Backlight a TI-83+ Graphing Calculator

Posted by: mhespenh
April 25, 2008


Starting from the top: The objective here is to add some sort of backlight to a TI-83+ graphing calculator.

Theory of operation: The idea is simple; A photoresistor is connected to another resistor as a voltage divider, which is connected to the base leg of a transistor. As the ambient light decreases the resistance of the photoresistor increases, thus allowing more current to flow to the base of the transistor. When the darkness reaches a certain point (my LED cut on point), the resistance is ~50k ohms, which allows 1V to flow to the base, turning the transistor on, which turns the backlight on. This whole assembly is connected to the collector-leg of another transistor, whose emitter is connected to ground. The base-leg of this transistor is connected to a voltage source on the calculator's mainboard which is only on when the calculator is on.
Thus- When the room is bright, the backlight is off. When the room gets dark, the backlight turns itself on, and this whole thing will only happen if the calculator is on. A complete circuit diagram is provided a few inches below.

License terms: I very much believe that creativity and invention is best served by the free exchange of thoughts and ideas, therefore every part of this project is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License, for full terms please see my license page and the Creative Commons site for this license.


Bill of materials:

  • TI-8x Graphing Calculator
  • 4 x green (or whatever color you want) SMD LEDs
  • 2 x 2N3904 NPN transistors
  • Photoresistor
  • Resistors
  • Very thin wire
  • Plexiglass
  • Tin foil
  • Electrical tape
  • Glue (I recommend spray on glue)

    • Other things you'll need to have (or have access to):
  • Soldering iron
  • Helping hands w/ magnifying glass
  • Drill
  • Sandpaper
  • Quick-setting glue

    • A few things before we begin:
  • The resistor values I've used are not absolutes- you will need to change them depending on the darkness you want to trigger the LEDs at, the resistance characteristics of your photoresistor, the voltage provided by the switched source you use, and the Hfe of the transistors you use
  • I'll apologize up front for the terrible quality of the pictures, I have a crappy digital camera from the late 90's
  • The pictures and video do not do justice to the brightness of the backlight- it is easily viewable in complete darkness with the backlight only.
  • This will shorten your calculator's battery life, probably by about 25%
  • Click on any of the images to view them in full-size
  • If you break your calculator attempting this I cannot be held liable in any way, so please, BE CAREFUL.

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    Step One: Tear your calculator apart!
  • You can find instructions on how to do this very easily all over the internet, just do a google search.
  • BE VERY CAREFUL WITH THE BLACK RIBBON THAT CONNECTS THE SCREEN TO THE MAINBOARD. YOU CAN NOT REMOVE IT OR CUT IT IN ANY WAY. IF YOU SCREW IT UP IT CAN NOT BE FIXED
  • Once you get it apart you have to separate the glass screen from the metal backing it is glued to. DO THIS VERY VERY CAREFULLY. If you're going to break your calculator, you're going to do it here. Too much pressure at any place will cause the screen to crack. You can't fix that. I slid an exacto-knife under one side and slowly worked my way around the edge cutting, but not lifting. Eventually the screen will seperate from the metal piece under it.

    • Screen and metal backing still attached to mainboard
    An observant user at TICalc.org has pointed out that this is not an 83 mainboard- he is correct. This is my old broken 84+ which I used as a "dry run" when removing the screen, however the procedure is exactly the same for the 83+

  • Next, remove the reflective film on the back of the glass screen and discard it.
  • Remove the metal piece the screen was attatched to from the main board of the calculator and discard it. It is held in place with a whole lot of glue, but you don't have to be as careful with it, so it comes off easily.
  • Measure the height and width of the glass screen, you'll cut a piece of plexiglass to fit behind it in the next step.


    • Weird metal backing
    Step Two: Diffusing the light -
  • Find the thinest piece of plexiglass you can (I got 0.080" thick from Lowes) and cut it about 1/8" wider than the measurements you took in the last part of step one.
  • Cut four notches (two on each side) in the side of the plexiglass for the LEDS to fit into.
  • Rough up all sides of the plexiglass with sandpaper to diffuse the light from the LEDs
  • Grab your roll of tin foil and cut a piece to cover the back and sides of the plexiglass and glue it on, making sure to get all air bubbles out.
  • Glue the LEDs into their notches and wire them up according to the circuit diagram. I found it very helpful to glue the wires to the side of the plexiglass to keep them from getting yanked off their solder points- krazy glue (or another instant bond glue) is great for this.
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    Diffuser

    Step Three: Wire up the innards-
  • Find a suitable place to put the photoresistor where it can soak up all the light it needs unobstructed. Drill a hole for it, and hold it in place with some glue (I used a hot glue gun). Here's mine:


    • My photoresistor placement

  • Using the circuit diagram given, wire up the guts of the backlight, to do this, you will need to find "+Vswitched" on the calculator mainboard, a place you can solder a wire to that is not on when the calculator is off. Since this will vary from calculator model to calculator model, and mainboard revisions I will not go into detail here, except to say: Pull out your voltmeter and find one! Once you have found your +Vswitched, use its voltage to calculate values for R1 and R2
  • The diffuser you just made is going to take the place of the weird metal backing you removed and threw away a few steps ago. Slide the diffuser assembly under the glass screen and tack it down with some glue taking care to ensure it is square under the screen and fits inside the mainboard (ie doesn't run off the sides).
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  • When you're tapping into the batteries I found it easiest to solder directly to the + and - pads on the bottom of the mainboard, you can see them in this picture:

    Battery solder pads

  • Take all of your wiring and tuck it neatly in the edges between the mainboard and the plastic body taking care to ensure you haven't left any wires/metal exposed which might cause a short.
  • Close it all up and put your calculator back together and you're done!

    • Crappy photo in the dark