back....

So, all 0V/Earths/Grounds should be connected together via nice fat cable and as short as is practicable.

Your frustration, my exasperation is a result of someone not listening! Read each word, take in each concept, if still in doubt, ask.

Steve A.

Ah, but I'd built those rectifying circuits before you said that there should be a common ground, and at the point where you pointed this out, I was building the motor control circuit. At that stage I had not really thought about it, but had you asked me I would have said there was but one ground to the single board... it was a surprise to me when I discovered there wasn't Not "not listening", just inexperience. I pay careful attention to most things mentioned on the board

[Andrew Davie]

The magic smoke literally streamed out of my lovely now ex-array. One of the resistors was burned brown, and at lest 75% of the LEDs have clearly burned out. Damnation!

Nothing a little retail therapy can't fix! I have just bought 100x ultrabright LEDs from the same eBay seller as before. I won't need 100, but then again I thought, when I bought my first lot, that I wouldn't need 50. Cost this time, US$30 delivered. Should be here in a few weeks.

[Roland]

I suggest you to use different stabilisers (e.g. 7812) for the different circuits. They can then still use the same unstabilized voltage. In my PCBs for the LED-driver I used a stabiliser on the driver board, which is ment to power only the LED-driver and the sync separator. For other circuits another stabiliser should be used.

Yes - I might yet do this. So far the motor is driven from a 7805 (6v motor) and I have a regulated PSU driving the video. I have plenty of spare 7812s and I'm sure I can find an unregulated PSU in my collection. Though as you say there is no good reason not just to have seperate PSUs.



Roland.