DIY Video Projector

[jjasniew]

Last night, I hooked up this 21" 640X480 monitor someone gave me at a flea market. After wrestling with the PC to get 60hz refresh, I tried to project the image on the wall.

I dissassembled the arm and overhead optics from my dukane 4003. Holding by hand in the relative position (monitor facing up) I was able to see an EXTREMELY FAINT image on the wall. In fact, the ambient light from the screen was competing (and winning, BTW) with the projected light. I used NORMAL brightness for this test (as I couldnt find the brightness control on this unit. Hey, it was late)

This demonstrates a few things;

1. It is possible to make a projection unit from a CRT, using std overhead projector optics. The optics I used, while designed for a smaller "stage" nevertheless captured the entire 21" CRT image.

2. Flat screen is a must, not only for the obvious focus requirement, but to minimize a tangental light component coming off the curve of the tube, which ends up as ambient light on the screen. I suppose some sort of "bellows" could be used, to mask off this problem.

3. The CRT will have to be turned up really, really bright - while still maintaining things like focus and contrast.

4. The light output, being relatively small (compared to a projection panel / OHP combo) makes it necessary to have a screen with a gain better than "1". These, as you will find out, are not cheap.

If you can surmount the "making it really bright" problem, it is realistically feasable IMHO. It may even be possible to align 3 color CRTs (they do it somehow in the 3 gun RGB projectors) and lenses so that the image brightness is added to by each successive projector. Look for three, new, flat screen 14" XGA monitors. Or for TV, 3, new, flat screen 13" color TVs...

FWIW,

[Timtimes]

The first I heard about this project was the 100 inch projected crt (big woody), which reminded me of a similar project (probably in popular science/electronics mags) when I was in high school (70's).

If you are going to project a CRT, I think you'll do better with a SMALL CRT. They are just BRIGHTER to begin with and I would think working off a 5" tube would make the optics more managable. I see a couple folks who are trying 14" monitors, which probably isn't going to be as bright/easy to deal with. You can buy a 5" TV with tuner built-in for less than $200.

For what it's worth, most of the rear projection TV's use five or seven inch CRT's. I'm curious whether the quality of a 5" CRT projected to about 40-50 inches (big enough for my tastes) wouldn't be just as clear/bright using a cheap fresnel setup as the more complicated LCD stuff most are working on? It'd be easy to invert a five inch tv set, and I can live with the left/right inversion by switching my l/r stereo leads to match.

Enjoy.

[Myren]

crt's are a bit problematic. any projection system worth a damn should cause almost instantaneous maximal pupil dilation, but its not very often you feel like you have to squint when your looking at a CRT. CRT projectors use MadCrazy (tm) gear to do their magical whoobajoob. I'm not sure how feasible it is to try it with a normal CRT.

Good luck though. If someone had told me i could make a LCD projector a year ago, i probably woulda laughed. There is a way.

Myren

[lokost]

Tim:

Thanks for the tip. I have just exhausted myself explaining to the local glass hawkers what a front surface mirror is. (Seems so strange some of them are actually clueless). Perhaps you can just post a contact and I will take it from there.

[Myren]

i dont suppose anyone can recommend a company i could contract for LARGE to-spec reflectors for moderately reasonable prices? having it be a dichroic cold mirror would be a big plus. depending on price, willing to order up to 1000, but will probably want about 20 initial samples first.

gonna get something that should be just prefect for everyone here working with LCD's. If it goes through, i'll make sure to make the reflectors available to people here.

myren

[Bitch]

Lokos managed to misplace his post so I'll put it in:

Posted by Lokost:

Chris

The Delta Lens from Surplushed has a focal length of 110mm; the Fujinon (L1812) has a focal length of 90 mm and will handle objects up to 4-1/2" diameter. In normal convex lens lingo, you need to place the object within 1 and 2 focal lengths to get a real and magnified image. Compound lenses like the Delta and Fujinon TV lenses allow you to place the object right at the surface of the lens and still get great magnification. Check Gunawan's post earlier on in this thread for the formula to help you calculate how much magnification you can expect for a given focal length and object( your LCD) position. Hope this helps.

[Axeman]

How did soldering your ribbon cable go?

[alank]

I'm getting ready to put a TV projection lens up on Ebay for sale. It's a US Precision Lens model Delta 77. These were used in rear projection televisions with 7" CRTs. It's a flawless lens, but it doesn't fit my project requirements. Might work for some of you though. Thought I'd give you all here first stab at it. If anyone wants it, let me know.

[Tim]

I used city glass and paint here in Bloomington,IN
USA. There phone number is 1-800-678-0138.
Maybe they can tell you what to ask for locally.
They knew exactly what I was asking for and had it in stock. Theu said they could order larger ones.

Good Luck.

[fender4]

alank,

I might be interested in that lens...my LCD comes in today, so if my current lens isn't large enough, it sounds like that one might be. Is the diameter of the rear lens 7"?

I will post soon if I need it. Thanks.

-f4