A Light at the End of the Tunnel

For those who want to use their Palmtop in the dark, there may be a solution in the works.

By David Sargeant

Editor's comment: David presents an illuminating article that describes the "why's and why not's" of producing a backlight upgrade for the HP Palmtop. At the end of the article he mentions that several people are working on a solution. We do not like to print articles that pre-announce future products. The products all too often do not make it to market because of "technical difficulties beyond our control." We chose to print this article because, if nothing else, it shows that avid users of the HP Palmtop are still interested in making something that works well, work even better.

During all the years that I've owned an HP Palmtop, the one thing I've wished for was a backlit display.

I often use my palmtop in conditions where lighting is not the best. But isn't that the whole point of a palmtop: to boldly go where no other computer can go? Yet, when I'm in a darkened classroom, my dimly-lit living room or trying to read an e-text in a car on a poorly-lit freeway, I can't help but think that this is an all too evident deficiency in the palmtop's design.

I realize that there are already some ways to bring portable lighting to the Palmtop. For example, the best-selling solution has 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 discussions on the HPLX-L mailing list have focused on the Photon LED (light-emitting diode) as an external light source. The interesting thing about this choice is that it's economical, if you have a Palmtop serial cable. You just insert the leads from the LED into the cable, and plug the other end of the cable into the Palmtop. The Palmtop's batteries supply the power. Stefan Peichl has even written a custom program to turn the proper serial port signals on and send power to the LED. However, in my opinion, external light sources are not ideal. You have to carry something extra with you and, no matter how you adjust the light; you always wind up with "hot-spots" and dim regions.

What is really needed is a full-fledged backlight. However, this is easier said than done.

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

There is also very little room inside the display part of the Palmtop. There is a one or two millimeter clearance between the glass part of the display and the printed circuit board behind it. Any backlighting solution is going to have to be extremely thin. If it's too thick it will press against the LCD glass producing the characteristic discoloration we're all familiar with when liquid-crystal displays are put under pressure. Even worse, the display might crack.

It should also be noted that any light source would 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. (Most of the backlight kits developed in Japan use an external power source to get around this problem, but this means that you have to carry around a power pack.)

The greatest problem is that the screen is not designed for a backlight. To produce the typical light gray background on the screen requires an aluminum sheet stuck to the glass. This aluminum sheet reflects the light that hits the screen and allows you to see the images on the screen. The dark gray text and images come from polarizing filters that block the reflected light. Simply putting a light behind the display won't do anything since light would not penetrate the aluminum sheet. One way around this is to remove the aluminum sheet and let the dark green circuit board act as a reflective surface (imagine black letters on a dark green background!). Better yet, it might be possible to substitute a "transflective layer" that would allow light from behind to come through while reflecting light like the regular screen does. This would be a better solution since the backlighting 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 is "yes."

Since the HP Palmtop originally came to market, the producers of screens for PDAs have come up with at least two ways to make them more readable. The two techniques are LEDs and electroluminescent panels. Let's take a look at these.

LEDs are the little red, green or yellow lights that glow on your desktop computer or TV and VCR control panels. This type of lighting device is too large to fit in the cover of the Palmtop. However, surface mounted LEDs come in packages small enough to slip under the screen very neatly

A big advantage of surface mounted LEDs is that the power requirements are low and there is very little need for extra circuitry. Theoretically, all you'd need would be some wires and a switch to turn the LEDs on and off. However surface mounted LEDs have a drawback. They produce uneven lighting. A point of light in the middle of the screen just won't cut it. On the other hand the HP 320LX, a Windows CE handheld device, uses just two surface mounted LEDs to light the whole screen. The LEDs are mounted sideways on the right 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, such "light-pipe panels" are not sold separately. You have to buy the whole display which is two to three millimeters too thick to fit in the Palmtop. A custom solution might be possible but this could cost quite a lot to develop.

A better solution to backlighting is the electroluminescent display.

Many people are familiar with this technology. It is used in wristwatches and display panels for bedside radios. EL panels produce a very even light in a wide variety of colors. 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 don't use much battery power. EL displays are rarely used to light areas as large as the 200LX screen (about 10 inches square), however there are EL panels of this size available and they will fit behind the screen with minimal fuss. Could this be the perfect backlighting solution? Well, not quite. While EL panels are power efficient, the power requirements are unusual to say the least. Rather than a simple, 3-5 volt, direct current, EL panels require a high-voltage, high frequency alternating current. To convert the power in the palmtop to something that would drive an EL display requires something called an inverter circuit. These circuits consist of a bulky transformer and some transistors and other components, which would not fit in the Palmtop. The solution to this problem, common to most of the current backlight upgrades, is to carry along an external power supply for the backlight. The power supply uses several AAA batteries. As an experiment, I installed an EL panel in a Palmtop and can report that it works quite well. I ran wires from the screen into an external earphone jack on the side of my palmtop and 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.

Some newer versions of EL backlights from Japan use an inverter circuit that is small enough to fit inside the Palmtop and is powered by the Palmtop's batteries. This may be a good solution for the palmtop, but there is still room for improvement. With this in mind, several people are working to produce a commercial, backlight upgrade for the Palmtop. With the talented engineers working on this, it may not be too long before we have a low-power, high-brightness, software-controlled backlight. We promise to keep you posted about the progress of this endeavor in future issues of The HP Palmtop Paper.