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Interesting Question — Parallax Forums

Interesting Question

piguy101piguy101 Posts: 248
edited 2011-07-16 13:52 in General Discussion
I was taking apart a scientific calculator and I found two flat objects in it. I thought "This might be an LED" and sure enough, when I got home I put a few mA in it and it lit up red. Here's my question: Why was there a LED or an object that acts like a LED in the calculator? It was at a position that you'd never see it light up unless you opened up the calculator, so what is the point of that object being in a calculator?

Comments

  • mindrobotsmindrobots Posts: 6,506
    edited 2011-07-14 13:33
    POC/Confidence test for the boards once they are complete but before they get put into the case? Why waste a case assembly if you have a bad board.

    {just a guess}
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-07-14 13:56
    It's possible it was just being used as a diode. I found one like that inside a Radio Shack CB radio once.

    I once built a power supply with 188,000 uF of filter capacitance. I put an LED and a resistor on the capacitor board, just to remind me if it still had a charge. You couldn't see it from outside the case.
  • piguy101piguy101 Posts: 248
    edited 2011-07-14 14:23
    That makes sense to be a diode, but LEDs cost more and handel less current than dark-emitting diodes.
  • xanatosxanatos Posts: 1,120
    edited 2011-07-14 17:13
    It wasn't connected to a light-pipe type of plastic was it? I was thinking a keyboard backlight maybe?
  • wjsteelewjsteele Posts: 697
    edited 2011-07-14 17:26
    More than likely it was a shared design board with several different calculators.

    Bill
  • piguy101piguy101 Posts: 248
    edited 2011-07-14 18:17
    xanatos wrote: »
    It wasn't connected to a light-pipe type of plastic was it? I was thinking a keyboard backlight maybe?

    It believe it wasn't connected to anything like that and the red light emitted from it was probably too dim for that.
    wjsteele wrote: »
    More than likely it was a shared design board with several different calculators.

    You know, I had never thought about that and that is probably why there was a LED in it.
  • wasswass Posts: 151
    edited 2011-07-15 23:28
    Was this in a calculator that has a solar cell as well as a backup button cell for power? If so, I believe that the LED is in there to drop the voltage from the solar cell so that it won't blow the low voltage calculator chip. The chip is made to run off of 1.5 volts and the solar cell can output 4.5 volts when well lit. HP uses this arrangement in their little 6S solar calculator and I've seen this on other small solar calculators as well.
  • piguy101piguy101 Posts: 248
    edited 2011-07-16 06:52
    It was from a calculator with a solar cell and a backup button cell. The solar cell did give off about 4.5 volts and by subtracting about 1.2 volts * 2 is about 1.5 volts. One problem: I THINK that the button cell was 3V.
  • wasswass Posts: 151
    edited 2011-07-16 09:28
    I THINK that the button cell was 3V

    Yeah, that would sort of kill my theory at least for your calculator. However the HP 6S solar uses a 1.5 volt button cell.
  • LawsonLawson Posts: 870
    edited 2011-07-16 11:55
    Naw, Wass, I think you're on the right track. But in this case I think the LED is being used as a cheap shunt voltage regulator. I.e. it limits the voltage the calculator chip sees by dumping any extra current the solar cell produces. With most red LEDs it'd start drawing a few uA at about 1.3v and be drawing 5-10mA by 1.7v. Basically the LED is used similar to how a zener diode is used, but the LED is cheaper.
  • piguy101piguy101 Posts: 248
    edited 2011-07-16 13:52
    wass wrote: »
    Yeah, that would sort of kill my theory at least for your calculator. However the HP 6S solar uses a 1.5 volt button cell.

    I just did a quick Internet search and this is the model: TI-30X IIS. I could be wrong about a 3 V cell, if anyone knows what cell that model uses please reply.
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