Workbench Creations

Editor's Note: This is the second in a two-part series. Part 1 ran yesterday.

Now let's talk about the OHP bulb. When I looked online, I learned that the one I had didn't produce the correct color temperature. A metal-halide bulb would give me the high-quality results that I wanted. One of the things I don't like about the video projectors that you can buy is that good bulbs are very expensive and hard to find. So, I decided to design my LCD projector around a more standard bulb that you can get in any hardware store. Because I already had a transformer for a 400-W mercury-vapor light, I bought a 400-W metal-halide bulb. At first, I considered re-designing the OHP to work with this bulb, but that didn't turn out to be feasible.

DIYers understand the role of trial and error. As I soon learned, the light bulb that I wanted to use wouldn't fit the OHP unless I made some major modifications to my design. The projector's inability to adjust the size of an image also posed a serious problem. OHP / projector optics are designed to focus when the projector is close to the screen, but I wanted to be able to adjust the image size at different distances while keeping the image in focus. Through experimentation, I discovered that removing one of the two lenses in my OHP made the image a lot smaller; however, the change in focus point caused a loss of light from the Fresnel light concentrator. To solve the light loss problem, I had to position the LCD and Fresnel at the proper distance from the bulb. Because I wanted the image size to be adjustable, this spacing also had to be adjustable.

In the end, I designed my optics box for maximum flexibility so that I could adjust the distance of the LCD screen to the optics. The LCD and Fresnel lens are mounted in a frame that can slide up and down inside the box. The OHP lens is mounted above the box with a cover that keeps light from escaping to the surrounding area. The bulb is mounted at the back of the box with a fan that blows air over it. I have tried numerous reflectors behind the bulb, but still haven't found one that is good enough. The bulb I now use helps to produce an image that is bright enough to see in a dark room; however, it's hard to see when the lights are on. I know that other projectors with the same-wattage bulb are much brighter, so the correct reflector could make all the difference.

Believe it or not, I haven't used my LCD projector in a while. Soon after I finished this project, I was given a bunch of older, ready-made projectors that provide a fantastic image. Today, I have plenty of other projectors that do what I want. One day, maybe I'll find the right reflector so that I can use my DIY LCD projector again.