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Homebrewed LCD Projectors
pseudofrog writes: "Seems the new thing to do may be to build your own LCD projector. For a couple hundred bucks, some guys are making projectors similar to the professional ones that cost thousands. And it looks pretty simple, too."
Er.. I thought the bulbs alone that powered the projectors cost several hundred dollars? Unless the manufacturers ridiculously over charge for the bulbs, I don't see how an entire system that costs several hundred dollars could compare to a system in which the bulb alone cost that much.
Well everything looks simple.
The coolest part about all of this is that in a few years time, the technology will be there so that the walls of your room are LCD/oLEDs and with the touch of a button you can make your walls have any wallpaper you want.
Just think, you could have the Windows "Clouds" wallpaper all over your room! Imagine that! Gee, if that were my wallpaper, I know I'd feel like I was actually in the clouds.
Hargun
Think nothing is impossible? Try slamming a revolving door.
well if its dark in the room and you have enough windowless wall space anyway and enough room to put the projector and .. and.... Well on second thought, back to the drawing board
"The United States has no right, no desire, and no intention to impose our form of government on anyone else." - Bush 05
Combine these projection systems with white LED's and we might have somthing here! =)
Ad Majorem Dei Gloriam
Interested in AI? MACR
I guess they should also build a server to go with that LCD projector, just for such an occasion.
It seems as though it is down already so here is the google cache
No picture of image result....that suck.
Is this another 50 feet Giant TV like project.
Without seeing the end result I won't spend money trying it.
Interesting. Now is this confined to hobbyists because the LCD companies are too slow-moving to have thought of it first? Or is the whole idea fundamentally flawed?
What I'd really want to see in the article is: Joe Blow built his own LCD projector for $350, and the image quality is better than what a good projection-screen television delivers.
Instead, there are lots of plans from hobbyists making their own, but no clear word about whether even the best of these designs produces an acceptable image.
One key thing: most of these designs call for a miniature 800x600 LCD monitor. Once projected onto a wall, how does that compare to the dots per inch of a regular projection TV? I bet it doesn't compare favorably at all.
The big question I came away with after reading this is: why aren't the big LCD companies developing this kind of product? Maybe it's an idea that, even with big-budget R&D, won't produce an acceptably good image.
I'm generally "Interesting," "Insightful," and even "Funny" here. What the hell happens to me at parties?
A lot of dotcoms did digital projections for their clients... and if we use my company as a comparison, they could have saved $12,700 if they could have had two projectors for $300.
;)
Now, they only had two projectors... imagine how many some of the bigger dotcoms must have had, and how much money could have been saved had this been out earlier!
libertarianswag.com
Build Your Own LCD Video Projector
However,
"And it looks pretty simple, too"
By simple are we talking about some of our fellow slashdotters hook the rest of us up, or are we talking simple for millionaire phds.
[o]_O
I bought a panel projector from a junk dealer for 10$. This is an older model, 3M 6150, so it's TFT 640x480 18 bit color.
Then I bought a smashed laptop screen for 5$ to get the backlight out.
Put the two together and I got a 15$ LCD monitor, this is fine for messing around with older gear like the Commodore 64, or a PC in 800x600, which the LCD panel can scale.
I'm pretty happy.
CAUTION: Raw LCD panels are very sensitive to static discharge! Use a wrist strap.
Here I thought the headline was "Homebrewed LSD Projections"... having to do with homemade LSD visions.
WWJD.... for a Klondike bar?
See this page cached@google :)
Great, now how do I install one of these for each wall in my house, and get a customised version quake where I can go running from room to room before getting zapped by a bouncing eyeball...?
http://meshier.com/mirrors/www.audiovisualizers.co m/madlab/lcd_proj.htm
Two Words... "Google Cache". Of course, they specifically requested permission from the site before the blatantly copied the material from this site to thier caching servers, didn't they ?
