This document describes phase I of our home theatre projection system: building the prototype. In phase two, we'll be creating a permanent enclosure for the whole thing but that's for another day.

Ever since we decided that an item as mind-numbing as a television set wasn't something we actually wanted to have taking up space in our home, we've been stuck watching DVDs and the occasional CSI (via an ATI hauppauge base TV tuner card) through the computer. Hardly comfortable. So we started considering various projectors, which could provide a badass wall-sized screen without taking up so much space or costing a fortune.

There are a host of LCD projectors available, that offer a lot of features in a nice stand-alone package. You can expect to pay more than $1000 for these, which I feel is acceptable for converting your living room into a projection theater. A little research, however, uncovered a disconcerting fact: the bulb replacements are really expensive! Do a quick search, or have a look at any random replacement bulb at LCD Projector Bulbs: replacements lamps will cost anywhere between $300 and $600 (USD). Unacceptable! It is insane, and must be nerve racking, to never know when you'll next have to dish out $400 just to watch a movie.

A little more searching uncovered a number of DIY projects for those with a more hands-on bent. These are split into two main camps, the fresnel lens and the transparency projector techniques.

Fresnel

A Fresnel lens is, most often, sold as a sheet of plastic with a series of circular grooves and cheap versions are sold as wallet magnifying glasses. Each groove functions as a prism which refracts light at an specific angle. If you can get your hands on an appropriate lens and, depending on the type of projector you are going to build, a front surface mirror (this is a mirror that has its reflective surface on top of the glass substrate, rather than behind it like typical mirrors) then you can build a simple fresnel projector or even your own rear screen projection TV. From our, admittedly incomplete, experiments the fresnel approach didn't provide the clarity or brightness we were looking for so we instead went with the transparency overhead projector.

Overhead Projector

This technique basically involves putting two components together: the classic overhead projector (the kind used to show transparencies in conferences or school, if you're old enough) and an LCD screen. When looking for an overhead projector (OHP), you'll want to find the right compromise between:

• Projector cost (don't skimp here, it is a one time investment)
• Projector quality, specifically the projector lens
• Brightness (measured in the lumens output of the projector lamp, which is a unit of illumination on a surface)
• Lamp replacement cost and average bulb lifetime

Look for an OHP with at least a 3000 lumen output, though if you can get your hands on something greater than 6000 you should have something bright even in a partially illuminated room (note that the projector output isn't the same as that for the bulbs it uses). The projector lens may be described singlet, doublet or triplet, which indicates the number of lens elements in a single assembly (here, more is better). Also, since you will need to give the LCD a little breathing room to avoid overheating, try to find a projector with a varifocal lens (which is a lens for which you can adjust the focal length by an inch or so). Overhead projectors include a fresnel lens, just beneath the stage aperture (the glass on which transparencies are placed) in order to uniformly spread the light from the bulb below--high quality versions of these lenses are expensive: if you want a nice uniform and complete illumination, this is another reason to avoid being cheap on the projector.

We settled on a 3M 9550, which is an nice looking, easily storable projector (the head folds down for transportation), has a 4100 lumen output and has a built in system where you can insert two bulbs and switch between them on the fly, should a lamp expire in the middle of your Babylon 5 episode. The replacement bulbs, ANSI code EVD 400 Watt/36 Volt halogens rated at 50 hours (but this is variable and we've already surpassed that), will run between $5 and $20 depending on where you shop--a welcome relief compared to LCD projector bulbs!

LCD Screen

Selecting an overhead projector is little more than smart shopping and is only a prelude to the real "do it yourself" part of the project: the LCD screen.

