Building Cheap 100 Inch TVs

Nastar writes "If you visit eBay and such places there are guys selling 'kits' so that you can easily build your own 100 inch projection screen. There are websites such as 100InchTV selling the instructions for around $10 a pop. They say "this is the only product of this kind on the web" and "it is now possible to convert any type of television or computer monitor into a 100 inch video system that's truly amazing!". I don't like the idea of these people selling this information, especially when you can get it free from the good people at BSTV BSTV. Ihaven't built mine yet, but the reports of quality differ from so-so to fantastic! I suppose it depends on perfecting the technique involved. "

Comments

Magnigfy

[E1ven]

I remember reading about one of these a while ago.

I can't get to the linked site right now (I'm presuming it was slashdotted allready), but the way it worked was to basically use a magnifing glass. The screen emmits through a box of a certain size (the screen size), If you put a lense over that, in theory, you could magnify that light, so that it would be large enough to fill a "100 inch" screen, but it would look horrible!
I would think it would be very blurred, very hard to see (they don't give off THAT much light), and the colours would wash out.

I'd be curious to hear of anyone's actual experiences in building one of these.

Just MHO, of course.

you know what 640x480 on a 21" monitor looks like?

[carlcmc]

Now take that and multiply it times your worst possible dream to get pixels the size of green peas across the wall in your 100" display.

Did this years ago

[Scutter]

My dad had a setup years ago that was basically a wooden box with a lens. Inside the box, he put a 13" TV with the picture reversed and upside down (I have no idea ho he managed that). He projected it on the wall. It looked fantastic. The most expensive part was the lens.

The only real downside was that you could really only see it well if the lights were turned down (or off).

Forget lenses, what about scanning LED projectors?

[dschuetz]

I experimented with a similar lens arrangement like 15 years ago with a fresnel lens from an overhead projector. Neat idea, but, well, it sucked.

I've been wondering for years, and have wondered aloud here before but gotten no response, about the possibility for building a scanning projection TV out of LEDs and mirrors.

Basically, rather than projecting an entire image at once, like an LCD projector does (and, thus, limiting your resolution to how big your LCD or DLP array is), this would take the output of red, green, and blue LEDs and bounce them off a mirror vibrating in two directions (horizontally and vertically) to provide a raster scan. With today's high brightness LEDs (ever notice how blindingly bright the new LED traffic lights are?), I'd think this could, with the right focusing system, give you a quality image on a decent screen.

All that remains is to decode the video signal for processing by the projector. In a simple mode, you might even be able to simply take the HSYNC and VSYNC signals and, essentially, use them to mark the edges of your scanning motion, then simply vibrate the mirror back and forth within that time frame. (this is hard to describe, but hopefully it'll make sense to some of you).

For something like this, the most expensive bit would be the lens at the front. You'd have a bunch of $2 LEDs (running cool and quiet, too, unlike the bulbs in DLP/LCD projectors), a simple electro-magnetic mirror mount (speaker coil for a prototype, maybe?), and maybe $50 worth of electronics.

Any EEs out there who think this makes sense? Or should I just keep waiting for HDTV projectors to come down to a kilobuck?

Re:Experience building these.

[taniwha]

I was using one lens. This turned the image upside-down. This meant I had to turn the TV upside-down to get a usable picture. This made the TV image turn funny colours. I have no idea if this happens to most TVs or not. A well-made TV *shouldn't* have this problem - it _should_ only be gravity-sensitive if some of the focusing coils are loose inside it. The electron beam certainly doesn't care about gravity. YMMV.

No - but it does care about the angle of the earth's magnetic field .... which is in effect reversed when you turn it upside down. Degausing the tube will help - but really you need either a helmholtz cage or a TV designed for the southern hemisphere (where of course they mostly use PAL so I guess you're SOL :-).

Back when I worked for a company in the computer monitors biz I learned that monitors for the northern hemisphere are alligned in Japan all facing in the same direction, those for the southern hemisphere are aligned in special cages (virtually facing the same way I guess) - we learned this the hard way after selling some monitors down south and having some really pissed customers

Re:Reason for lack of brightness

[Technician]

For anybody who wants to know why they are so dim..

A CRT projector of the 3 tube variety uses this setup for several reasons related to brightness. Number one on the list is no slot mask! Each tube is one color. You are not blocking 80% of the electron beam to the phosphor with a matrix shadow mask inside the CRT.

Number 2 on the list is F stop. A large CRT giving off light with a lens far away gets very little light to and thru the lens. Most (about 80% or more) of the light hits the inside of the box instead of the lens. A projector set uses a small set of CRT's so the lens is very close to the CRT getting most of the light thru the lens. The smaller CRT's can easly have flat faces taking care of the focal plane problem also.

Raw Power.. The small CRT's in a projection set are not limited in beam current as there is no shadow mask to worry about overheating. The face of the CRT is gel or liquid coupled to the lense to reduce interelement reflections and aid in cooling. They can put out brightnesses on the face of the CRT's that can be painful to look at unlike a conventional tube.

The last item is when the distance to a projection surface doubles, the brightness goes down with the square of the distance. Doubling the distance to double the size decreases the brightness 4 fold. This is true for both a real projector and the home made variety, but the home made doesn't have the brightness to sacrifice on the larger immage.

With all these factors working for a 3 tube projector and against a single tube, the diffrence in projected brightness is typicaly more than 200X brighter. Translation.. a room with a couple candles in it will typicaly wash out the image on the home made projectors.