Backlighting the screen

    Maybe your English teacher passed some films in class, and you wished you have your Phœnix in your pocket. But even if that was the case, unfortunately you can't see the screen in the dark. Well, guess it's time to learn how to backlight your calculator.
    This assembling is intended for confirmed "hardware geeks", since some of the manipulations are quite delicate to do, but the result is sooo nice.

    On this page, I will give my example :-) Some of the steps described below may not apply to you.

First step: Tools
Second step: preparing the screen
Third step: preparing diffusing surface
Fourth step: preparing the LEDs
Fifth step: preparing lighting surface and installation
Sixth step: assembling and finishing

First step: Tools

Yellow items are mandatory, other are optional.

- Ether, at least 50mL to have enough (beware this stinks, open the windows or work outside)
- Cutter, razor blade, scalpel, or ideally a X-Acto, roughly anything with a very thin and sharp blade + spare blades for your tool (optional)
- soft cloth which doesn't get linty, reusable cleaning cloth (available in hardware stores and sometimes in grocery stores), at worst some top quality paper towel
- long nose pliers (optional but advised)
- 1,5mm thick Plexiglas (ideally), not over 3mm (at worst) (different manufacturers often give different name to the product, such as Lucite or Optix), 76x53mm
- something to take measurements to the millimetre, ruler or a calliper
- Sanding block (optional but advised) + sanding paper grade 150 (and some 80 for meticulous people)
- A Dremel, mounted on the drill press attachment if possible, with cutting disk (+s if you're clumsy), and a high speed cutter #194 (1/8 inch cylindrical bit, thinner if desired).
- small metal saw to cut the Plexiglas (optional)
- small file (for nails or metal)
- LEDs the color you want, the most powerful possible
- electric wire (very fine) (at least 10 cm)
- A switch or microswitch, the model you want according to what you want to do
- small soldering iron and solder
- cyanoacrylate glue (Super Glue)
To your choice:
- double-sided foam tape, foam tape (one side), or firm flat white foam (1~3cm thick)
- paper glue stick
- two of the above :)

I won't tell how to take apart the calculator, it's already on the overclocking page.

Second step: preparing the screen

    Now you put out the printed circuit board out of the calculator, turn it over to have the screen facing you. If you watch to the side of it, you will see a metal plate between the printed circuit board and the glass screen. This plate is tightly secured to the circuit and the back of the screen by a kind of gelified, very strong glue.

Picture 1: Close-up on the layers "sandwich" as mounted.

1- Removing the plate from the printed circuit board
    You need to remove this plate in order to continue. To do this, take the X-Acto and gently slide its blade between the circuit's epoxy (the green thing on the photo) and the metal plate, and slide it along all possible sides, until the plate comes off. The printed circuit under the screen is quite fine and has a small chip on it, so don't twist it too much. If the screen+plate set doesn't comes off the board easily, slide the X-Acto again.
    Remember it's vital not to force the blade ! If it gets too stiff to slide the blade, rise it closer to the edge and continue. It is sometimes possible to slightly enhance how well they come off each other by pouring some drops of ether where the blade cuts. Don't forget to close the ether bottle tightly, as this liquid is highly volatile, you could end up with a dry bottle if you don't.

2- Removing the metal plate from the screen
    It is now necessary to take the metal plate apart from the screen itself. Don't try to do as we just saw, it's the best way to break the screen, made out of a quite fragile glass.
    You will notice that the metal plate has some large holes in it, you can see the reflector stuck on the back of the screen. In each of these holes, pour ether until the edge, wait a few seconds, then try to insert the tip of a X-Acto blade on the edge of the plate, facing a hole. If it doesn't insert properly while sliding gently, don't force, and repeat the operation while keeping the same amount of ether in the holes, until it comes off. Once it starts coming off, as a miracle, it goes like in butter on each side...

3- Cleaning up
    Now the plate is freed, chances are you should clean up the back of the screen from any glue trace.
    Soak a cloth's corner with ether (not the whole cloth, it will be useless in addition of wasting product), and wipe out the remaining glue traces, always in the same way, from inside to outside, until the surface is smooth to the touch and doesn't stick, soaking the cloth as much as necessary.

