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Backlighting Issues With the 200LX

In the years I've owned a palmtop, the one thing I've always wished the 200LX had was a good backlight. I often use my palmtop in conditions where lighting is sub-optimal; that's the whole point of a palmtop, isn't it? To go where no other computer can. But when I'm in a darkened classroom, or trying to do a little word processing in my poorly-lit living room, or trying to read an etext while riding in a car along a poorly-lit freeway, I can't help but think that this is a glaring deficiency in the palmtop's design. With this in mind, I set out to find the ultimate solution to this problem.

There are several different options to lighting the palmtop screen. The most obvious method is to use an external light and shine it on the screen. Several solutions have been proposed in the past; the best-selling solution has probably been the Flexible Pocket Light sold by the Palmtop Network and more recently by Thaddeus Computing. While this solution is decent, the lighting is not very even and the light itself adds bulk to a portable setup. Recent discussion on the HPLX-L mailing list has discussed the Photon LED (light-emitting diode) lights, and connecting LEDs directly to the palmtop serial port. Stefan Peichl has even written a custom utility to turn the proper serial port signals on to light the screen. However, these solutions are not optimal: it's something you have to carry around, and the light of the screen is fairly uneven.

What is really needed is a full-fledged backlight. Unfortunately, this complicates the situation a great deal.

The screen of the palmtop is not really meant to be opened. There is a metal retaining clip around it which must be removed in order to do any backlighting. While it's not terribly difficult to remove this clip, it does require bending pieces of it back and forth when removing and closing it, and at any point any one of these metal clips could snap off, which would render the entire screen useless. So even opening the screen up carries risk.

There is also a problem in that there is very little room inside the screen to put anything. There is one millimeter of clearance, two at most, behind the glass of the display and the circuit board behind it. So any lighting solution is going to have to be extremely thin. Too thick, and it will press against the LCD glass, producing the characteristic discoloration we're all familiar with when liquid-crystal displays are put under pressure-- or worse, the display might crack. It should also be remembered that any light source will require some support circuitry, whether it is merely wires to a power source or a full-fledged digital circuit board. This, too, must be small enough to fit inside the palmtop somewhere. (Most of the backlight kits developed in Japan use an external power source and avoid this problem, but who wants to carry around a big power pack?)

The next problem is that the screen is not designed for a backlight. The grey-white color behind the black pixels of the screen is caused by two things. The white color is actually a silvery, and is an aluminum sticker sealed to the back of the glass. This sticker reflects the light that hits the screen and allows you to see the images on the screen. The grey is caused by two polarizing filters, one on the front of the screen, the other on the back directly in front of the aluminum sticker. The interaction between these polarizers and the liquid crystals in the display allow the pixels of the screen to turn black when electrical signals are applied.

How does this figure into the backlighting project? Well, the aluminum sticker makes the 200LX LCD a "reflective display." Simply putting a light behind the display won't do anything; no light gets through the metal sticker. This must be either removed (which would let the dark green circuit board behind the screen show through, and black pixels on near-black circuit board is very bad contrast) or replaced with a "transflective layer," which would allow light from behind to come through while also reflecting like the regular screen does, so the backlight would not have to be on all the time.

So given all the problems with backlighting the 200LX screen, is there any way to get the job done? The answer, perhaps surprisingly, is "yes." Two of the most promising technologies are LEDs and electroluminescent panels. Let's take a look at those.

LEDs used to light the screen from behind would obviously have to be smaller than the big round, square, or tombstone-shaped blobs most of us are familiar with. Surface-mount LEDs are just right for the job. They come in packages small enough to slip under the screen with little fuss. A big advantage of LEDs is that the power requirements are low and the whole package is very simple in theory-- hook up the lights to a power source (like the screen power contacts), add a switch, and you're ready to go, no complicated support circuitry required. But LEDs also have a big problem: even lighting. A single point of light in the middle of the screen is not going to be sufficient to see much of the rest of it. What can be done about this? For the answer to this, one can look at the 200LX's backlit Windows CE descendent, the 320LX. It uses two green LEDs to light the whole screen, but they are mounted sideways on the right side of the display, and shine into a plastic panel with a frosted back. This panel diffuses the light evenly across the screen and produces a bright, clear backlight.

Unfortunately, light-pipe panels like this are not generally sold; they are usually manufactured for a particular display, and they are usually several millimeters thick, which rules out use in the 200LX. A custom solution might be possible, but this could cost quite a lot to develop. So if LEDs are going to be used to backlight the palmtop, the light will either be uneven, or some hefty development money will have to be spent.

More promising is electroluminescence. Many people are familiar with this technology, since it is used in wristwatches like the Timex Indiglo and Casio Illuminator series. EL panels produce a very even light in a wide variety of colors (although the most common color is a blue-green, because this is the most power-efficient color for EL technology). In addition, EL panels are thin and fairly power-efficient, especially recent low-power versions. Although it is rarely used to light areas as large as the 200LX screen (about 5x2 inches, 10 inches square), panels sized for the 200LX are available, and can be inserted right behind the screen with minimal fuss. The perfect backlighting solution!

Well, not quite. While EL panels are fairly power efficient, the power requirements are unusual to say the least. Rather than a simple, 3-5 volt direct current voltage (like those available in the palmtop), EL panels require a high-voltage, high-frequency alternating current. To convert the power in the palmtop, or from batteries in general, to a usable form, an inverter circuit is required. These typically consist of a bulky transformer and some support transistors and other components-- obviously impossible to fit in the palmtop. The general solution to this problem, used in most of the Japanese backlight upgrades, is to use an external power supply for the backlight, which runs off of several AAA batteries. I have installed one of these in a backup palmtop of mine and can report that it works quite well: I ran wires down from the screen into an external earphone jack on the side of my palmtop, then put an earphone plug on the external inverter pack. Plugging the power supply into the jack and hitting a switch brings up a brilliant white light, suitable for operating the palmtop in any lighting conditions. It's hardly ideal, and I haven't installed it in my regular palmtop because contrast without the light on is terrible, but it's nice to know that backlighting can be done, at any rate.

Some newer versions of EL backlights from Japan use an inverter circuit that is small enough to fit inside the palmtop and is powered right from the palmtop. This may be a good solution for the palmtop, but there is still room for improvement. With this in mind, several groups are working on producing commercial backlight upgrades. With the talented engineers we have working on this, it won't be long now until we have a low-power, high-brightness, software-controlled backlight. Watch for news here.

UPDATES SINCE THIS ARTICLE WAS PUBLISHED: Yes, things are going very well as far as backlights are concerned. Beta testing has begun for the commercial backlight upgrade (sorry, we've already selected the beta team members). As soon as the product is determined to be viable, expect news here and from Thaddeus Computing. In the meantime, mail me personally at hplx.net if you have any questions that just can't wait. A few tantalizing details to get you drooling: the backlight is a nice rich blue EL panel(not the sickly blue-green of Indiglo watch fame), it is completely internal, provides better contrast than the original screen when the backlight is off, and is all around cool.


Copyright 1999, David Sargeant.
Last Updated 23-Aug-1999
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