A Backlight for the HP Palmtop



This article is related to the Palmtops HP 100LX, 200LX and 1000CX. The HP OmniGo 700LX may be compatible enough, too (untested). For the 95LX, some info from this article may be used, but in general the 95LX is not compatible.

Please note that I don't offer backlight upgrade installation service anymore. But backlight do-it-yourself kits are still available for sale. Please refer to the HP Palmtop product catalog.

An alternative screen lighting device, which is much easier to build and install than the backlight, is the LED-light.

Contents of this page:

• Introduction
• How to get the palmtop upgraded
• Detailed technical information
• Some pictures and data points of my research work
• What about the other HP DOS palmtops (95LX, OG700LX)?
• Special thanks to:

Related pages:

• The backlight installation instructions
• http://www.backlight4you.com - the place where the backlight upgrade kit has been sold
• http://www.palmtoppaper.com - Thaddeus Computing, a sponsor of this project
• http://www.rundel.net- Thomas Rundel, a sponsor of this project

Introduction:

One of the biggest disadvantages of the Hewlett Packard DOS palmtops is that they don´t have a backlit screen, thus they are unsuable in dim light conditions and in the dark.

There are ways to illuminate the screen without upgrading the palmtop with backlight: For example I developed the "LED light", which is a quite convenient alternative to backlight. Some people use battery-driven LED lamps, which can be mounted on the screen of the palmtop or on your head. But all these solutions require you to carry another device around and most of them even with extra batteries.

The backlight is the ideal solution for palmtop users who often need to illuminate the screen without much trouble. You simply switch on the palmtop, press the backlight hotkey Fn-X and the screen is readable, whereever you are.

How to get the palmtop upgraded:

Since I have stopped backlight inbuild service, the only way to get your palmtop upgraded with backlight is to buy the backlight upgrade do-it-yourself kit and - well - do the upgrade yourself. Here are the details:

The upgrade kit (basically, an EL foil and a driver module for it) can directly be ordered from backlight4you.com for about 60 EURO.
English: backlight4you.com HP backlight upgrade kit
Deutsch: backlight4you.com HP backlight upgrade kit

If you want to install the upgrade yourself, please consider that a quite high skill level is needed: You have to take the palmtop entirely apart, even opening the screen module. Before you decide to do the upgrade yourself, read the installation instructions (see next paragraph) carefully to see if you will be able to do it yourself! And please consider that replacement screens are extremely hard to get, and if you get one, it will be expensive.

Please read that installation instruction sheet carefully before you decide to do the upgrade yourself!
Click here to read the installation instruction sheet!.

The backlight kit does not contain any tools necessary for the upgrade process.

Detailed technical information

Further technical information can be found on the installation instruction sheet, technical info about the upgrade is in the introduction, and a usage guide for the software control of the backlight can be found in the driver section.

Here are a few pictures so you can see how the backlight upgrade affects the contrast and overall-readability of the palmtop screen. Please be aware that it is extremely difficult to take pictures of SCD screens and light sources which represent the real contrast and colors of the items, so I took one picture which shows the actual color of the backlight upgrade:

(Actual look of the backlight upgrade)

The following pictures are taken with another camera than the picture above and thus the color of the backlight is falsified and it looks darker than it really is, but the pictures show the contrast relations quite well:

Since the original reflective foil on the back of the LCD screen is replaced by an Electroluminescent (EL) foil, the crispiness of the screen is reduced a bit. This may be considered a disadvantage, but it can even be convenient for the eyes in very bright light.

1: unmodified screen

2: modified screen - backlight off

3: modified screen - backlight on

Compare the appearance of an upgraded palmtop to the appearance of an unmodified palmtop under different light conditions:

(in bright light - left: modified, but backlight off; right: unmodified palmtop)

(in dark light - left: modified with backlight on; right: unmodified palmtop)

(in very dark light - left: modified with backlight off; right: unmodified palmtop)

(in very dark light - left: modified with backlight on; right: unmodified palmtop)

You can see that the backlight upgrade decreases the contrast of the screen, which makes the screen less readable under a certain light intensity threshold, but as soon as you go below that threshold, you can switch on the backlight and the screen is very well readable again.