NZ Electronics Enthusiasts: Check out my Trade Me Listings
As if these guys are going to mount legal action against slashdot ... posting it here SAVES them money, its not like they've got a site full of banners.
Keep up the good work, Taco
I never claimed the above post was illegal. Reread my reply. Troll.
Just in case the main site gets /.'ed, It basicly involves a powerful bulb being sent through a LCD. He is using 2 fresnels to help get rid of some of the distortion, but I'll belive the quality when I see it.
A link from the main site: DIY LCD Projector
BTW...My office just picked up a NICE LCD thingy. It has 4 video inputs, 2 computer inputs (displayed PIP-style or side-by-side), and a 100mbps ethernet switch. We paid close to $10k. In my opinion, it was worth every penny. I'd rather spend $10k on something worth it than $500~~$1000 on something not worth it.
I'd rather you do it wrong, than for me to have to do it at all.
can you imagine a Beowolf cluster of these?
If you take your average LED, and compare it to the output of a 500W spotlight, no comparison. The LED is thousands of times weaker(lightwise), so you would need thousands of them to light up a point, not to mention collumnating them(hit the same spots for the whole area). :(/ LC_H andbook_v11.html
The only advantage you would get is heat dissipated, because 1000's of the buggers would chew, oh say, hundreds of watts anyway.
Someone made a projector from lasers and mirror systems, but they are again, hundreds of times brighter, and can be directly modulated, but the 'pictures' are 'liney', scanlines showing everywhere. Unless you have a screen that amplifies the light, the LED idea just doesn't work.
http://www.nofs.navy.mil/about_NOFS/staff/cbl
Navy specs on light sources, look halfway down
This mind intentionally left blank.
The KKK a bunch of sheetheads? You decide!
A nice thread on DIY projectors, a lot of them are done with LCDs.
diyAudio
The sites seem to mention the results (when they succeeded) were kinda pixelated. Others have pointed out that the bulbs they use tend to have a short life. With an image of the projected image it may be able to make a better judgement. How does it compare to the lower end projects that cost about 1700 or so?
As a side note, I'm looking at setting up my home theater to proper levels, and if these projectors display a good image, they seem to be a much more viable solution. Any one have suggestions regarding this? It seems that getting a 1000 lumens projecjtor that can do up to 720p isn't too unreasonable compared to any CRT or projection based TV solution. Is 1000 lumens sufficient? I'm specifically looking at the Mitsubishi SL1U Projector. I'm aware that watching TV/movies with such a system will naturally require a low level of ambient light, but two things are very cool about this.
1) Pretty much as big a screen as I want, provided distance in the room. Speaking of which, how far back is needed to get, say, a 60-70" image? Again, having seating so as not to get in the way of the projector is another issu...
2) When I move, large TVs are so unmanagable. The projector, and possibly a screen (maybe just use a white wall, provided the wall is smooth and white enough) Projector is on the order of 6-10 pounds, and the screen would be also manageable..
Who all has experience with this? My current rig is an old 24" console TV from the 80s with gaussed spots all over and annoyingly cropped image, so it wouldn't take much to impress me.
XML is like violence. If it doesn't solve the problem, use more.
You can get 800X600 lcd projectors on ebay for less than $500.00
and these dont need a computer, just plug in composite video.. (I can hear it now the videophiles that have their 2048p projectors that use fiberoptic digital video and HDTV ready HD-DVD players will whine that it's grainey,low light because it's lower than 95,000 lumens and doesnt have glass lenses made by Plossol in germany... Go to hell videophiles..)
The dayton hamfest is coming up very soon, you can get a (GASP) old technology video-tube projection tv for probably less than $300.00 that works fine. (granted, it's a coffee table, but hey...)
the golden rule is that you scroung for a used one first, then look for cheap new, and THEN create it by hand.
