NBTV without any moving parts. THE NIPTRIX

Forum for discussion of narrow-bandwidth mechanical television

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Postby gary » Sat Oct 06, 2012 4:02 pm

M3DVQ wrote:Also very important to make sure that whatever is driving your matrix can't ever stop clocking with the led driver still active! It would be much like burning a dot into a CRT only there might be a bit more smoke :shock:


Urk! yes indeed... :shock:
Perfecting an NBTV system is like trying to slam a revolving door...
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Postby Steve Anderson » Sat Oct 06, 2012 4:47 pm

Just for reference, Kevin Hadfield's 'Carousel' scanners are in V30 #4 (monochrome) and V33 #2 & #3 (colour, in two parts).

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Postby gary » Sat Oct 06, 2012 5:00 pm

Steve, I hope you haven't spent a significant amount of time searching for those - you appear to have missed my first post... never mind you picked up that the colour version was in two parts ;-)
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Postby Steve Anderson » Sat Oct 06, 2012 5:32 pm

Quite right Gary, 'skim-reading' again...a habit I've been trying to get out of!

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Postby gary » Sat Oct 06, 2012 6:35 pm

Steve Anderson wrote:Quite right Gary, 'skim-reading' again...a habit I've been trying to get out of!

Steve A.


Well that's one way to avoid "bloat"... ;-)
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Postby Harry Dalek » Sun Oct 07, 2012 10:37 am

Hi Albert have you seen this circuit on Grants matrix tv circuit idea?
newsletter 20.. 3 seems to have worked no moving parts either.

It will be interesting to see how yours gos.
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Postby Steve Anderson » Sun Oct 07, 2012 5:02 pm

OT I know but, a couple of items after Grant Dixon's contribution in V20 #3 called "New Products" is this...

"Wireless World" reveals that a new blue LED rated at 1000 mcd is available from Hero Electronics Ltd. for something around £30.

30 quid for one blue LED...and that's at 1995 prices! How things have changed.

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I just checked with my local supplier, 5mm blue LEDs, four times as bright, 500 of 'em for the same price...at today's prices.
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Postby Steve Anderson » Sun Oct 07, 2012 6:07 pm

Interesting to note in Grant's newsletter item that once he sorted out a few LED driver problems the intensity was sufficient, even at this very low duty-cycle. So maybe this is a go-er.

I think that it would be a worthwhile experiment though to knock-up a 555 to drive a LED at this very low average current, a simple breadboard exercise that should only take a few minutes.

Grant has a follow-up in the next issue where he was having all sorts of problems but it would appear they stemmed from the NBTV source coming off-tape.

The other problem Grant mentions is leakage currents from all the driver transistors. He was pulsing the LEDs at 80mA, at 1/1536 = 52µA for full intensity. It doesn't take much leakage and the LEDs will not extinguish.

I've looked through further issues up to 2001 and there appears to be nothing further...unless I've missed it/them...quite possible! Does anyone have anything further on this?

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Postby Viewmaster » Sun Oct 07, 2012 6:30 pm

Grants timing used a 4046pll which he was still developing. He also had 4 bit addressing. I too cannot find if he cracked it but that photo I posted of him at a convention seems to suggest he did. Pity he used green LED's though.

I am now inclined to use Graham's idea of a free running oscillator re triggered all the time.
Two 555's can be made to work as a push pull retriggerable timer and is very stable, so we shall see. At 19.2kHz a 2% error in freq per column results in a one pixel error so one should be OK, with a fine pot adjustment.

The 48 pixel columns and also the 32 rows wiil require 4017's (or similar) to be cascaded. There is a standard method of doing this with a few AND gates thrown in ! It isn't as straight forward as it first may appear, but nothing in NBTV ever is. :lol:
If anyone wishes to see the cct I will post it here.

That missing pulse frame adder of Peter Smith's will certainly be required,
the pulse use to start off the first column and trigger the first row.

edit.......Just found out that diodes can be used instead of AND gates. takes up less room too.
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Postby Viewmaster » Mon Oct 08, 2012 5:31 pm

Steve Anderson wrote:Interesting to note in Grant's newsletter item that once he sorted out a few LED driver problems the intensity was sufficient, even at this very low duty-cycle. So maybe this is a go-er.