And here are some results of current measurements I have done to calculate the backlight´s impact on battery life. All values have been measured in the battery circuit, with a battery voltage of 2.6V (1800mAh NiMH rechargeables), no AC adapter attached, in pure MS-DOS mode, i.e. without System Manager running. Light sleep mode is entered automatically by the palmtop after a very short period after the last keypress, so the "normal" mode is active during a key is pressed permanently. In light sleep mode the current sometimes changes heavily and periodically, so I give current ranges rather than specific values:

You see that the current draw is in average about doubled with activated backlight, so if you always use the backlight, the battery life will be decreased by factor 2.

Some pictures and data points of my research work

I began to work on the backlight project in August 2002.

First I bought some components to build an inverter module for the EL panels which I got from Thaddeus Computing for research. As the main component I used the Sipex SP4403EU chip and built the surrounding electronics very similar to the suggestion on its data sheet. The first working circuit was still a bit large for the palmtop, but I compressed it when it worked and that smaller one fitted where it was intended to fit: Into the hinge tube.

This solution worked well, but it needed very much power, so the batteries only last 1/6 of the time they last without the backlight switched on.

Then I found out that Stefan Kächele (www.backlight4you.com) also worked on a backlight solution for the 200LX palmtop, and we combined our knowledge and experiences and optimized the inverter / EL foil combination, so the final backlight product would shorten the battery life approximately only by factor 2 in case the backlight is always switched on.

Another issue were the optical charateristics of the involved screen layers. Simply removing the reflective layer of the screen and replacing it with the EL foil did not work, because due to some reason, the contrast did suffer very much in that case, the light of the EL foil was darkened very much when penetrating the screen.

The solution was to put a thin transparent foil in between the screen´s polarizing foil and the EL foil, this established a certain distance between the two, and the contrast problems vanished. I tried different kinds of transparent foil, and then we choose the one which offered the best contrast with and without backlight switched on.

What about the other HP DOS palmtops (95LX, OG700LX)?

The screen module of the OG700LX is identical to the screen module of the 200LX, so the backlight upgrade should generally also work with the 700LX. However, there are the following problems, which may be solved some day, but they aren´t yet:

• The case of the 700LX is totally different, so the technician who installs the upgrades has to learn to upgrade a 700LX before he can do that reliably. This requires a 700LX for trial. If you can offer one (no guarantee that is will survive that), please do so!
• I don´t know if the CPU pin we use on the 200LX for software control of the backlight is also free in the 700LX. Since the 700LX has some additional features, it could be that that I/O pin is used for something else. If that´s the case, the backlight can only be switched via hardware (SMD switch)

Regarding the 95LX: No research has been done regarding the 95LX, and I won´t do anything into that direction. There are many unknown factors; in general, it seems possible to use the backlight kit also for upgrading the 95LX, but I don´t know if the screen is similar enough to the screen of the 200LX (its light transition properties for example, the general design and so on). Also software control using a GPIO pin of the CPU and a special driver would probably not be possible. Another unknown factor is where we can get the necessary power to drive the backlight, maybe the PCMCIA 5V supply can be used (as on the 200LX).

Special Thanks to:

Thaddeus Computing for supporting me with left-over material from their own backlight project and with replacement parts for my research work. Without them, I wouldn´t have been able to come up with a working backlight solution!

Rundel Datentechnik for supporting me with hardware parts for research

Stefan Kächele (backlight4you.com) for sharing some of his research results with me, and to be so cooperative that we can finally offer the long-awaited backlight upgrade to the palmtop community!

Stefan Peichl for writing the backlight driver, which makes it so convenient to use the backlight

Eric of battlequest.com for proof-reading the installation instructions

The HPLX mailing list and its very knowledgable and creative community, which helped me in many ways to get this project running.