4- Removing the reflector
    The reflector's back you just cleaned up is not much permeable to light, so you need to remove it in order LEDs' light can pass. Caution, maybe you'll want to reuse it (that's why you cleaned it), so proceed with care to remove it. Avoid to tear it !
    With a well sharpened side of the X-Acto's blade, slide it under a corner of the sheet while taking care of the screen. Continue the farthest you can, plunging a few millimetres each time you slide the blade. When you start to feel a stiffness or it becomes perilous to continue this way, take the unstuck part of the sheet between two fingers, and pull gently while holding the screen with the other hand. Place your fingers the closest possible to the place where the sheet becomes unstuck to reduce to a minimum risks of breaking the screen and the sheet.

    Bravo! The screen is now ready to accept further operations. Place the calculator's circuit in a safe place while taking care of the screen; its sticky surface (ether doesn't change anything, unfortunately) should not touch any part of the circuit, or worse, falls in dust...Use a large opening jar (a clean one!) with lid if you have one.

Third step: preparing diffusing surface

    LEDs are economic to the use, but only produce a small dot of light. So there's a need to make a diffusing surface to distribute light on all the screen's area. At this point Plexiglas takes its use.

1- Cutting the Plexiglas plate to the dimensions
    If you asked to have the Plexiglas at the prescribed dimensions (76x53), chances are the edges are flawlessly cut, not talking about time you will gain. If this is not the case, you will need to mark off a rectangle to the right size, then cut it out with the small metal saw or a Dremel cutting disc.

2- Plexiglas' "light holes"
Fig. 1: Plate's dimensions

    Important: if you messed up the reflector, skip there:

   Once you have the plastic rectangle, you must cut the notches indicated on the scheme. To do this, it is recommended to use a Dremel mounted on the drill press attachment perpendicularly to the column. At first speed, cut along the vertical lines. To make the notch itself, cut a bit more to the inside of the marked off area, and continue until you reach the other end of the notch. this method may seem slow and imprecise, but it is not. The plastic is soft and is easily cut with the Dremel, even at minimal speed. To avoid a "sawtooth" effect on notch's edge, use the file. Cardboard nail files do a great job.





Fig. 2: Details of the "sawtooth" effect after cutting out the notch





3- Preparing light's diffusion
    The plastic plate must be prepared to diffuse light. Remove the protective protection sheets on the plastic. As is, Plexiglas lets light flow as through a window. It has to be prepared to make it translucent and allow it to diffuse it (!). On a sanding block, place a piece of 80 sandpaper, and sand the two sides of the Plexiglas. Finish with the 150 sandpaper. If you don't have a sanding block, you can use either your hand :), or a flat piece of...anything, in fact :-).
    You should now have a translucent plate, with a frost finish on the two sides. If this is what you've got, well, that's good since this is what we wanted to have.

4- Directing light the good way!
    The plate you just prepared has an inconvenient however, it diffuses light on the two sides. A LED is not very powerful, so you need to make one side of the plate reflective (or at least non-absorbent). First method: if you kept the original reflective sheet, see scheme to guide you, forgetting the tape (just replace it by any thick material to make pressure-holding possible); glue all the reflective surface evenly, otherwise you'll get ugly traces when drying. Why glue it although I'm talking about pressure holding ? Because if you unstuck the sheet as indicated, chances are it will roll up when freed, at a point where additionnal support may be needed. Second method: follow the scheme, and forget the reflector :) stick the foam tape it on the back of the Plexi plate, all of the "layers"  holding by pressure. Additionnally, according to the adhesive colour and if you want to use it to keep the LEDs in place, you'll either stick it to the screen or the epoxy. I don't recommend gluing it on both sides, because if you misplace it, it won't be easy to correct the situation. Third method: uses a combination of the two we saw, and is represented below, all layers holding by pressure. I will advise you to use this last solution for its versatility. Fourth method: maybe the most barbaric, but results are not that odd: the point is to replace any of the two sticking foams seen before by a rectangular foam piece (same dimensions as the plexi) cut in relatively thick foam, the goal being eliminating the inconvenience of the wrapping reflector evoked in the first method. Why do I only think about foams ? Simple. Physically, foam is the perfect material to distribute pressure evenly, which is essential when pressing a fragile structure such as a glass screen. When the thick foam is compressed (from 1~3cm to about 3mm), it it exerts an even pressure on the epoxy board and under the Plexiglas while attempting to take back its original size, and this pressure is normally enough to flatten the reflector (if you use one).