Do not look at laser with remaining good eye.
diyaudio has a 100+ page discussion of this. Most people are using an overhead projector with their standard lightsources, or mercury vapor or metal halide bulbs. The OHP's fresnel lenses get around the problem of having an LCD panel and lens with different sizes and not wasting a lot of the light(but they're kinda big). Pretty good results have been obtained. However, there are some caveats, the contrast ratio of lcd panels from desktop monitors are pretty poor(under 300:1), decent commercial projectors have contrast ratios > 1000:1. Although I haven't built one yet, I think this can lead to washed out images. Also, nobody has found a supplier for the nice 1-2 inch LCD's used in commercial projectors
My main interest is playing movies on the wall of my living room.
This project looks nice if I can find someone to do it for me that is.
I have a few concerns though.
1) Aren't dvd players using Macrovision to forbid a signal to be output on anything else than a TV? The signal going to a video capture card seems to be a slight problem. Is this a reason why every one in the article is talking about LD and VCD, these two older media not suffering of the Macrovision "virus", er copy protection.
2) Is this really cheap? I mean a dedicated P800 in the living room (cpu speed to cope with descaler complex algorithm)+capture card+LCD panel, etc... I don't even mention the electric bill.
3) Noise level: Getting a Pentium noisy as an air carrier next to a TV and adding the noise of the fan(s) for cooling down the bulb(s), does that meant that I will need to listen to the movie with a pair a noise cancelling headphones?
4) Space: I imagine that the distance from the projector to the screen needs to be consequent. I can't find data regarding the minimum size of a room to use the projector.
I still like the idea though.
PPA, the girl next door.
-- I feel better now. Thanks for asking.
I don't know where the first component could be found, but I could probably make a working model for around $200...
1: 6mmx7mm LCD screen (don't know if they can be made that small with decent resolution for a reasonable price)
2: Beseler 67 Photo enlarger
3: Bank of 4-8 halogen lights (or a single car headlight?)
4: Maybe a lens to focus the bank of lights
The correct wiring and such to make all of it work. The Beseler will already have the correct lens for focusing a screen of that size, and if you replace the standard enlarger lamp with something like a couple of car headlamps or something, you can probably squeeze out enough light (with a lot of life to the lamp too) to get a decent display in a dimly lit room...
Approximate cost (given a good day on ebay): $200
wackyballs
That's one thing that is cool about projectors, they don't devaluate very rapidly (check on ebay for anything 1-2 yrs old) compared to high-end TV. Plus it's not a pain in the ass to move around, plus you can have the size of screen that you want when you have 800+ lumens and good screen. (well obviously the more lumens the better if you're to do 15' diagonal :) )
:).
When I see people buying HDTV TVs at C$5000+ I don't understand why they aren't looking a medium range projector with HDTV support. Okay you don't buy anything under XGA resolution because with all the resampling it'll screw up the quality big time, but still, at 5K you have a nice tv, but at 5K you have a BIG refurb projector that can do both progressive playback of your dvd, give you an image that has easily 4 time the area covered, and best of all, you can play quake at wall size!.
In my case I've been trying to grab a cheap DLP XGA projector for a while, I don't want a 60 inch tv that will be a pain to move around, I want a 90 inch "tv" that I'll be able to plug my computer on it and also have fun watching movies like in the theatre
The replacement lamps are very expensive, but then again, when you look at the "kit" they sell you for 400$, it's basically a specific lamp with specific properties (metal halide, etc etc) at a specific voltage, plus a little crappy plastic holder... there's no optics (you read "lamp module" you'd think it has some collimating lenses or something) and you can buy these same lamps from a third party at 1/5th of the price, and you just have to mount it back on the plastic thing that was attaching the old lamp. If you have to break it, so what, nothing a high-temperature epoxy can't fix.
Anyways, nice to see articles like that, but LCD sucks, DLP is the way to go for video projectors, too bad parts are still expensive, anyone here knows a 3rd party supplier that won't only sell developper kits at 3K$?
--- Metamoderating abusive downgraders since my 300th post.