The easiest and cleanest method would be to use an LCD panel specifically designed for use with overhead projectors, such as the Proxima Ovation, Infocus or other LCD panels. These are simply nicely packaged LCD screens without a backlight, that rest atop an OHP and usually provide their own cooling. The problem with these units is that with the advent of stand-alone LCD projectors, they have fallen out of favor and are usually older models. This means that most have low resolution (between 240x120 and 800x600, most at 640x480), low color depth and bad refresh rates--all characteristics that make them OK for the powerpoint mongers but terrible for home theaters. If you can find a good model and are willing to spend the dollars to avoid the trouble, then check out ebay, froogle or similar sites. Real hackers will crack open an LCD monitor ;-)

An LCD monitor is just a transparent liquid crystal screen with a powerful backlight, so the trick is to remove that LCD screen from its enclosure and convert it to an LCD panel for use on your projector.

Extracting the fragile LCD screen is delicate work. If you aren't gentle, you could crack the screen and render it useless. Even more likely you could detach some of the connectors, many of which a simply soldered on like any other surface mount component--and don't expect to solder these back on manually or try to stick the whole contraption in your home made toaster-oven reflow oven: if this happens you'll need to try, try again with another monitor (this has happened to me, ahhg!).

The hardest part in the whole affair is selecting an appropriate monitor. You want a smallish screen, 15 inch or 17 at the most, with support for at least 1024x768 and a good refresh rate (anything under 30ms is good, but less is better). The main problem is that there's no way to guess what the guts of your device will be like. Though all LCDs share most of their features and layout, certain monitors are designed such that internal components are stacked and the connectors used (often little more than wire traces on a piece of plastic) force components to overlap. If this is the case, you'll either need to find or create a new connector (unlikely) or lose a portion of your visible screen.

Disassembling the LCD monitor

Our first attempt was with a Samsung SyncMaster 570VTFT 15" monitor. Warning for speed readers: this is not a good choice. Though the screen was clear and perfect for playing movies, the internals were not. As described above, one of the connectors was positioned such that it formed a right angle which overlapped the backside of the screen, extending to about 7 cm from the bottom and right of the screen. Though it is possible to simply mask the entire right side, it would make fullscreen projections impossible and mean the loss of about 20% of our surface area. Sad.

In any case, it was the screen we'd purchased so we moved forward and I got a prototype working. However, repeated manipulation of the bare circuits soon caused one of the many soldered connectors to detach. This was the end game move, as it caused a vertical section in the center of the screen about 3 centimeters across to go completely dead. The LCD was scrapped, in light of our limited SMT abilities.

With a little luck, I was able to locate a pair of Hitachi CML 151 XW2 monitors. Neither was functional, but at $15 a pop, it was worth a try. For starters, there was no power supply provided. As expected, however, the 12V/3.5 A DC converter from the dead Samsung had the right specs (if not the right connector) and a quick snip & splice into the Hitachi's power input connector did the trick.

One of the monitors had a power supply problem that I've yet to resolve. The other would power up, but do nothing else but flash the yellow LED. Blink, blink, blink, "there's a problem"... hmmm. It turns out a little frankensteining by combining components from the two was enough to get it working. Opening it up, as pictured above, showed the usual Faraday cage with its "you have no business here" air. A few screws later, the guts of the system were revealed as pictured below as seen from the back of the monitor.

LCD Monitor Components

This Hitachi is a very clean unit, with the components nicely joined together by easy to use connectors (no the horrid soldered on thingies, but actual multiple use connectors that humans can detach). In the photo, the colored rectangles were added to simplify discussion.

VGA to digital and LCD Control. On the upper right, in the blue square, is the most important piece of circuitry. This component accepts VGA input (15 pin connector at the bottom) and takes care of converting it to a signal the LCD can interpret. It is connected to the LCD panel via the two flat connectors on the right, which we'll come back to later. From top to bottom, on its left side, are connectors to:

• The power supply input;
• The inverter;
• The user control panels.

Power supply input. In red, almost centered in the image, is the power supply input. The function of this little board is simply to hold the power jack. Though it has four wires, they are connected in pairs to the connect to the jacks ground and supply pins. Owing to our incompatible power supply, we just got rid of it and spliced the wires directly into the adapter's plus and ground.