Fig. 3: Third method to direct the light - Most versatile.

     If you messed up the small sheet, you can use the method 2 to 4 described above, or another one (which is not as ugly you can think :)), a bit less durable however. If you don't have this double-sided foam tape, you can either paint the back of the Plexi plate with silver paint (be sure to paint the plate before cutting the light holes !), while shaking the can sometimes to avoid lumps, or simply use a white surface behind the back of the Plexiglas plate. To do this, you can use a mere sheet of cardboard or paper.



Fourth step: preparing the LEDs

    This step is optional, but if you couldn't choose the LED size you wanted, like me, or work with Plexiglas less than 2mm thick, chances are even the thinnest LED's side will be too thick for the Plexi. So you will need to resize LEDs' case to adapt it to our needs. Here, I strongly recommend, if you chose blue LEDs, considering their price, you first try on a less expensive one, such as a red one.
    Admitting you have a 5mm dia. case, which I do, you must reduce the total thickness by 3.5mm (1.75mm each side) to use it with 1.5mm thick Plexiglas. With a Dremel mounted across on the drill press attachment, start it to speed 2½~3, and start to trim on the side, as indicated by the black arrow on the first photo. Cutting disk path should be similar to the one indicated below in dotted line. I recommend you hold the LED by the leads with long nose pliers, very close to the case to ensure the most straight cutting (I done it a bit crooked :-p).

Once you cut both sides, file (with side of the cutting disk, never forcing) to have the flattest possible surface, but don't exaggerate: when you reach the electrode as below, it is time to stop !

Picture 2: Close-up on the LEDs

    If the LEDs are too long or you want to give a particular shape, proceed the same way. They will remain usable unless you hit the active part, which lets a good margin considering the relative size of the case reported to active part's one.

    You obviously have to repeat the operation afor every LED you have...






Fifth step: preparing the lighting surface and installation

    Now you have all the parts in hand, you must assemble them. This will be pretty easy  if you followed up my advice to the letter, particularly concerning work neatness (not skill :-p). In this step, you only have to settle the LEDs in place, and to link them by wires.

1- Installing the LEDs
    There are many ways to proceed; if you used double-sided foam tape, use the excess adhesive material covering the notch to stick the LEDs on it, while doing a small notch in it to let leads go over. If you used paint, reflective sheet+paper glue, thick foam with our without white paper under the screen, place LEDs in their notch, and fix the sides with a drop of cyanoacrylate. Tubes are small, and their content is toxic, don't put too much glue, just enough to secure it. Don't forget to let exceed the leads at the back.

2- Connecting the LEDs
    Here is no rule of thumb to follow, it depends on the diodes voltage, essentially. Calculator is supplied with 6V max. battery power (alkaline batteries). If you want to avoid using a resistor, you have to know the maximum diode's voltage (the one where the luminosity is maximum for the lowest consumption). If you have enough luck to be in my case, the LEDs are 6V ones, you only need to link them in parallel respecting the polarity. If you don't know anything about your LEDs (I advised you to ask...), a 3V power supply is generally enough to light them up, at worst 2V if they heat up a bit, but nothing fatal. For those which have a 3V requirement, link two of them in series to divide the voltage, then link each set in parallel with one another if you have more than two LEDs. For example, if you have six 3V LEDs, you should have 3 sets of 2 series LEDs each..
    The same goes with 2V LEDs, group them by series of 3.
    For the terminal wires, don't forget 1- to respect polarity, 2- to cut them to a sufficient length.

    On the other hand, if your LEDs require a precise voltage to light up, ask for ALL the technical specs you need to calculate the resistor value (if any) to place in series in the circuit.

    Once finished, park the wires by putting them behind the foam tape or the reflector, connection wire should be visible at the plate's side or bottom.

Finishing lighting surface and case before definitive mountPicture 3: It's easy to distinguish the 6 LEDs linked in parallel. The Plexi plate needed some cutout (represnted by the yellow marks) to fit in its housing without effort. With the same goal, the latter has been thinnered with a small cylindrical drill (yellow dotted lines).

3- Installing lighting surface

    When plate is ready, here's time to fix it behind the screen. If you used the double-sided foam "reflector", it will be a piece of cake, just remove the other side's protective layer and stick it (line it up properly) onto the circuit board where the old glue was (optional). If you used single-sided foam or standard thick foam, go to the next step; screen's glass will be folded over and the whole assembly will hold simply by pressure once reassembled.