I think that it would be a worthwhile experiment though to knock-up a 555 to drive a LED at this very low average current, a simple breadboard exercise that should only take a few minutes.
Steve A.

I have to apologise to you Steve, I previously said that your concern about brightness was not valid as Jeremy's LED's ran at 28 kHZ whilst the NBTV only ran at 19.2 so would be brighter.
The penny has now dropped because Jeremy's LEDs were on for 1/60
of each frame whilst the NBTV's only on for 1/1536.

In camera stop terms ( in that a halving in exposure time is one stop), we coulsd say the the NBTV LED compared with Jeremy's is 1/60th...1/120th...1/240th...1/480th...1/960th...1/1920th(near to 1/1536th)
.........
5 stops down in brightness?

Re popping over run LEDs , maybe a pot in th supply line would enable brightness to be kept to a minimum all the time.
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Postby Harry Dalek » Mon Oct 08, 2012 6:40 pm

Hi Albert

Could you post the`development of the project if you draw up any circuits i for one am very interested how you go.
I would like to learn how its done and how you go about it ...i am not good on digital electronics :oops:

The`4017 is an ic i have played around with on its `own i can see string them together as you think might work ...i was thinking about an easy way to modulate the the string of leds while also scanning...do you think this might work ? simple like me .
Could you use the sync from say the club circuit and use that to sync the clock i am thinking the clock has to be faster than 400 hz ...is it that times the number leds you use per line ?
Dam my small XT processor in my head . :roll:
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Postby Steve Anderson » Mon Oct 08, 2012 8:27 pm

Viewmaster wrote:I have to apologise to you Steve


No you don't Albert, I hadn't really clued-on anyway as I haven't read Jeremy's item as yet.

I have thought of a way such that the LEDs are driven for 1/32 of the time, it's still a form of multiplexing but it is quite complex once you start drawing little boxes on paper. Such that I don't think it's viable, but I'll keep staring at the scribbles and maybe a 'eureka' moment will happen...then again maybe not.

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Postby Viewmaster » Mon Oct 08, 2012 10:52 pm

harry dalek wrote:Hi Albert

The`4017 is an ic i have played around with on its `own i can see string them together as you think might work ...i was thinking about an easy way to modulate the the string of leds while also scanning...do you think this might work ? simple like me .
Could you use the sync from say the club circuit and use that to sync the clock i am thinking the clock has to be faster than 400 hz ...is it that times the number leds you use per line ?


The problem with your circuit Harry is that the LED current is limited to what the 4017 can deliver....about 20ma I believe. So if we are to drive each LED harder then both the 48 column and the 32 row outputs from the 4017 will require transistors to drive the LED's harder
...pnp 's for the columns and npn's for the rows.....see Grant Dixons diagram if you have the club's newsletter archive CD.
And yes, as previously mentioned the column clock must run at 19.2kHz
and the row clock (each line) at 400Hz, sync ed to the line pulses on the NBTV video.
BTW, I may appear at times to be knowledgable but don't be fooled. :lol:
I have had some digital experience when I built an electronic organ controlled from the BBC micro, but I kept it as simple as possible.

In trying to design a simple approach to this LED display I shall no doubt go off the rails :)

Steve...I too am busy scribbling but not to the depth you appear to be in getting those LED's to brighten up.
The LED's will not be on full for much of the time....in a typical test card/video they are not full on for more that 50% or even less.
In your avatar, for exaple, the LED's would be near off in the black hair and never fully on in the other parts.

I'm trying to get a good layout for the large veroboards that would be required... 10 x4017 + 80 transistors PLUS the 1536 LED's
Ain't NBTV just.
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Postby M3DVQ » Mon Oct 08, 2012 11:17 pm

That is quite a lot of LEDs isn't it :shock:

Even buying surface mount LEDs on tapes of 1000 it's going to be dear. What colour were you planning on doing it?
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Postby Viewmaster » Mon Oct 08, 2012 11:40 pm

M3DVQ wrote:That is quite a lot of LEDs isn't it :shock:

Even buying surface mount LEDs on tapes of 1000 it's going to be dear. What colour were you planning on doing it?


One can buy 5mm ultra bright LEDs at 2 1/2p each on eBay, so that's £40
for 1,600...........that gives me 64 spares.

My colour would be white or soft white.

If I ever did go ahead and try to build it would you visit me afterwards in the house of the white coated men? :lol:
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