What about a 100” Big Screen TV at minimal expense?
Build a Projection TV!

This article explains the operation behind DIY projection TV. It should give you enough insight into the subject to enable you to design and build a projection TV. 

In the process, we also highlight the limitations behind a DIY projection TV design - technical limitations that arise by way of the design principles employed.  

Even more important, we hope that the information contained in this article will enable you to determine if the 'DIY projection TV route' do represents a possible big screen TV solution to your needs.

Once you build a projection TV, you will soon realize that it is not all true what  some sites say about their DIY TV projection plans ...build a projection TV and save thousands of dollars on a video projector!

It is rather unfortunate that many hardly mention these limitations associated with DIY projection TV designs. DIY projection TV units are capable of producing big screen projections, but these are not a direct replacement to a video projector costing a thousand dollars more.

This does not mean that you cannot build a projection TV capable of delivering big screen images worth watching.  Some LCD-based DIY projection TV designs can produce remarkable results at a fraction of the cost of a commercial unit. 

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DIY projection TV: Operational Principles, System Limitations & Design Improvements

In  principle, it is possible to build a projection TV through two basic design approaches:

	The Cathode Ray Tube TV - Fresnel lens setup If you want to build a projection TV, this is the simplest way to go - it is also the cheapest, but no big screen miracles here!
	The LCD-Panel DIY projection TV Do you want to build a projection TV that is closer to the real video projector? The LCD panel DIY video projector is the answer. Some elaborated designs can produce remarkable results at typically a third of the cost of a commercial video projector unit.

 

The CRT TV - Fresnel lens type DIY projector:

In its basic and most simple form, you are just placing a magnifying glass in front of your TV to project the image on to a wall. In the process, you have to direct all light emitted by your TV on to the screen through your lens - otherwise the whole process fails.

The best way to build a projection TV based on this principle is to construct two 'open-ended' cardboard or thin plywood boxes - one designed slightly smaller than the other such that the smaller box can slide into the other. 

The larger box should be designed to house a TV or computer monitor. A projection lens is then fixed on the front of the smaller box and the two are inserted into each other. Sliding the smaller box in or out of the bigger one will help focus the image on the screen. This two-box set-up allows for ease of focusing and image size adjustment.

Usually, the lens used in the process is a Fresnel lens. While it is possible to use any double-convex lens (also known as the magnifying lens), yet due to the curved nature of these lenses, the projected image will be heavily distorted around the edges.  A Fresnel lens is unique in that it is completely flat - it takes the curve of a lens and reduces it to a flat shape by sectioning the lens into a large number of concentric rings. Hence there is no real distortion or bending of the image because every part of the lens projects the whole image.

The internal sides of the boxes are best sprayed with black matt paint to avoid undesirable effects as a result of internal reflections. 

Focusing and projected image size are correlated - both are dependent on the lens-to-TV (or monitor distance) and the lens-to-projection-screen distance.

Setting up this type of DIY projection TV can be a timely process as adjustments have to be made in small steps at a time to properly the TV projector and adjust the image to the required size.

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This is all that it takes to build a projection TV using a Fresnel lens and a CRT TV (or LCD TV). This type of DIY projection TV unit works 'fine' as long as you accept the design limitations

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Simple DIY Projection TV - Design Limitations

Image brightness:

The room must be in complete darkness for these projectors to function. You are relying totally on your TV brightness here; no matter what you do, the light from a standard TV is just not enough to give you a clear bright big picture. 

Keep in mind that image brightness from a fixed light source falls in proportion to the image area - hence the bigger the projection, the dimmer the image will be.

While most sites featuring DIY projection TV plans advertise 150-inch and bigger projections, yet do not expect anything over 80" to look acceptable when you build a projection TV using a simple CRT/Fresnel lens set-up.  Why?

A bright TV image can have a brightness level (luminance) of around 60 foot-Lamberts; LCD TVs may have a slightly higher level of brightness. A projected image in low ambient light should have a luminance level of 12 foot-Lamberts to achieve a minimum image contrast of 5:1 that is necessary for the eye to perceive the image as having adequate brightness. In total darkness, 6 foot-Lamberts may be sufficient ...but keep in mind that it is extremely difficult to achieve total darkness.

More details on the issue of image luminance, ambient light, and screen size are available here.

It is therefore clear that for those who want to build a projection TV, projected image brightness represents a serious constraint.