Sixth step: assembling and finishing

    Maybe you don't want the backlight to eat up your batteries like I drink Coke...So that's why a switch becomes useful. For the most rudimentary, you can recycle a slide switch and make a hole on the side of the calculator to access it, but it's not very beautiful, nor much practical. Those of you who care a bit more about their calculator will sacrify the practical aspect installing the switch in the battery compartment, the same place as a noverclock (it is possible, however, to use one switch of the microswitch).

    On the other hand, you have the hard but aesthetical and practical option I'm going to describe.

    First you need a momentary microswitch. The case is usually pretty large compared to the size of the trigger itself. Its small size and the very short run required to change its state are very interesting here. The point is to let the trigger exceed from the calc's case in a way that a small pressure on it produces a click. But there should be no need to depress the whole way to make this click, and we are going to see why. The trigger is normally pushed a bit downward when you put the slidecase onto the calc's back, in a way that the backlight can light up automatically when you are in operating position.

    For those who cannot stand to play with the slidecase slid behind, I'll tell you a place where to put the trigger so that it can be pushed while holding the calc. It is possible to do that if you let the trigger exceed on the case's side, level with the [ALPHA] key, at the bottom side of the case. You only have to make a hole the exact same size as the trigger (for an aesthetical purpose only) at the mentioned place. However, with most microswitches in sale, the projecting you can obtain won't be long enough to allow depressing as desired. If this is your case, you must attempt to thin up the case's inside wall to narrow the distance between the outside of the switch's case and the outside world :-). If you use a Dremel, you need the high speed cutter #194; it has a cylindrical shape, the same diameter as the axis (1/8").

1- Preparing the case
    Begin the work by marking out the length of plastic you need to thinner; chances are it will be pretty important; actually the same length (at least) as your microswitch. Rough out firmly holding the Dremel with one hand, the cutter parallel to the working surface, working by small touches not to wear out the plastic too fast. On the enlarged picture below, original plastic level is shown under the double dotted grey line, and the level after work, in simple grey dotted line. Be careful when working close to the case's recess (to the left  of the yellow dotted line, I know it's quite thin...), it's less thick than the other case's walls, and it is easy to underestimate the remaining plastic's thickness and to start making a hole in the case (next picture; it's not as serious as it looks just by looking at the flaked off paint; it is easy to cure, the real hole is coloured in tan – I made it bigger to scare you...;-)). Use hard filler (modeler's one) to fill up the crack.  Finish up using the X-Acto or a nail file. On the outside, use caution if you need to remove excess filler.
Picture 4: Lower half of the case prepared

Picture 5: A crack in the case - easily filled

    At last, all that treatment may be insufficient to make the trigger exceed enough from the case. Don't give up now, in that case, you must work on the switch's case. A drill press mounted Dremel will work fine if you mount it across with a cutting disk. The switch's plastic is particularly hard, at speed #3, slightly grind out the side where the trigger is, without forcing (cutting disks are very fragile). Don't worry if the trigger comes out, it doesn't change anything to our needs, since it will be took in sandwich between the calc's inside wall and its location in the µswitch. Just pay attention not to lose it, since a reliable replacement is almost impossible to make (too small for ordinary tools).
    Once you have the correct size, tin the switch's connections, then glue it down (don't forget the trigger if it came out!) with cyanoacrylate on the inside wall of the calc's case. Put some more drops of cyano on the switch's worn side for better adherence.

    When you'll try to reassemble the two parts of the case, maybe you'll need to make a very thin notch in the plastic, as on the photo below; otherwise this part may exert pressure on the switch's side, which is not desirable as it can wear out the bond between the two cases. Use an X-Acto to remove some plastic. On excise l'excroissance excessive...

Picture 6: Upper half of the case notched




2- Connecting switch and LEDs together
     Now you have to solder a wire from the "+" labelled pad to one switch's connector. Then link the "-" pad to the LEDs' network "-". Carefully reassemble the calc once you have reinstalled its circuit and the batteries, then make a test: press the trigger, you should see the LEDs lighting, and turn off if you release it. Also if you put the slidecase in operating position, you should see the light. If it doesn't work, find your mistake !

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