Narrow-Bandwidth Television Association

wonderful work ! very well done sir !

Yesterday I bought at the local drugstore (Kruidvat) an LED pocket lamp with 28 high light output white LEDs, for € 4,-. This is more or less the price that I paid about 8 yeard ago for ONE amber high brilliance LED. I bought 32 of them and connected them in series for my monitor. So the regular electronics shop is no more the best source for LEDs.

How good are those "kruidvat-LED"s? My wife is working next to a "Kruidvat".

Thank you Klaas for an idea to search in non-specialized shops!

I bought in a local small hardware-store exactly what I have been looking for. A camping flashlights with 20 straw-head high output leds.

With leds, I can't stop surprizing. For some reason, there were no such leds in ANY specialized shop, including Farnell, which is VERY expensive in terms of shipping, at least to Israel.

Another surprising fact. In the lamp i bought, the leds are divided in 2 branches of the same type leds, one containing 5 leds, another - 15, all parallel connected. The switch commutate between them and also acts as a resistor, it has 3 Ohm in itself. The surprise is that the branch with 5 leds runs at 0.45A, 0.09A for each led. And the 15 leds branch has 0.7A, which means about 0.05A for each led.
Sorry, it was a mistake (0.9A), so nothing unusual...

BTW, why not to use a fuse instead of resistors in series with leds? Fuse of correct value is safer then resistor, it guarantees that amperage won't rise above definite one. And it takes no energy opposed to resistors. The correct gamma could be obtained by means of some source resistor in the amp. Isn't it a better solution?

Well, there are three problems with a fuse:
1. a fuse is far more expensive than a resistor,
2. a fuse has too little resistance. A resistor is needed to limit the current.
3. a fuse is likely to blow. This is an enormeous problem for too many people.

But nice to hear that you also found a cheap source LEDs

I am looking for a good magnifying glass for my tv now and everything found so far is quite disappointing. The original picture is 2" and quite bright for making it 4".
What happens if I use Fresnel lens? All this stuff is expensive and problematic to find in shops here in Jerusalem. And before ordering something by internet it's better to know what drawbacks this Fresnel magnifier has, for NBTV in particular? Viewing angle? Moire pattern? distorsions?
Any thouoghts greatly appreciated!

Hi Holtzman,

I got a selection of Fresnel lenses from somewhere but I can't remember where - just to experiment with. I suspected they'd cause problems with paterning with NBTV but have found they work very well. In my colour televisor there is one which magnifies the 1" or so picture up to about 2" with no objectionable distortions.

There's loads on ebay - try motor shops (they sell big ones for rear-view magnifiers in cars) though they may be a bit coarse. If I remember where I got my kit I'll let you know!

Dom

Hi Dom,
I mean those rectangular sheets of frames for elder people with impaired vision. There are cheap ones and expensive ones, cheap are floppy. All of them are very large, about A4 sheet. Smaller fresnels are gadgets like "credit card" magnifier etc. Automotive shops is a good idea - I never realised these strange objects on minivan rear glass are fresnels. BTW, I don't understand why they use lens for reverse driving?
But why do you use fresnel in your 2" tv? With 1" picture I was quite happy using regular 3" lens. And for something about 4" regular round 2x lens is expensive/cumbersome/hard to find and needs to sit too far from the disc.
Thanks for your info!

I don't like the magnifying glasses. I have two, one is 9 cm diameter and circular, the other rectangular and 7 cm wide, but both give reflections from the room behind me. I have the idea that fresnel lenses also give reflections. I never found a magnifying glass with anti reflection coatings. That would solve the problem.

The original Baird magnifying glass had 2 lenses. I have also seen that in old slide viewers. There are 2 lenses, one large, the other smaller. They were designed to watch a picture of 24 x 36 mm, that is about 2" in diameter. At the Convention I have seen members using such a viewer in front of their Nipkow disc. But they as well give reflections. And they have a narrow viewing field. Two eyes at the time.

I used the Fresnel for three reasons.

1) I had used a normal lens and it cause a fair bit of a "fish-eye" effect
2) I had some fresnel material to try out
3) being a flat sheet was a lot easier to cut to the required size and mount in a little box.

It does reflect light from the room but not too badly, being flat it would be easy to add a layer of non-reflective film to when I get some. I'm currently restoring a Pye V4 television and this has a purple plexiglass implosion screen - it looks strange when the set is switched off but when playing it reduces glare and gives a very good picture.

The stuff for cars might be a bit coarse the reading lenses would probably be better...I'll keep an eye out for some. I too have a credit card one somewhere that came with the magazine Elektor - it's very useful for reading small component numbers!

Fresnels are used for rear-view magnifiers because a normal lens 50cm diameter would be difficult to stick to the back window and very very heavy!

Another idea you could try is to make your own lens like the old TV magnifiers - http://www.tvhistory.tv/TV-Lens.htm fill something see thru and the right shape with ground nut oil?

Dom

In 60 lines disc tv, the image is so dim that you have to darken the room before watching. Even with 30 leds it's not brighter than my four-led first mockup tv! Once it's twilight around, reflections are much less visible, if there are any. I tried to place a conic shield around lens, like that "tunnel" in Baird's commercial tv, it eliminated some reflections but was not that nessesary. And the viewing angle is already narrow because disc is made of thick plastic and I didn't want to counter-sink the holes.
Another point - the lens add some reality illusion effect, it's like a window to another world What I don't like indeed are LCDs and plasmas - no reflections, but no life...

Hi Maksim

Wow, I love the flexible drive between the 1500rpm disc and the interrupter drum, I never would have guessed. I saw your youtube video which kept me guessing. I had to dig through these old threads to find it.

i am keen to try a two or three turns Nipkow disc. I am thinking of using a disc interrupter though.

I think you have two of the best mechanical TV's I have seen. Do you plan to build anymore or have any ideas that you would build if you had the time? I would be interested to hear.

Cheers
Keitj