How about setting up this configuration, not with an LCD screen, but an old CRT monitor? Would the output be too dim? Could you crank the brightness, or would you then loose contrast?
i have dozens of old CRT's around for hacking.
could i just put a fresnel lense on one and aim it at a lense, then at a wall?
I hold very few opinions. I hold information based on observation and fact. If you wish to disagree, please use facts.
These projects and the directions look great, but there is something missing... None of these people have posted pictures of or described the quality of the output. As in, if you built one of these projectors, would you regret having laid out the $400, or would you be psyched and loving the high quality video image on your wall? I mean, I would be willing to make the investment for my own apartment, and would even bring one of these into my classroom (I am a teacher) to use with my classroom computer. But will the result be worth it?
Derek
------- "One of the joys of travel is visiting new towns and meeting new people." -- G. KHAN
I was going to ask why the article has all that equipment, but I guess it might be required if you're doing hi-res computer stuff on the screen (but then what's the point of hi-res? Why not just scale it down?). But if you just want to watch TV, all you would need is the overhead, panel (provided it has s-vid or RCA in) and a VCR or DVD player. Most of them nowadays have s-vid or RCA out (and s-vid to RCA is just a dumb converter away).
funny munging
This is giving me some rather good ideas..
:) Now if someone would only approve that.. :)
I've been wanting to get an LCD projector for my living room, after playing with one from a club a year or so ago (the unit was already 3 years old). In average lighting in a friends house, we projected a TV image that was beautiful, from about 10 feet away, which made an image approx 5 feet tall.
I don't want to spend the $3k+ to buy one from the store, but I'm more than willing to spend less than $100 on a small handheld LCD TV and junk store movie/slide projector.
I'll post to Slashdot if it works. "Hacker Makes LCD TV/VGA projector for $100", with results.
Serious? Seriousness is well above my pay grade.
- Buy a typical overhead slide projector (the one your college has hundreds of)
- Get a flat panel LCD @ 14-15"
- remove the backlight from your flat panel
- slap the panel stripped of the backlight on top of your overhead projector
- Heh. There is no step 5!
Am I missing something or is this a better approach to the problem?Your pizza just the way you ought to have it.
Okay, a little better than the Big Screen with a CRT and Frenzel, but if you want good TV this is not it. If you want a good cheap LCD just go on ebay and buy a used 800x600.
Neat DIY, but really, it pushes forward all the things that are wrong with consumer AV. Low contrast rations, no idea about proper screen materials, poor color, bad scaling...you could go on and on about it.
Please note that the monitor suggested for the experiment (at this link), claims to have a resolution of "960(H) X RGB x 240 (V)=230400" which can be a bit misleading if you're not too tech-savy on these numbers.
Note that the horizontal resolution is NOT 960 pixels, but rather 320, since they're counting each RGB pixel as THREE pixels (very sneaky indeed).
So just be warned in case you thought this was the deal of the century.
WOOOOOHOOOO! Finaly! Now a cheap bastard like me can afford me some BIG SCREEN pr0n! Pass the quart of immitation butter flavoring please... /=^)
Dr MPF
The seams would irritate me.
Random and weird software I've written.
If you can get decent quality 1-2" LCD screens, why not just rig them up on a pair of glasses, run the hardware off either a cable or a short range video transmitter and watch a seemingly immense screen with no noise, no bulbs...granted this is only good for one person... Shrug
We've secretely replaced the Enterprise's dilithium crystals with Folgers crystals. Lets see if they notice.
The resolution would suck ass. For LEDs you need a very large room and a lot of distance between you and the wall. LCds would be ok, but laying out the floor plan of the room would be made difficult with mounting the projectors in an advantagous position. "To be or not to be? Not to be, definately" -~* The lazy pessimist
...a tree falls in the forest and no one hears it does anybody care? Whats the difference if they did save 12,700 (a ridiculous price)per a projector? It wouldn't have kept them around any longer, 12,700 USD per a projector is such an insignificant percentage of the total buget expenses it is humorous to think that any person capable of operating a keyboard even considered that it could have any measurable impact on their overall survival.
