I'm guessing the easiest way to do this is to to just solder it right to the LCD power connections. My question is, which of the connections is positive and which is ground, so I know which way to put the LED?

Well you'd most likely need a least a resistor in there with the LED to keep it from draining your battery to quickly, but I'd also suggest you include an on off switch for the same reason. Also for the calc a front light might even be a better option though back lights have been done.

Also you may want to have a look at http://www.michaelv.org/programs/calcs/calc_mods.php to see how the calc god Michael V did his very well done backlight mod to an 86.

Thanks, that's very informative. Would an LED right up against the glass of the LCD, but under the case of the calc, work?

Let me see if I can hunt down my detailed information from the Ultimate Calculator 1 about backlighting... Also, I definitely don't recommend you take from where the ribbon cable attaches, as that has a small voltage regulator. Take from where the battery terminals are at the left side of that image.

Info: http://www.cemetech.net/projects/item.php?id=22
Build log: http://www.cemetech.net/forum/viewtopic.php?t=2217

I don't see the exact details, but I used rubbing alcohol and a thin plastic card to pry the big metal ballast chunk off the back of the LCD, removed the silvered paper backing, and used more rubbing alcohol to remove the adhesive on the back. I then cut a screen-shaped plexi sheet to put behind the LCD, sanded the back of it, and put an insulated foil sheet behind it as a reflector. My LEDs are at the bottom edge of the plexi sheet behind the LCD, pointing up into that edge and across the underneath of the LCD.

I was hoping to have the light power on and off with the LCD. Could I use a small transistor attached to the LCD power as a switch?

Yes, you could. However, I believe that LCD power is not necessarily switched at that cable. I'll test tomorrow with a multimeter to see if that is indee the case.

Im just throwing this in there... But with a backlight, how could you see the keys? Its not like a gameboy with 6 obvious buttons. If it was possible, I think somone should some how fabricate clear buttons and have a light behind the keys and have it connected to however the screen's light turns on. Just a thoguht.

An excellent point, but I know at least for me back in the day, I had my calculator's keypad way more than sufficiently memorized to touchtype. I could have programmed with my eyes closed; I knew every key sequence like [PRGM][left][8] == ClrHome. It would be cool to fabricate backlit keys though; I wonder how that could be done.

Some CAD, some clear plastic (or thick fiberglass) and a three-axis mill? I only ever played around with wood and machinable foam, but I cranked out models with a lot more intricacy (and detail) than a few buttons.

And I have access to all but the plastic there.

I probably do too, since I'm on very good terms with the shop teacher at my old high school.

Alternatively, Shapeways (if we wanted to bulk order)

I wonder if I could make a worm-light and just wire it to the battery compartment with a modified battery door?

That would be the easiest way to do it, definitely.

Jonimus, Elfprince: I am going to have to go back to solidworks'ing the Ultimate Calculator 3 case, then, and have a chat with you guys.