Hi guys, I've been trying to make a cheap LDC overhead projector, very much the same as the one seen on youtube here...
http://www.youtube.com/watch?v=n6n6jW0XIV0

But I've had a few issues. Firstly the stage glass was totally smashed on the OH-projector, I'm guessing that this will not matter? Can I use it without the stage glass?

Second, I managed to get the screen appart (a mission without a screwdriver I can tell you!), its a Hyundai L72s, which is a 17 inch screen. Its a bit big but I figure I'll screw around with my computer display settings and make the borders smaller. Anyways - the screen seems not very transparent. It looks dim on the top of the projector, but its pretty much black by the time the light is aimed at the wall. Do I need a more powerful bulb?

The LCD panel itself is a very black film, do you guys normally just ramp up bulb sizes? If it's bright enough for a transparency, should it be ok for an LCD? From what I can see the bulb is a 130V 400W, which seems like it should be good enough? I can try post some photos if needed, so far this has turned out quite frustrating!

Thanks for any help!!!!

Ohh I should mention that I have all the various polarizing films and sheets that surrounded the LCD panel, should these be added into some sort of sandwhich between the panel and where the stage is (or was supposed to be)?

LCDs don't pass a lot of light. (Look in the FAQ.) They just don't.

You do need the polarisers from the LCD. It's best to leave them on the LCD, and not strip them off in the first place.

The stage glass on the OHP isn't a problem.

A 400W halogen lamp puts out MAYBE as much light as a 150W metal halide lamp. Upgrading the lamp will help, but these things don't tend to be super bright. It will help a lot to mask off any light from going around the side of the LCD panel.

I think he was talking about all those extra diffusing sheets in between the backlight and the Lcd.

And if you were , No they are not needed.

Yeah thanks guys, I was thinking they didn't seem very "polarising", I kept flipping them over and spinning them round but no darkness lol.

So this youtube video is way off then? To get it to a workable projection level I need to get a MH bulb and ballast then? Would they guy in the video have changed the bulb or do some projectors come standard with MH?


So a 400 W MH will be enough? (bearing in mind its currently so dark I can't really see anything on the wall harldy). And I'm assuimg the MH bulbs are not directly interchangeable with the one in the projector?

Thanks!

Right. I was talking about polarisers, not the extra sheets that are included in the LCD frame.

Even with your 400W halogen, you ought to be able to get a decent projection. I'm a bit concerned about the stage glass. I wonder if you also lost a fresnel lens with that. Does the projector still bring the light to a bright spot at the head of the projector? This is needed in order to get much of anything on the wall.

A brighter lamp helps, certainly, since an LCD blocks 90-95% of the light that hits it. This is unavoidable, because of the way that the LCD works. Since any light that goes past the LCD tot he sides is going to be 10 to 20 times brighter, it will easily wash out the image. Because of this, you need to be careful to block this light out.

The polar films are sandwich and glued to the LCD on both sides and painful to remove and as Supraguy mentioned shouldn't be touch. The antiglared polar on the front of the panel is a different story unless it's a nonantiglare type. Modding antiglare polar is another topic that you should read more on and is a very very risky mod. These 'loose' films your flipping are enhancement films used in conjunction with the flourescent lights within the monitor and should be disgarded. You MAY find some reflective film glued to the rear of the LCD on top of the rear polarizer and again these are additional enhancements (normally 2 sheets) and are not neccessary but are a challenge to remove just as equal to modding antiglared polar. They can be left alone on the panel and you will get projection but can affect brightness and/or even clarity as some have claimed.


Might be because of the loose enhancement films you still have tucked behind the panel but, yes, a 400w bulb is the standard here. Some have used 150-250w bulbs based on power consumption preference but bear in mind the arc of the lamp also makes a difference. The smaller the arc is on the lamp is best overall because the rear fresnel points to a dot just like a magnifying lens. To large an arc and you have stray light affecting clarity. To give an idea, a 400w arc is somewhere of 25-30mm between electrodes, a 575w HMI lamp's arc is 7mm.

And no MH bulbs are rated to be used with their matching ANSI ballast most especially magnetic core ballasts. A electronic ballast can be used with differing ANSI bulbs as long as they have matching wattages but check manufacturers for this. Overclocking bulbs for example 400w ballast + 250w bulb has been done but there is a high risk of shortening the life of the bulb or even blowing it up and again another topic you should or shouldn't read more on.

There is an acrylic lens under where the stage was(between the bulb and the stage), which is kinda like a bubble, when I look through it it inverts the picture, like a magnifying glass, which must be the fresnel I'm guessing.

Thanks for your help guys I'll take a picture later today and I'll just make sure before I go and buy a 400W MH with ballast.