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Originally Posted by Chriszuma
I know this isn't exactly related to FIRST, but I don't know of any other place to get the opinion of hundreds of electrical engineers.
I'm going to mod my calculator, and light up the screen with a blue LED and some fiber optic cables. I can get the fibers from a cheapo Ikea lamp of something, but I'm having trouble finding a suitable led; the calc runs off 4 AAAs (6v, in case you're anti-multiplication). I really don't want to pay $3.29 for Radioshaft's 6v-max-rated led, but I can't seem to find any other 6v ones (and i'm beginning to suspect that they are liars). What would you suggest? Possibly only wiring it to 3 of the batteries somehow, or putting a voltage regulator on there? Maybe wiring two leds in parallel (I'm en electronics newb, if you can't tell).
Thanks for your help.
~Chris Hammond
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Use Ohm's law to figure out what size resistor you should put in the circuit. I assume that the LED wants 10mA, and the drop over the LED is somewhere around 2.5V, leaving 3.5V for the resistor. Thus, you need a resistor that is 3.5/.01 = 350 ohms, which you would put in series with the LED. If you have actual ratings for the LED, I would suggest using those, but this should be a good enough approximation. Also, a word of advice from
someone fairly involved in the calculator scene: be very careful when you're doing this. The inside of graphing calculators is very delicate, and while you're trying to remove the adhesive foam between the screen and the circuit board, you may very well rip off the ribbon cable that connects the two, and there is no easy way to repair this. Once you get past that stage, removing the reflective backing from the screen is also a pain, and you'll never quite be able to get all of the adhesive off of the screen, so it will always look smeary. I gave up, and ended up buying a
backlit TI-86 from someone who knew what he was doing. Nonetheless, good luck with your project.