Yep heat is the problem. Most of the pannels have special back glass to protect from heat and have colling fans.
COP: Tell me son, why exactly were you purchasing a metal halide lamp?
GEEK: Well sir, I am a hobbyist, see
COP: interrupts A hobbyist, huh?
GEEK: yes, I am making a homegrown
COP: interrupts homegrown, huh? Son, you have the right to remain silent...
GEEK: LCD PROJECTOR! SIR!!! A HOMEGROWN LCD PROJECTOR!!! YOU KNOW, one of those things that projects computer images on the wall.
COP: Images on the walls. Hmmm...you must be pretty good at your "hobby"
cat
Anyone know why I couldn't rip apart a handheld LCD TV, tear off the backlighting and use that?
Sure, you can use a fairly normal lamp and a fairly normal LCD panel to produce large images on a wall. Heck, you can do it even cheaper with a bunch of colored markers, some saran wrap, and a light bulb.
If you absolutely do not care about video quality, it's easy to get/build a projector on the cheap.
However, today's typical $3500 projector includes:
- A truly full-spectrum lamp. Retail price: $425. Wholesale price: $300. Manufacturing price: $250.
- Built-in line doubler. Most LCDs have 768 vertical pixels; some have 600, some have 1024. DVD's have 525 vertical lines. VHS has something like 240 lines. How do you get from 525 or 240 to 768 or 1024? Anyone who knows anything about computer grahpics will realize that the answer is not "double every 1/X line". Line doublers interpolate lines on the fly.
- Progressive scan support. Again, not a huge big deal, but the way I read the article, not supported.
- Component video in support. Like progressive scan, increasingly critical for decent video.
- Distortion correction, especially trapezoid. It's very rare to be able to project from the geometric center of the screen. Most of us have to live with projecting from the ceiling or floor, and use optical or digital means to correct the image for that.
Sure, you can build a "projector" for $400. Heck, you can probably build one for $100 (see earlier lightbulb, saran wrap, and markers note). But if it were really that easy to produce a home theater quality projector for $1000, don't you think any of the mass producers would have done it? For less than the cost of a one-off? (Anyone who responds that all 8 major projector manufacturers are colluding in price fixing should be laughed at).
Cheers
-b
LCD projectors have one panel for each primary colour. They get mixed together so that white pixels look white. If you use only a single LCD panel, white pixels might look like separate red, green and blue stripes.
What is the model of the Active matrix Nview projector panel you use? I can't find one capable of 1024x786 native resolution for the price you quote. Where did you get it?
OK, so these guys really expect me to believe that these cheesy $350+ hack jobs are going to have a better picture than a DLP projector, or a Barco? Please, don't waste my time. I can go on to E-bay and buy a Sony 1272 projector for about 700 bucks, which has RGB inputs and can handle at least the refresh rate of a 1024x768 SVGA resolution output if not more (I never tested a higher resolution on one). I used these things faithfully for 6 years on Corporate presentations when the power of a Barco or a DLP wasnt necessary. This ridiculous hack job reminds me of those $10 lenses you buy for your TV that is supposed to turn them into a projection set. Oh well, wasnt it PT Barnum who said "A sucker is born every minute" ?
Here'e the lowdown on the diffrence in a home projector and a better commercial projector, just to show the comparison as really apples and oranges and not apples and apples.
Single LCD projector will never be as efficent as a 3 LCD projector because....
