Jakes Garage

DIY Projector

This section of my site is an archive of my previous site about building a projector.

 

I keep getting email asking for more information so I made this little write up about how to make a projector, this should be able to answer most of your questions.
LCDs:

Well first off most all new projectors use lcds some have one and some have three and their easy to work with because you can find them everywhere with different options like s-video, composite, or vga input so you can connect it to a pc, vcr, satellite, DVD player, or game console. One thing you need to look for in these panels is the response time usually measured in milliseconds or “ms” a decent lcd has around 30-40 ms response if it gets much higher than that when you watch a movie that has a lot of action it can be jerky. Another thing is the resolution bigger is always better I suggest 1024×768 but I would not go below 640X480 there are some panels out there that are 930×234 and those would not look good at all imagine you have a image 8 foot wide by 6 foot tall the pixels that make the image would be 1/3 of an inch tall which makes for an effect called screen door its like looking out a screen door you can see a lot of little black lines everywhere. There is also the contrast ratio this determines how dark black is when projected on the screen in store bought projectors this can be up to 3000:1 which is very high it makes it so that if the lcd was totally black no light will hit the screen most normal lcds are around 150:1 to 500:1 here again higher is better I suggest to go no lower than 300:1 or your blacks start to look like dark blue. The final thing with lcds is a trial and error step, for a projector to work it needs a very bright light to shine through the lcd in order to do this you have to take the lcd apart and take the old backlight and cover off the back of the lcd so you can get a new light to shine right through it. Not all lcds will work some of them you can’t take it apart at all or it’s not easy and if you’re not careful you could tear one of the ribbon cables and those are almost impossible to replace or repair. The most simple way and most popular is to use what is called a projection panel I’m sure you have seen these being used in school, you just set it on an overhead projector and presto you got a big TV. These can be found all over eBay and some inventory surplus sites but do research on it before you buy it and make sure the specs are within an acceptable range. If you got a big wallet you can actually buy real .7″ projection lcds with custom driver boards for around $3000 these work GREAT but as you see quality comes with a big price tag.
Light Source:

It takes a lot of light to make a bright projected image, I’m guessing about 60-75% of the light is actually blocked by the polarizer’s in the lcd about 50,000 lumens going through the screen will make a very bright picture. The tricky part is if you get a 400W metal halide bulb that puts out 50,000 lumens and just set the bulb behind the screen the actual light going through the screen is about 5-10% of the rated lumens because the light is going in every direction. You need a way to make all the light go through the lcd, this is done with a reflector this again is a trial and error step not all reflectors are going to work you just have to try and see what happens. The best type of light to use is a Metal Halide these come in all kinds of shapes and sizes you can go to the “GE” or “Sylvania” website and see what kind would work for you.

Lenses:

Sitting behind the lcd is a lens called a fresnel lens this is used to converge the light onto the projection lens its important to get one that has the same focal length as your projection lens so it focuses most of the light onto the projection lens. If its not the right focal length it could only let half of the light through the projection lens and will make your image half as dark as it should be.
The final and a very important part is a projection lens the best ones to use are triplet lenses it is 3 lenses made in one casing and makes it easy to focus the image, what I mean by this is if you use a regular convex lens when you try to focus the center of the image it will be in focus but the outside edges will be blurry because they are a little bit further from the lens. A triplet lens fixes this and makes the outside edges come into focus when the center is in focus. This next step is another trial and error step there are some calculators you can use for this but its hard to get an accurate measurement, the magnification of the lens or also called focal length determines how big your projected image is say your projector is 10 feet away from the screen and you have a 200mm lens it makes the image 8 feet wide and if you use a 400mm lens at the same distance it makes the image 6 feet wide (these are not accurate measurements) you just have to find the right size for your needs. Another factor for this is the lcd size if your lcd is 10″ and you have a 300 mm lens and its 10 feet away from the screen you image is 8 feet wide but if your lcd is 1″ and you got the same 300mm lens and same distance your projected image could be about 3 feet wide. There are so many variables in this that its very difficult to tell what size you need an easy way is to go buy a set of like 10 convex lenses that range from 200mm – 600mm and try them out and see about what size you need you can get these very cheep at places like www.surplusshed.com.

I hope this has got the ball rolling for you I am no expert in this field so I really can’t give a good answer to most of the tough questions I have received but this should give you the basic idea behind this. It does take a lot of trial and error I have been trying all kinds of things I can think of to make my projector better and I have been searching for a cheap small lcd source and a small light, when I find reliable sources you can get them here and Im hoping to make a kit that has about every thing you need to build your own projector.

theory1

This is a basic setup like in my first projector you can use mirrors to direct the light to make a more compact design.


theory2

This way you can save room and if you can find small parts the unit can be made very small.


Here are some links that will show you how the optics work.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/refln/u13l1a.html The basics of optics.

http://www.education.eth.net/acads/physics/light-I.htm More on optics.

http://ourworld.compuserve.com/homepages/awi_industries/Optic_reflectors.htm How reflectors work.

http://www.tecdis.com/product.htm How LCDs work.

http://micro.magnet.fsu.edu/primer/photomicrography/colortemperature.html Color tempature of light.

http://www.nasatech.com/Briefs/June01/NPO20824.html Recycling light.

 

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