Based on the above considerations, the table below gives the maximum projected screen size possible to ensure adequate projected image brightness under different ambient light conditions for different CRT TV screen sizes:

 

 

 

There is hardly anything you can do to enhance the image brightness with this type of DIY TV projector. Try using a different light source to light up your TV picture and you will find that the glass screen of your TV or monitor causes enough of a glare to overshadow the picture. 

However, there is something one can do to improve the projected image brightness and overall picture quality. 

Directly associated with the projected image brightness is the projection surface. If you were to go through the projection screen guides included under our Home Theater Screens section, you will see that the projection screen surface can do a lot to the end result. A high gain reflective screen surface is essential if you want to enjoy the best results from a TV/Fresnel lens projector. 

Most well-known projection screen manufacturers have a number of high gain screens within their product portfolio. At the same time, it does not make sense to build a projection TV costing a few dollars and then spend a few hundred dollars on a projection screen. Good quality projection screens do not come cheap. It is therefore not recommend investing in a high-quality screen at this stage especially if you intend to replace your DIY projection TV unit with a branded video projector unit sometime in the future. 

A projection screen has to match the video projector unit you are using and therefore it is best that you consider your projection screen purchase once you decide on your video projector. 

Once you build a projection TV, a cheaper and more appropriate alternative is to build your own high gain silver screen. Specially formulated projection screen paints are available from Goo Systems. The use of ordinary metallic silver enamel spray paint over a smooth surface - spraying several light coats using an appropriate spray can - will also improve the projected TV image brightness. 

Picture quality:

This is not a miracle product. The picture quality of the projected image is dependent on the TV you are using as your image source.

Standard TV reception normally supports around 330 lines of horizontal resolution. You will be projecting these 300 to 400 lines on a 100" screen - each line of picture information will be approximate 0.25" thick!

As such, while picture clarity is generally acceptable when viewed from the appropriate distance and in complete darkness, yet a perfect image is impossible to achieve especially when using a standard TV. Worst still, the projection process itself will not only magnify your TV picture, it will also magnify the flaws within the TV picture itself!

Substantial improvement in picture quality is possible if you use a DVD coupled with a high resolution flat screen computer monitor. This will help eliminate the flaws normally associated with standard TV reception while benefiting from the higher resolution and superior picture quality associated with DVDs and computer monitors. 

Projected Image Size:

Apart from the limitations on the maximum projected image size directly related to the issue of image brightness already referred to above, there is also the issue of projected image size adjustment.

The resultant projected screen size from a DIY projection TV unit is a function of the TV size, the projection lens focal length, and also the distance from TV to lens and the distance from projector lens to screen.

Setting the required image size is therefore the trickiest and most time-consuming adjustment to make once you build a projection TV based on this simple set-up. The projector to screen distance and the lens to TV distance need to be correlated to bring the picture into focus at the desired image size.

Projection Unit Size: 

D.I.Y projection TV units employing a CRT type TV as the picture source are generally bulky and heavy in that these will have to accommodate the TV itself within the projection unit enclosure. These are generally at least as big as the TV itself and almost 2 to 2.5 times in length with respect to the depth of your TV. It is like having two TV sets lined up in front of each other. 

Any TV size between 13" to 30" is generally suitable as a picture source for your TV projector. For practical reasons however, the most suitable TV size to build a projection TV is 21"; the limiting factor being the overall size of the DIY TV projector unit.

The ideal TV should have a flat or close to flat screen surface, good brightness, low glare, and has good clarity. 

Other limitations:

The TV has to be placed upside down to counteract the image inversion produced by the projecting lens. Placing the television upside down will not hurt your TV though color and contrast adjustments may be necessary with CRT TVs. 

Care should also be taken with respect to the ventilation slots on your TV - there should be still enough air circulation within the unit to ensure that your TV will not overheat.

Before proceeding to build a projection TV, it is best to check that your TV functions properly while turned upside down and also when set with the viewable screen facing the ceiling. If it works properly once inverted, then you are in business! 

Placing the TV upside down when you build a projection TV will still result in text appearing in the wrong direction (backwards). A solution is to place your TV Projector facing the ceiling with a mirror directly above the lens and placed at 45 degrees to the lens to reflect the projected image on the wall.  The lateral inversion produced by the mirror will counteract the backwards text problem -typical for any basic Fresnel lens type of DIY projection TV design. 

This 'mirror solution' will also enable you to correct - by adjusting slightly the mirror angle - the resulting projected image distortion when your projector is not placed in line with the normal to your projection surface. This is somewhat similar to the Keystone correction feature found on most modern video projectors.

