Calculator Backlight Project

My eyesight was never 20/20.
I recently purchased a new calculator (a TI-89 Ti) that does all sorts of fancy stuff that I need for a graduate class.
The trouble is, I can't read some of the small fonts on the LCD screen.

So, I was thinking of adding LED lighting internally to the calculator to make it more readable, especially in dim places.

There is very little information available on the internet for this type of project, and most of it is more than 5 years old.
Has anybody in CPF ever done this?
What color LED's should I use?
How do they light up the screen on an ipod?
---Marc

This has been done before, or at least I have read about attempts to do so. You might try TI's discussion group on their site, www.ti.org or some other sites that I don't remember. I think they have a links section at ti.org.

Depending upon what you are doing, the HP is usually a better calculator. I have the 49G (the next-to-latest blue one). It has a coice of fonts, all of which are easier to see than the '89's.

Scott

I added LED backlighting on two bike computers. In both cases for power I made a wire with a plug at the end to fit a matching receptacle on my headlight which was connected to the headlight batteries. The CR2032 in the bike computer obviously wouldn't have lasted very long powering a backlight.

The problem with doing a project like this is that most LCD displays are reflective, meaning that they have something like aluminum foil to reflect the ambient light. In order to backlight such a display you must peel off the aluminum foil and replace it with transreflective material. I got mine from dead transreflective taximeter LCD displays. This material reflects ambient light like the foil, but also allows some light to pass through. After that you can decide how and where to mount the LEDs for the most even lighting. I used hot melt glue as a diffuser. Clear silicon works well also. As to color, that's up to you. One of my bike computers uses yellow-green SMD LEDs. For the other I cut down a few 5 mm ocean blue LEDs so they would fit. Pretty much all that's left is the lead frame and a little epoxy around the die. Ocean blue is also called Tokyo blue, with a wavelength of about 490 nm. I think it's a very pleasing color to backlight a display with. I would suggest you avoid red, orange, and yellow. The display won't appear as constrasty with red or orange. With yellow the contrast will be OK but I think it looks too much like an incandescent. You can't go wrong with blue, cyan, or green. White will look OK also, and in this use I think the bluish whites will look a little better since the display transmissive film tends to yellow the light a bit. The net result of bluish-white LEDs and the yellowing effect should be a pure white.

I can't help with specifics on that calculator, but hopefully what I wrote above will be a good guideline.

Here's what the ocean blue backlight looks like (the white parts are where the LEDs are located):

Things are pretty tight inside the calculator. There is NO way to place LED's or an electroluminescent sheet behind the LCD. The LCD is glued to a block of aluminum that is glued to the the PCB. It's not going to come off.
Therefore, the only way to get light on the subject is to put LED's at the edge of the LCD. I'm going to get a bunch of LED's and give it a go.