DIY Projector Guide - Part 2 - Assembly
From Lumenlab
Contents |
Putting it Together
Look at this illustration:
Number 1 is our point light source, the 65K T15 lamp. It is important that the lamp center (arc) be on center (line X) with the entire enclosure and at the focus of the first fresnel (3a). The fresnel lenses (3a+b) and projection lens (5) can be purchased from the Lumenlab store. The Lumenlab S15 fresnels have been specially made for a 15" projector. They are already the proper size, groove pitch and thickness. The fresnel lens on the lamp side has a shorter focal length of ~220mm, and the fresnel lens facing the projection lens has a focal length of ~317mm. This is advantageous because we can have the lamp a little closer to the lens, which means less light is ‘lost’. Our piece of isolation glass (2) makes a great mount for the collimator fresnel. Never mount the fresnel on the lamp side of the glass, it will melt! The isolation glass (2) should be a piece of 1/8” tempered glass cut to size. Lexan can also be used. These are readily available at most hardware stores for a few dollars, but tempered glass can take a week to obtain so plan ahead.
Be sure to tell the hardware store that you do NOT want your tempered glass to have a stamp or bug (safety glass label). If they stamp it the stamp might be in the way of the projection.
Size the isolation glass 1/2” shorter than your enclosure height. This is so that the glass can sit off of the bottom by 1/2” which is necessary for #Keeping it Cool (explained later).
The S15 projection lens (5) is a 320mm focal length lens triplet. It is important that the projection lens be centered to the other lenses and the enclosure (line *X*).
So we know that 1 is our lamp, 2 is our isolation glass, 3a+b are the (a) collimator and (b) collector fresnel lenses, 4 is our lcd panel and 5 is our projection lens. The collimator fresnel has a rear focal length of 220mm, so we would position the center of the lamp 220mm away, as represented by line X. The collector fresnel has a forward focal length of 330mm, but we would position our projection lens ~320mm *away from the collector* as represented by line Y. The reason for this is because we want to use the entire projection lens’ area, not just the center. The illustration shows that the projection lens is positioned slightly before the focus of the forward fresnel lens.
Our projection lens may have a focal length of 320mm, but the if the lens were only placed at its focal length from the LCD panel we would be focused on infinity, so in practical usage we would place it around ~340mm from the LCD panel as represented by line Z (note this distance will vary with different screen sizes). We leave a gap of ~15mm between the condenser fresnel lens and the LCD panel and a gap of ~20mm between the collector fresnel and the LCD panel. This is desirable because if a fresnel lens is too close to the panel, the grooves on the fresnel lens will be projected too. The larger gap for the collector fresnel is to give some room for keystone tilting if needed. If you have circular lines or Moire patterns in your projection your fresnels are too close to the panel.
If you are making a focusing mechanism for your lens, you’ll want about 50mm (~2”) of movement in the projection lens which would accommodate virtually every usable setting in which you would use your projector. This means your projection lens range would be 320mm to 370mm from the LCD panel.
Also of note is that there is some tolerance in the focal lengths of the lenses. If you have lines in your projection because the fresnel is too close, you should be able to distance it more without a problem.
See Lens Mounting Techniques, Focus Boxes
Because the optics invert the image on the LCD panel, the panel will be mounted upside down and flipped from left to right. Basically you want what used to be the front of the panel facing the light, and turned upside down.
Advanced Design Concepts
- Light Flow
- Throw to Screen Size Ratio
- Un-Split Optics
- Split Optics
- Keystone Correction
- Lens Shifting
- Folding the Light Path
- Pre-Condenser
Keeping it Cool
In the 15” projector I designed a cooling system that only needs one exhaust fan (1). The best fan we've found to date is the turbine lateral cross-flow fan. These fans move the most amount of air and are very quiet compared to other styles of fans. It is critical that the lcd panel does not exceed 105f during operation, so have your meat thermometer handy when testing. I specified that you should have your tempered glass cut 1/2” shorter than your enclosure height. This is to allow cooling air to pass under the glass at point 2. Cut a 3/8” slot most of the way across your enclosure lid to admit the air (point 3). Not much light will escape from the slot (3) but use a piece of air conditioner filter cloth on the underside of the slot to block light leakage and keep the panel from getting dusty from the incoming air. In designing the cooling system this way, the lcd panel gets top priority. A large volume of air travels through the gap between the fresnels and the panel, cooling both simultaneously. DO NOT put the fresnel lenses on the lamp side of the glass, they will melt! With this design I can keep my panel at a constant 95f during operation. I suggest putting your meat thermometer in the top slot to observe the panel temp over several hours of operation until you are CERTAIN it can maintain a constant temperature.
If you’re using a wood enclosure I would recommend that you use aluminum flashing on the interior of the lamp side of the enclosure. This is to prevent any scorching from the lamp. Keep your wiring away from the lamp. Secure all wiring to make sure it doesn’t move and end up on the lamp. Leave a space of 2” at least between the back of the lamp and enclosure. Pay careful attention during the first few hours of operation for fire hazards. If carefully designed and maintained the projector should run for many years safely.
It is also advantageous to use a DPST switch for your wiring; one pole for the lamp and panel, and another for the fan. Used in combination with a thermal switch, the fan can be made to run automatically after you shut down the projector, until the projector is cool. A power entry module is a safe way to get power to your box, and a circuit breaker provides the needed fire and safety protection. All of these items and a power cord are available at the Lumenlab Store.
Cooling Resources
- Isolation Plate: Tempered Glass and Polycarbonate
- Heat A closer look at removing heat
Building the Enclosure
Enclosure Resources
- Simple Projector Plans
- Component Layout
- Parts List
- Simple Wiring
- Timber and how to work with it
Conclusion
I hope this project is as fun and rewarding for you as it was for me. My family and friends have enjoyed our prototype projector for months without a single problem. The total costs for this prototype projector was $400 with all new components. The CMV1515 lcd monitor was $225. Of course you could spend much less or more depending on your selection of components. Some have built decent projectors for as little as $100. This projector can be completed in about 2 days time if you have all of your components and materials at hand. Even at $400 this projector is a great bargain. A comparable XGA projector could cost thousands, and the bulb for it alone could cost near $400. I won’t have to change my $15 bulb for many years! You also get a big ‘wow’ factor out of this projector. Your friends and family will be impressed to say the least. Of course the most fun is sitting back with some friends and a big bowl of popcorn and enjoying your own movie theater.
Please be safe and use your wits when engaging in a project such as this. Always put your safety first!
