Tutorial: Build an LED light for the Palmtop

Archived 2013-12-08
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Tutorial: Build an LED light for the Palmtop

Contents of this page:


What is the LED light?


The LED light is a device that illuminates the screen of your HP Palmtop 100LX, 200LX, OmniGo 700LX or 1000CX.
It gets its power from the palmtop´s serial port, so no external power source is needed.
You simply plug the LED light into the serial port, load the Software ´LIGHTON´ (by Stefan Peichl), and you can switch the LED light on and off using the hotkey Fn-L (or any other hotkey).


Since the backlight upgrade project by Thaddeus Computing has been cancelled due to technical problems, such an LED light is in my opinion the most comfortable solution if you need to read the screen of your palmtop in bad lighting conditions or even in total darkness.
UPDATE: I have taken over the backlight project in 2002 and continued the great work the people of Thaddeus computing have done, so now there is a backlight upgrade available for the 100LX, 200LX and 1000CX. If you prefer backlight instead of a LED light, please click here!


If you bought an LED light from me (so don´t have to build you own one), please jump directly to the usage instructions!


This picture shows my LED light in action: This picture was taken in nearly total darkness. As you can see, the screen is perfectly readable and even the keyboard is a little bit illuminated!


First a note about the electronics: You may wonder why no current limiting resistor is needed. The reason is, that the RS232 port driver chip in the palmtop contains already current limiting resistors for each line, which are needed to protect the port from all kinds of short circuits. The RS232 specification says that an RS232 serial port has to stand short circuits for an unlimited time. If you insert an additional resistor for safety here, you will drop the current so much that the light will not be bright enough anymore to be useful. The 6.8 mcd, which the LEDs should produce, are only reached at a current of about 20mA. Here we have a current of only about 10 mA, so the LEDs are not as bright as they could be anyway. So don´t worry about the serial port - it will probably be safe, even without an additonal resistor. I use my LED light every evening for several years now, without any sign of a damage to the palmtop.
However, I do not guarantee that it is totally safe. Damage may occur to your palmtop, I will not be responsible!


Also, all LEDs are different, even if it comes from the same manufacturer, has the same part number and so on. So you may notice a slight difference in brightness or colour temperature of the two LEDs of the LED light. This effect is enforced by the fact that the LEDs are not driven at their nominal current, but at a much lower current.



Building Instructions


What you need:

  • 2 white LEDs. I used Wustlich WU-7-750 SWC. Specs: 5mm white, GaInN, viewing angle 20 degrees, 6200 mcd at I=20mA. Available from Segor Electronics. Other kinds of LEDs may also work, but only white or blue ones. Red, orange or yellow LED light is not reflected well enough by the reflective layer of the screen. Also in the meantime there might be LEDs on the market with better suitable characteristics. A wider angle would be preferable, for example.
  • A connector for the serial port. For example part number 739251 from Conrad ElektronikClick here to see pictureIt is a 2*20 pin connector, which has to be cut to 2*5 pin. The connector of a notebook 2,5" IDE hard drive also has the correct hole spacing, but it doesn´t have the pins on the back, so it cannot be easily soldered to the circuit board.

The following items can probably be found in any electronics store:

  • a 1cm * 2cm piece of circuit board, both sides copper-coated, 2mm thick
  • 23 cm of stiff copper wire 1,2mm diameter, Segor order number: K4030-12501.
  • 25 cm of flexible insulated wire
  • 20 cm of heat shrink tubing 2.4mm diameter
  • 3 cm of heat shrink tubing 12-14mm diameter
  • 2 cm of heat shrink tubing abt. 5mm diameter
  • hot melt glue
  • a soldering iron
  • a rotary grinder or something similar to remove some of the copper of the circuit board. Of course if you have the tools, you can also etch the circuit board according to the plan below.


Here is a step-by-step guide to build the LED light:

    • Cut off a 2*5 pin piece of the connector, which fits into the serial port of the palmtop.



    • Use a rotary grinder or an adequate tool to build the needed copper circuits on the board. On the bottom side, no pin has to be connected to anywhere. They must all be insulated from each other. On the top side, pins 3 and 5 are used: Pin 3 is +Vcc for the LEDs and should go to the big copper plate with the holes, where later the stiff copper wire is soldered into, pin 5 is ground and should be connected to the second, flexible wire. Also drill the two holes into the circuit board. They should have a diameter of 1.3 or 1.4mm and the hole next to the connector side of the board should be at least 9 mm away from the connector side.


      If you have the equipment, you can of course etch the circuit board according to the diagram below, but you should not use the graphic as the template, because the lines may be too thin to make a good insulation gap.


    • Then solder the connector to the circuit board. Take care not to procude short-circuits!



    • Bend the stiff wire using pliers the following way:



    • Stick the stiff wire through the holes of the circtuit board and then bend the tail of the stiff wire around the edge of the board, so that it has a firm hold, even without soldering.



    • Now solder the stiff wire to the large copper plate. Use much soldering tin.



    • Solder one end of the flexible wire to Pin 5 of the connector.



    • Test, if the light works: Plug it carefully into the serial port of the palmtop, do a

serctl /w

          to enable the port, then take an LED and connect it with the short leg to the stiff wire, the long leg to the flexible wire. The LED should shine brightly.



    • Push the 20cm 2.4mm heat shrink tubing over the two wires and shrink it.



    • Carefully apply hot melt glue to the bottom and top side of the circuit board, so that it gets a little more voluminous, and so that everything is fixed (esp. the flexible wire). Let the glue get cold. You could also use Epoxy glue here, but that one gets so hard that corrections, if necessary, are difficult. If you use hot melt glue, you don´t have to take care that it doesn´t look ugly, because that kind of glue will be melted again when heating the tubing later and adapt itself "automatically" to the tubing.