An even neater solution is to proceed with a simple wiring modification on the field coils mounted on your CRT tube such that it will be possible to build a projection TV without having to turn your TV upside down. Although most often, the necessary wiring details are included with some TV projection plans available on-line, yet the information contained in this respect is always generic. Our recommendation is to consult with a qualified TV technician - it is just a few minutes job - so it should not cost much.

The LCD-Panel DIY Video projector:

Whatever is your motivation to build a projection TV - budget restrictions, DIY do-it-all type, etc., if you are really serious about DIY projection TV - this is the way to go.

Why?  As already indicated earlier on in our discussion, the CRT TV/Fresnel lens DIY projector is bulky and heavy. Furthermore, no matter what you do, the projected image quality based on the use of a standard TV gets relatively poor and dim as the image size gets bigger. 

More elaborated DIY projector designs using an LCD as the picture source are capable of projecting an even bigger image size at relative good picture quality.

These DIY projectors are not only compact and light in comparison to the CRT-TV type, well built LCD DIY projectors are even capable of achieving an image quality that is closer to that produced by commercial video projector units.

In order to get started and possible build a projection TV based on an LCD panel, you need to know how these projectors work.

An LCD video projector uses an alternative light source other than the LCD panel. It magnifies the picture with one or more lenses and then projects it for viewing using the power of the alternative light source.

There are two basic types of LCD-DIY projectors.

The first method uses transmissive projection techniques - a rather more complicated set-up in terms of optical elements where light passes through a clear LCD image-forming display panel prior to being projected on the screen. It is similar in operation to the classic photographic slide projector with the LCD panel replacing the photo slide itself.

Most LCD-based DIY projector designs available on-line make use of this arrangement for the best results.

The second type is simpler in design and is based on reflective projection techniques. A light source is used to shine on an image forming LCD panel to be reflected and projected through the use of mirrors and lenses, on to a projection screen.

Basically, the principle of operation here is very similar to that of the artograph. Originally developed over 50 years ago, the Artograph projector is used by artists and designers to project an image of a pattern, design, or photograph, on to a wall or easel for fast and easy viewing or tracing.

Forced ventilation using a small cooling fan is in most cases essential when you build a projection TV using this type of LCD panel set-up. This is required to dissipate the heat generated by the light source - the LCD panel may be easily damaged by the heat accumulating within the projector if a cooling fan is not used.

Picture quality is dependent on the available LCD resolution, as well as the response time of your LCD panel, and the quality of the optics used in the set-up. Try to get an SVGA or XGA display. The latter supports a 1024 (horizontal) by 768 (vertical) pixel resolution.  

LCD panels are becoming relatively inexpensive and easily available on bargain sites such amazon zshop - where fast, high contrast XGA panels are selling for less than $200 new or $100 used.

Try to get a return policy on used LCD panels in case you discover that the LCD panel is not in good condition - in particular if the number of burnt-out pixels is quite noticeable. Burnt-out pixels on used LCD panels are quite common. These will normally show as either black or white spots on a static white or dark background - not that much annoying during a movie if the number of burnt cells is minimal. 

A main advantage of DIY LCD projectors is that the metal halide bulb used as the light source is significantly less expensive than that found on commercial video projectors. These bulbs will last for some 20,000 hours and costs about $20, a far cry from the $250-$400 and the 2000 to 4000 hours lifetime for color corrected projector bulbs found on commercial units.

A serious problem you may encounter with DIY LCD projectors is the response of the LCD panel in that some panels are not capable of displaying the full range of contrast and brightness levels associated with a normal video signal. Bright scenes get washed out to a solid white while dark scenes turn to solid black. 

You would need a video signal compressor to solve this problem - basically it will have to dim the white while brightening the darks. In this manner, you will be able to see the details in the dark and bright areas.

Expect to spend around $300 to $500 to build a projection TV using an LCD panel; this depends on whether you use new or used parts.

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A final remark

It is hoped that the information contained in this article will be of value to you and will help you in your quest to build a projection TV.

Keep in mind however the limitations highlighted in this discussion in particular with the CRT TV - Fresnel lens type. A Liquid Crystal Display DIY projection TV unit may be a more expensive option if you want to build a projection TV, requiring an even greater effort on your part. But then the results are closer to the real video projector in terms of image quality. 

YOU can really build a projection TV for the fraction of the cost of a commercial projector unit and enjoy the results but there is no substitute to a good quality video projector worth a thousand dollars or so.