Red light must pass thru a red pixel in a single color LCD. This means all the white light that hits green and blue pixels is NOT adding to the brightness of the red. This absorbtion of the 2 colors not passed by a pixel filter means 2/3 of the light is lost in the filter and turned into heat at the LCD where it is not needed. This alone limits bulb size and projected lumens. In a 3 LCD projector, the light is split into primary colors with beamsplitting dichoric mirrors. Therefore all the red of the white light does hit the red LCD (actualy a B&W LCD without a color filter). The LCD then only changes the polorization of the light. The polorizers take the heat, not the LCD. The polorizers are spaced away from the LCD allowing cooling the polorizers while not heating the LCD unlike a color filtered single panel LCD. The same holds true for green and blue. The 3 beams are then recombined into one beam and exits the lens to the screen. This overlaying of the colors gives true full color pixels, not a color stripe matrix display of adjacent red green blue pixels. The heat not removed by the cold mirror at the lamp is now spread out over 6 polorizers, (one each in front and behind each LCD) not in the one LCD panel. This allows a brighter light source to be used.
Now the simple math..
Light not absorbed by pixle filters, but routed to proper LCD = 3 X brighness. Point source arc lamp with cold mirror = 4 X more visable light per watt. 6 polorizers instead of one pannel to lose the heat = 6 X brighter bulb can be used. Polorizers seprate from LCD keeping heat away = 4 X more watts in heat can be safely absorbed without overheating the LCD's. 1/2 light absorbed by polorizers 1/2 (OK it does lose light)
The totals
Dichoric splitters 3X
6 polorizers 6X, 4X, 1/2X
Arc lamp 4X more usable light
Total 144 times brighter projected lumens.
Any incandecent light source single LCD projector will not come anywhere close to the 3 LCD arc lamp commercial projector in projected lumens for these reasons. A commercial one can be used on a trade show floor, where a home built will never overcome the ambient light.
The truth shall set you free!
I bought a used projector on ebay - no hack value but plenty of instant gratification.
Here's another one...
Like New Epson Projector, Remote, Low Reserve
I'll try to list them.
You need a metal halide light bulb and ballast. Metal halide light bulbs are many many times more efficient at turning electricity into light, and thus produce far less heat. They also provide a 'whiter' light than a standard bulb. These are about $200 from hydroponics shops, and range in power from about 200-1000watts.
You need a 'cold filter' to block out the UV rays the light produces. Without this, the ultraviolet rays will actually start killing pixels. This happened to my projector. These little peices of glass alone cost around $200. But without one, you will cook an LCD very quickly, especially with higher powered lights.
By the time you get a decent lense, so you can actually get a decently sized and focussed image, you are starting to get into the sort of money that a decent 2nd hand projector goes for, without all the pitfalls.
MacroVision works by putting false sync and colour burst signals into the interlaced fields in a composite video signal.
This fools AGC (Automatic Gain Control) circuits into thinking they have a very bright picture, and so they reduce the gain. By varying the signal you can make the picture brightness pulse, or in some cases cause it to loose track of the synchronisation all together.
Conventional display devices don't have to have such accurate control of the gain of the signal, so are not very heavily effected, although it is possible to see the effects on some devices. You could see the high amplitude bursts, but these occur in the 'off screen' section of the field that holds the sync signals, and stuff like teletext - if you have vertical hold then you might be able to see them.
(For a great technical and non-technical explanation check Repair FAQ for an easy explanation check How Stuf Works)
Now originally this was intended to specifically block VHS style recorders, but as things have developed there is another device now in common use that can be effected, that wasn't around in consumer products when MacroVision was invented - the frame store.
These are handy digital devices that read the composite video signal in and store it in real time. The video can then be read out in any format you want. Why would you want to do this?
1) Stabilise the signal
2) Change video formats from 50/60 interlaced fields.
Now the first one is done during video editing so that different sources can be synchronised and things like picture in picture and wipe effects between 2 video sources will actually work. They are also now common in good prosumer VCR's for this reason. Digital camcorders have them by default because of point 2...
The second point is that it allows you to do standards conversion in real time - such as in a capture card where you digitise the signal to a different frame rate.
And here is the point - digital projectors such as LCD and DLP tend to use progressive scan rather than interlaced signals, so they contain conversion technology including frame stores to do the de-interlacing (good notes at SourceForge)
So any device that uses a frame store approach can be effected by MacroVision, it just depends on how good the AGC in the framestore is.