    • Prepare the 3cm 12-14mm heat shrink tubing: use a small screw driver to prick a little hole from the inside to the outside of the tubing, exactly where the LED light arm will pierce it.



    • Bend the LED light arm directly above the board 90 degrees towards the connector. Be careful to not break the stiff wire!



    • Prick the arm with its wires from inside through the hole of the large tubing and slip it over the circuit board. Bend the arm up again (carefully!) and shrink the tubing. Begin at the connector side, so hot melt glue getting liqid again cannot pour from inside and fill the contact holes of the connector. This could destroy the connector! Do no apply too much heat here, otherwise the hole in the tubing can be teared open too much. Cut off hot melt glue which poured from the back of the LED light grip with a hot knife. Also cut off tubing which may be in front of the connetor holes.



Now it should look like this:



    • Take the two LEDs, bend their legs as the first picture shows (carefully! Don´t break the legs!) and twist the corresponding legs together (short led with short leg, long leg with long leg).



    • Shorten the LED legs. Cut them off so they get the same length, see picture:



    • Shove the 2cm 5mm tubing over the LED light arm and cut the two wires to an equal length, so that they are abt. 1cm longer than than the tubing which they are in:



    • Solder the LED leg which is made from the two originally short legs to the stiff wire and the leg which was long before to the flexible one. If you don´t know the right direction, simply plug the light into the palmtop and try it out by applying the LED legs to the wires manually.



    • Wrap a piece of paper around the legs to prevent future short-circuits.



    • Shove the 2cm 5mm tubing over the soldering points, apply a spot of hot melt glue onto the top where the legs come out to stabilize the legs and shrink the tubing.


That´s it!




Usage instructions



There is a little program (a TSR) written by Stefan Peichl especially for the LED light, called LIGHTON, which lets you simply switch the LED light on and off by using the hotkey Fn-L. Download this program from the S.U.P.E.R. site, unzip the archive, place the file lighton.com somewhere on your C: drive (for example in c:\bin) and add a line to your autoexec.bat:
Then reboot your palmtop. (You may need the COM file from the LIGHTON2 package. Try which one you need: Go into DOS mode and press Fn-L (if no LIGHTON TSR is loaded). If a pound symbol is created, LIGHTON is okay. If just a lowercase L is created, you need LIGHTON2, which has the hotkey changed to Alt-L, instead of the original hotkey Fn-L)
After the reboot, you should hear a beep sound everytime you press Fn-L or Alt-L.


If you received your LED light by mail, I probably have folded it up and put it into a music cassette box (picture shows box with two LED lights). If not, omit the following step.



Remove all adhesive tape carefully (or rip it open with a sharp knife for example) and unpack the folded LED light.
The first thing you should do is to remove the three kinks by bending them into the opposite direction, so that your LED light is straight.



Caution: Don´t bend the LED light too much. The stiff wire used inside the light is a copper wire which will break if you bend it too much or often. So be careful!


Adjusting the LEDs

Now plug your LED light into the serial port of the palmtop and switch the palmtop on. Does the light work? Try to press Fn-L or Alt-L a few times, you should switch the light on and off by that. If not, you either installed lighton incorrectly or you have another TSR loaded which keeps the serial port deactivated (like Buddy, for example).


Well, your LED light illuminates the ceiling now. That´s nice, but not what you bought the LED light for. Darken the room and see how bright the LEDs are! They may indeed illuminate your entire room a bit.


Now the most tricky part begins: You now have to adjust the LEDs so that they illuminate the screen in an optimal way (i.e. as evenly as possible). A few rules-of-thumb:


    • The LEDs must point from a similar direction than your eyes look, otherwise you will get only odd effects due to the light reflection and polarizing properties of the screen.
    • The distance of the LEDs to the screen must be at least 8-10cm, because the LEDs have a viewing angle of only 20 degrees.


First, bend the whole thing so that the LEDs point onto the screen in a way I just described. It should look as on the pictures:



Always take care you don´t press the contacts of the LEDs together, you could cause a short-circuit, which wouldn´t damage anything electrically, but you would wonder why the LEDs don´t shine and you would risk mechanical damage of the LED legs.



Now you have to fine-tune the LEDs´ positions. Be careful! First, adjust the angle between the LEDs in a way that one LED illuminates the right part of the screen and the other one illuminates the left part. Then bend the whole LED head again to finish the adjustment.


It will take a while until you will get used to the reflections of the LED light which you will always see on the screen. Maybe you even find a position where the reflections disappear without too much light loss. Simply try around!



This is a badly adjusted LED light:



And this is how it should be:



These pictures show only the center light dots of the LEDs, but the surrounding light will be fully sufficient to make the screen readable even in total darkness. And as a side effect, the keyboard will be well visible!


Have fun!




I'm not responsible for physical or other damages to your devices. The descriptions I made may contain errors which could cause serious damage of your palmtop or your peripherial devices. What you build here is a device directly connected to the motherboard of your palmtop. If you damaged the motherboard, it would become really expensive for you!

Do you like to support me?

Dear fellow Palmtopper!

If you like, you may reward me for the work I put into this website with a donation.


For a donation in US$:

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Your donation, even if it's just a single USD, will help me to:

  • finance the server that hosts the site
  • justify time I spend with maintaining the site and do not spend with earning money otherwise (wife is watching! ;D)
  • just continue to be motivated :)

Since my company "hermocom" does not exist for many years anymore, I am now spending my spare time and private money for all of this. Thank you very much! 🙂

Yours truly,
Daniel Hertrich