Inverter.
The inverter, on the left in yellow, converts the DC power provided by the power supply to the high voltage AC used by the backlight. The backlight consists of a pair of fluorescent tubes and a diffuser, which is just a semi-transparent piece of rigid plastic used to do just that: diffuse the fluorescents' light and let it spill through whichever crystals happen to be rotated in the LCD at any given moment. For our purposes, this component and the fluorescents are useless so you can chuck or sell them. Be careful though: there is a reason LCD power supplies are bulky and provide such high amperage--the power developed by these units can be downright dangerous, so disconnect them before applying power and be wary of where you stick those fat fingers.

User control panel. These are the buttons, green at bottom, used to control the monitor and access menus and such. Our unit works without these even being attached and we've yet to need them, though it might be interesting to affix them to the outside of our installation when we get to that point in phase II.

Extracting the LCD Panel

The first step for getting to the LCD screen was to remove all the components. The most delicate operation here was detaching the flat, ribbon cable type, connectors from the main circuit board. These are two series of parallel metallic tracks, sandwiched between transparent plastic sheets, the edges of which expose the wires on one side. To get them dislodged from the surface mount connectors (white rectangles in the image), you need to gently unlatch the very small black plastic locks on each connector by sliding a finger nail beneath and pulling up until you feel a click. If you break or remove these little latches, it won't be fun.

Having removed all the components, it was possible to extract the screen from its shell. As you can see, from the front there are (obviously) no components blocking the screen which is enclosed, to the left and bottom, by chips which control the actual rows and columns of pixels. The diffusion screen, immediately behind the black LCD panel, was sandwiched between the LCD and another PCB connected one one side to the VGA circuit board and to those pixel control chips on the other. Luckily, the two connectors on this board were flexible enough that it was possible to twist the smaller board to a 90 degree angle, thereby clearing the field of view for the LCD. Pictured below are the VGA PCB on the left, the LCD controller board standing vertically above the lower left side of the LCD panel.

Having successfully removed the LCD from the monitor, it was time to make sure everything was still functional. Moving the entire assembly over to the living room and onto the OHP, it was a joy to see Neo flipping us the bird. In real life the LCD should not be laid down directly atop the projector as the 400 Watts give off a good deal of heat, regardless of the OHP's fan. Note: when placing the LCD, ensure that you see it as if you were looking at a regular monitor, with the light from the project stage shining through the rear when facing the projector directly (no need to flip it over or put it face down, the projector lens expects it this way).

By recycling the monitors plastic front panel, and drilling a few extra holes on each side, it was possible to create a stand about 1.5 cm thick on which to deposit the panel. Affixed to the projector with a dash of tape, this support has to date provided enough airflow to avoid requiring the use of a fan (though we will probably use the low noise Ultra 80cm fan I got for this purpose, during phase II) while minimizing the amount of light escaping the stage.

Projector Placement

At this point, we just stuck the entire assembly into a big cardboard box with a few holes facing the projector's fans and to let cables through. The projected screen size itself is proportional to the distance of the projector from the wall. You've got a good deal of leeway here, especially with a varifocal lens, but there are still limits to how near or far you can place the projector. The formula for screen size is:

Screen size = ((distance from wall / focal length of head) - 1) * projection panel size

Final Notes

• Try to do better than us and give your projector a permanent home right away, so you don't damage any of the dangling circuitry;
• Make sure you allow for easy lamp replacement;
• When shopping for your LCD monitor, snatch up cheaper used models that have broken backlights/fluorescents or inverter (since we don't use these components anyway);
• If you see a brown spot appear on your screen, your LCD is overheating. This will eventually lead to permanent failure... Add more space between the LCD and OHP stage and/or use an 80cm fan;
• To ensure longer lamp life, use a UPS or at least a surge protector on the OHP power supply.

Enjoy your home built theater and be sure to share your experiences with us!