How do you avoid this? Simple really don't use a video signal that can have MacroVision on it. If you have RGB (component) then this won't have protection, and is the superior connection anyway for a projector. The S-Video source is normally ok as it seperates the chrominance and luminancne (colour and brightness) signals - although I've heard of a new 'level 2' MacroVision that can disrupt this - sorry no tech details on that I'm still looking, but I think it has to do with messing about with the chrominance.
Of course the fact you regenerate the signal from the framestore means a good one is able to strip the MacroVision out, but there are cheaper ways to do that, and no I'm not giving the links - spend 2 minutes on Google, and remember that MacroVision is specific to PAL/NTSC so don't go ordering abroad! A good legal reason to have such a device is to connect a non AV socket TV to a non RF output player via a normal VCR, or to connect a projector sensitive to MacroVision when you don't have RGB Component output. Of course in the US you will fall foul of the DMCA, but we already know what a mess that is!
I think the graphics on the website rocked!d plan_1.jpg) . Who needs web designers? Cool or what? Especially liked the crumpled pages :-)
(e.g. http://www.audiovisualizers.com/madlab/lcdproj/lc
(Great fun, I really mean this, true mad scientist web pages!)
Digital Micromirrors. Thats what I'm talkin' about!
This is where the high end projectors are going to. The black levels from LCD projectors are horrid. With micromirrors, you can get true black. Truer black than even with film!
So I wonder if its possible to homebrew yourself one of these? I guess it comes down to being able to buy the micromirrors and associated electronics out on the market. Anyone have any knowledge in this area?
mje0w!!!1!
A lot of people have commented on how these thing actually look, whether they actually work, and why anyone would want to do this when they could buy one used or new...
1. What do they look like? Depends mainly on the LCD and lens system, not to mention the "case" of the projector. If built right, with good components and a "light tight" case, it can look good. Not great, not HDTV ready, but good enough to watch TV or a video with.
2. Do they work? YES! Built right, they work as they should. LCD projection isn't anything really fancy - it is basically a slide projector with the LCD display substituted for the slide, and the backlight being a very bright lamp. You have to cool the LCD in some way (or polarise the light properly) to keep the LCD from "shutting down".
3. Why do this? I would say it is mainly a hobby, but I would also say it is because most of us can't afford a decent new or used projector. New projectors are hella expensive, and used ones maintain their value, and are thus not that cheap either. The only ones cheap enough are the large CRT projectors, which tend to be real heavy, need to be aligned after moving them, and need special support structures to hang the heavy weight from the ceiling (if that is how you want to mount yours).
I can't say I have ever built one of these projectors, but I can say how they probably look. I currently own a Fujix P401 portable LCD video projector. The thing is about the size of a couple of VHS tapes stacked upon one another, and it uses a small one inch LCD with a halogen lamp (it is a 6 volt halogen reflector lamp that is VERY difficult to find). It takes composite input, and has a system to either project on a small internal screen, or out to an external screen. Built in stereo speakers and the ability to run off of an 8mm video camera battery completes the system.
The quality is OK. At larger image sizes the pixels start to become apparent, but all in all it really isn't that bad of a projector. If I keep the image to around 40-50" diagonal, it is highly watchable (you need to be in a darkened room), great for videos. I have used a VGA->TV convertor successfully with it, and viewed VCDs under KDE.
I would expect a home-brew solution to be as good or better. I would imagine the larger LCDs to be higher resolution, and should give a sharper image at the larger sizes, with less "pixelization" (which really isn't a big deal on my P401).
I honestly don't understand why LCD projector manufacturers don't (or won't) make cheaper, lower-res units. I would think a 640x480 unit would only cost $500-700 - a lot of people would eagerly snatch it up. It could be made compact and lightweight. Make it easy to attach to a computer video card or composite/SVHS inputs. I think it could sell. I have a similar gripe regarding laptops (ie, why not a 640x480 cheap laptop), but now is not the place.
Reason is the Path to God - Anon