That's pretty cool, I've never considered the photo resistor before, very clever. I did something similar with my ti85 before it got bricked by leaky batteries, but I installed a switch, and like an idiot assumed that the screen would diffuse the light well enough for all practical purposes, it doesn't.

I've seen some other schematics for this kind of thing before, I think a mixture of this one, and instead of plexiglass, using the screen diffuser glass from a Game Boy Advance I saw used in another type of installation would probably give you the best and most professional results you can get.

Very well done, and thanks for this article Michael, it was definitely worth reporting on. :)

hmm, the photos of the calc in the case is an 83+, but isn't the calc you're prying the screen off of an 84+? did TI change the board layout of the new 83+'s, or am I just mistaken?

You are correct sir. That is the screen from an old (broken) 84+ I used as a "dry run" before I attacked by (working) 83+. I forgot to snap pictures of the 83 while I was working on it, so I went back and took some pics of the 84+, since I already had it apart and the screen assemblies are essentially the same (aside from the main board changes)

Now would this apply to any 8X calculator or just the 83 or 84? The 89TI would look stellar with some sort of backlighting. Now I believe that a sheet of plastic that can perform total internal reflection like a normal backlit screen would work too, such as salvaging the screen layer off a Game Boy Advance SP. The lighting isn't through enough when just using plexiglass.

I can think of no reason why it shouldn't port to any other 8x calc.

And yes, there are definitely better materials to use than plexiglass, if you can snag something better, absolutely use it. I was trying to work with common (and cheap!) materials.

Hmmm... Fiber optic sheet strips work wonderfully! And it is incredibly thin and you only need 2 LEDs instead of 4.

But I found out that cell phone chargers with Mini USB shouldn't be put into the USB port of the 84+... completely fried it.

"the backlight automatically turns on and off based upon ambient lightning."

I knew it was possible...could it handle 1.21 Jigawatts?

Seriously, though, this is a really neat project. I'm almost tempted to try it myself.

And here we thought you were dead!

?

Yes, Brandon isn't dead. It's an inside joke; you're not expected to understand it.

Thanks for posting!

mhespenh here,

If ya'll have any questions or comments feel free to shoot me an email or PM (mhespenh AT mhespenh DOT net).

Nice job I like how the photo resistor pokes out the case, in the photo it almost looks like an extra button.
if you powered the photo resistor voltage divider off of Vswitched then you could eliminate a transistor from your circuit because it would turn off when Vswitched turned off.

I thought about that, however the easiest place I found to tap into was a transistor on the calc mainboard which only provides +2V. I was worried about drawing too much current away from it. Using the battery as the power source ensured I didn't pull too much current away from other places it was needed.

Wow, actually, now that I think about it, thats a pretty good idea.

I might work up a schematic and post it as an alternate circuit. I hope you won't mind if I credit you?

I adapted these plans for my TI-86. Got everything working -great-, except I accidentally sliced part of the ribbon cable right as I was going to put the thing back in the case! Noooooooo :'(

Noooooooo...

At least you got it working, though.

How does the cut affect the display?

If you had read the article, you'd know that cutting the ribbon cable destroys the display!

I know that, but he said that he only cut part of it. It's possible that, for example, it would result in a few columns of pixels going dead.