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driving a 4x4 LED display — Parallax Forums

driving a 4x4 LED display

Armored CarsArmored Cars Posts: 172
edited 2005-09-15 15:07 in General Discussion
I have a 4x4 LED display and I am trying to run it on an SX52.· I cannot figure out how to control it.· I can find no data sheet and the way it behaves defies all logic.· I have tried many combinations trying to find a pattern but to no avail.· I havn't been able to get a single dot the whole time.·· Any help will be appreciated.

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-14 13:06
    Any markings on the part?

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    Jon Williams
    Applications Engineer, Parallax
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 13:34
    Yes,

    S LT5107E

    Google only brings up 10 hits. Apparently it is made by Sharp and I have not been able to find a single data sheet. I have looked at other data sheets from other companies bu they don't match up to what this one is doing.
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 13:41
    I also cant get more than four LEDs to light up at the same time. Sometimes I can get 2 and 3, but thats it.
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-14 13:51
    I'm sure the display requires multiplexing.· I found this:

    http://www.datasheetarchive.com/semiconductors/specsheet.php?specsheet=1161489

    It says the display is common anode; so you should be able to start with that and determine the column connections, then the row connections.· After that, writing code to multiplex it should be pretty simple.

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    Jon Williams
    Applications Engineer, Parallax
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 14:01
    Ok, new problem. I figured out the pin configuration, it is totally different than anything else I've seen. Now rb.6 isn't sinking correctly. I'll try resoldering it and see what that does, but right now the connections look perfect.
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-14 14:02
    Perhaps you busted RB.6 during your experiments....

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    Jon Williams
    Applications Engineer, Parallax
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 15:24
    What does that mean?

    I just bought this thing, soldered the socket for the display on and started messing with the program.· The only place the board has been is on my desk next to my computer.· It has never been shorted, it is impossible for it to be shorted because the only way it can get to something else is through the LED display.
  • ElectronegativityElectronegativity Posts: 311
    edited 2005-09-14 19:38
    Hi Armored Cars,

    Do you have a resistor between the SX and the display?

    If the display can draw more than the absolute maximum current rating of the SX52 then it could burn out the pin

    For the SX20/28 the AMR is 45 mA, and the reasonable limit is 30 mA.

    I don't know about the SX52, but you might want to look it up in the data sheet.

    There is also a maximum total output for all pins, which is 130mA for the SX20/28.

    This thread went over a lot of these issues:


    http://forums.parallax.com/showthread.php?p=548249

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    I wonder if this wire is hot...
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 22:27
    Ok, so if the chip·burned out can I resolider a new SX onto the board?· I have never solidered SMT parts, but know it can be done.

    As far as the resistors how do I know what one to put in?· Or do I just guess and put on in that doesn't dim the LEDs too much?
  • ElectronegativityElectronegativity Posts: 311
    edited 2005-09-14 22:47
    Quoting Paul Baker from the thread I linked above:
    Now for LEDs, they have a voltage drop across them when they are turned on (Vf), so to calculate the correct value for your resistor, use: R=(Vdd-Vf)/I, where I is the desired current, not to exceed the maximum current for the SX pin, then choose the closest value of resistor that is higher than the resistance you calculated. Never drive an LED without a current limiting resistor, even if you are doing switching to limit the overall current, if there is a bug in you program, or if you program hangs while the pin is on, you will fry your LED and perhaps even the SX. If the number of elements you have to drive requires more than the total current the SX20 can handle, you will have to use a driver for the LEDs such as a darlington array. said...
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    I wonder if this wire is hot...
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-14 22:55
    No need to guess when Ohm's Law lets us calculate the value.· The spec brief I pointed to earlier says the Vf of the LED is 2.5 at a current of 10 mA.· The rest is easy:

    · R = (Vdd - Vf) / If

    ...

    · R = (5 - 2.5) / 0.01

    ...

    · R = 250


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    Jon Williams
    Applications Engineer, Parallax
  • Armored CarsArmored Cars Posts: 172
    edited 2005-09-14 23:34
    Looks simple enough. What does Vf and If stand for though?

    Also how about soldering SMT stuff?
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-15 00:22
    Vf = Forward Voltage
    If = Forward Current

    These are pretty standard electronics terms; you may want to pick-up an electronics text if you haven't done much circuit design. Not knowing that an LED requires a current limiter will do damage to nearly any microcontroller.

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    Jon Williams
    Applications Engineer, Parallax
  • ElectronegativityElectronegativity Posts: 311
    edited 2005-09-15 15:07
    I'm guessing it's an SX52 protoboard, so why not just buy another one?

    The board with a chip on it is $9.95, and the chip alone is $7.

    Considering the cost to set yourself up for surface mounting, and the high probability that you would destroy components while learning the technique, it would seem to make more sense just to buy a few of these:

    www.parallax.com/detail.asp?product_id=45304

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    I wonder if this wire is hot...
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