Using negative transmissive lcd as information projector?

[GTechno13]

I have one of those clocks that projects the time onto the wall/ceiling and it's getting really dim. I don't want to fix it but rather build my own that displays my own information (time and weather info).
Would it be possible to place a negative transmissive graphical lcd in front of a high power led with lens to achieve this effect? I have never used graphical lcds so I'm not very familiar with their properties and I'm not very experienced with optics. Would the opaque portion let any light through? I figure a 10-15 deg lens for the led would project the right size on the wall but would this be in focus or is some sort of focusing lens needed?
Google and forum searches yielded no information on such a project but maybe my search terms were bad.
If anyone can provide some insightful input I would be grateful!
Thanks
-GTech

 

[Blueteeth]

It's how many commecial projectors work, albeit with very powerful light sources, and tiny TFT displays.

Most graphic LCD displays are STN, that is 'passive' not active, so the contrast ratio suffers as a result. This would mean that, when a pixel is off, it still allows some light through, and when 'on', does not completely allow it to pass. This also goes for TFT displays, although since they have active circuitry, the pixels can be fully turned on/off (each pixel having a transistor and capacitor assosciated with it) so the contrast ratio is in the oder of 400 - 1000:1, as opposed to 4-10:1 for STN's.

LCD's use polarization, the display will always absorb 50% of the light that hits it. There are clever ways around this, such as polarized light emitters (not easy) and polarized reflective films, which 'reflect' the unwanted polarization, rather than absorb it as the filter on the LCD would. This reflected light is then recycled back into the optical system.

Also note, the displays in projectors are very small. VGA/SVGA resolution, but at most a couple of inches diagonally. The size is not only for a smaller unit but also for easier /smaller optics.

I am sure you *could* do it, but you would need a very bright source, and optics with rather long focal lengths (due to the size of the display), making the system a bit on the big side. I have seen DIY LCD projectors using over head projectors and a modified TFT panel - one with the backlight removed. Making a mini one of those would be a great start

Apologies if I've gone over your head, or too simple, I'm never sure what people know

 

[Nigel Goodwin]

If you google round, there are loads of people using old LCD TV's, or laptop screens, as projectors.

 

[GTechno13]

I'm not looking for much resolution. A projection clock is what I'm after with maybe a 1-5 watt led as the source. I imagined a lcd such as this one to pass the light through. Would this have a very poor contrast ratio and light transmission? The wiki page for projection clocks mentions that the low brightness ones use a lcd or led display but how do they obtain such a laser like brightness?

Wiki says FSTN means "film compensated" STN for added contrast. Does this help me?

Thanks

 

[Blueteeth]

FSTN does indeed have a slightly better contrast ratio than standand vanilla STN. It also corrects 'colour' as I'm sure you've seen on character LCD's, the pixels tend to have a blue tint to them, as opposed to black. This is an artifact of super-twist-network displays, when it blocks light, it blocks the blue end of the spectrum lightly less than the red end, and the 'film' they use retards the blue end, making black 'black'

Crystal fontz have great datasheets, which can give you the sort of specs of LCD's that are available. Not saying you have to buy theirs, but at least it'll give you some numbers to compare to other displays.

https://www.electro-tech-online.com/custompdfs/2010/10/CFAH1602DYTIET_v21.pdf

That quotes the contrast as 5. But I have a few FSTN graphic displays where the datasheet quotes 25, namely some hitachi 320x240 ones (with a bright CCFL backlight). SP14Q004:
https://www.electro-tech-online.com/custompdfs/2010/10/sp14q002-a1.pdf

So I guess a 30:1 is more than possible, but who knows, if you get a small 128x64, and set up your light source with a lens, it might look great numbers can't always tell you what looks ok. You might try an old mobile phone LCD, but I'm not sure if they are negative.

Check out sparkfun, they always have great displays, and the cheapest prices. Also provide great documentation on how to controll them. They have a small TFT, with on board controller. I imagine that would look awesome as a mini projector, plus its small size makes your life easier with optics. 128x128, and pretty simple to control, just ignore colour information (but its always a handy option eh?)