How-To: Build your own HD projector (Part 3)

So, in case you haven't noticed, we're building our own LCD projector! In part one we specced out what we needed and got together some optics. In part two we really tore into our 19-inch Samsung monitor. Today we take a dive into the most difficult part of the projector: designing it to be built. It's easy to talk about something when you're just putting it down on paper. But now it's time to put it together and build it. It's crunch time. Jump into today's how-to; we'll show you how we're going to pull it off and the details that have to be considered.

The projector enclosure is the most important component, and it can be the most difficult to make. Optical alignment, light control, and cooling are all required for success. Before firming up the design in your head, it may help to look at some other enclosures. We can use some of those ideas, but because we're using such a large LCD, the stock designs won't quite work for us.

We want to have two solid, supporting sides. We'll use the same material to provide structural bracing, and finish it off with a thinner covering to keep the light inside and the weight down. We decided to use 3/4-inch birch plywood for the sides and the cross bracing. It's reasonably light, strong, straight and we can put a nice finish on it. For framing up the LCD, we picked up few four foot pieces of straightest 1x2 select pine we could find.

At our un-named hardware store, we had one of the "red shirts" cut up a four foot by eight foot piece of plywood for us. He cut it too fast, so we got some rough edges, but we'll clean them up later. (A four by four piece was $10 less, so we bought the bigger piece.) Most hardware stores will do these cuts for free or a minimal charge. They'll be straight, just don't depend on the measurements being very accurate. (Careful of those red shirts! Just after this, he was sucked up by the shop vac just like the kid in Willy Wonka)

Getting wood cut can become a chicken and egg problem. We had a rough idea of what we needed, so we had two 14 inch wide rips cut. The rest was cut into thirds for easy transport.

Oh so carefully, we measured the thickness of our panel. It's under an eight of an inch thick. Be careful not touch the LCD with the calipers!

Next we need to create a frame to hold our LCD panel. This is the first real task for building the enclosure. We'll determine the final dimensions of our enclosure from our framed panel size. The LCD is 10.5 by 16.5, so we cut two 12-inch and one 18-inch piece of 1x2-inch pine. Later, we'll trim them to the final dimensions.

The LCD is thin, so a single cut with a table saw blade is wide enough to accommodate the edge of the LCD. We set the depth to 1/4-inch and the fence is 5/8 of an inch from the blade. We want the lamp side Fresnel lens to be about 16mm (5/8-inch) from the LCD, so the frame will act as a simple lens spacer.

To keep from screwing up, we made a test cut on a scrap.

A quick test fit to the width of the LCD and we know we're golden.

Notice those little cables on the right side and the cables up top? We have to accommodate them in the frame. We'll come back to the stuff up top later.

We cranked up the blade height of our table saw and cut one side deeper to accommodate them. We eyeballed it for the cut, but it's about 1/2-inch deep.

Once we had everything tweaked, we measured our final framed LCD width. We'll be building our enclosure to be 17.5-inches wide inside. Knowing that, we got out the table saw again and ripped down one of the spare pieces of plywood to 17.5-inches wide. By cutting all the cross bracing to the same width, life should be easier later on.

To get a feel for the height and width of the box, we set up the sides with a 17 and 1/2 inch spacer in the middle. The finished product will be much smaller, but it helps to get a feel for what's coming.

Digging into the design, we have to consider and commit to our design decisions. We laid out our design on directly on the plywood. Photographing pencil marks on plywood isn't the easiest to read, so we'll walk through it step by step. (Use a pencil, it's not committed until you start cutting!)

The LCD is the heart of the projector, so we have to determine it's placement and locate everything else around it. To place the LCD, we determined the maximum distance we would ever want the projection lens at, added a bit for good measure, and came up with 25 inches. The is a bit long, but it'll give us room to maneuver.

We pulled out Focal Calc II from the Lumenlab forums to come up with our numbers. The LCD and screen width can be considered either diagonally, horizontally or vertically. Our screen is 104-inches wide, so we used that along with a 16-inch LCD width. The Pro projection lens has a 500mm focal point, the lens side Fresnel has a 650mm focal point and the lamp side has a 220mm focal point. Plunking that into Focal Calc gave a lens distance around 24-inches, and a 72-inch screen gave around 22-inches. We'll overbuild and provide for up to a 25-inch distance with the enclosure. (We'll get a bit more travel once we build the carriage for the lens.)

As we lay everything out, it's helpful to revisit the components. We measured our bulb and base. We care about the center of the bulb - it comes out around 7-inches from the bottom of the mogul base. (Note the small stub on the inner glass envelope.) We'll have to offset the ceramic base with a wooden base.

The drawing isn't quite to scale - the end result won't be 48- long, so relax. Now we add in our lens placements -- the Fresnel's on the lamp side will use the frame of the LCD as a spacer. It will be 5/8 (about 16mm) away from the LCD. The lamp will be 220mm from the Fresnel lens.

Alignment is key to the optics. Before determining where the center of the bulb and Fresnel lenses should be, we need to determine the location of the LCD's center. Obviously, the center of the LCD is easy. Remember the big cable on the top of the LCD? Now we have to consider where to route them and how much we'll have to offset the LCD.

Inside the projector, the image from the LCD will be reversed and flipped upside down. So the display needs to be mounted upside down and facing the lamp. The cables can exit forward or backward, so we've decided to place the interface board on the bottom of the projector, and flip the cables toward it for an easy connection once the display is inside the enclosure. Inside the case, we'll place blocks of wood to set the height of the LCD, while the LCD will rest in the channels of the frame we built.

Bringing that idea back to our layout, we need to note the upper and lower limits of the LCD, as well as the position of the interface board. Initially we provisioned for over an inch, but it'll probably end up between a quarter and a half inch.

Now's a good time to measure the ballast. It's 10-inches long, 4-inches wide and 3-inches wide. We're still toying with mounting locations, so we're going to get some of the enclosure built before making the decision.

Now that we've nailed down the basics, we'll start building up our enclosure. (We'd have a nice preview shot for you, but our intrepid how-to author forgot to buy wood clamps.) Next time we'll dig into the construction details and get up close and personal with each component as we mount, clamp, glue and wire our way to HD viewing pleasure.