3 digit LED muliplexed display to 1 digit displays

Discussion in 'General Electronics Chat' started by chrischrischris, Mar 30, 2012.

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  1. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Hi.
    Could someone please help me in converting a 3 digit multiplexed LED display to 3 single 7 segment LED displays.

    I've bought a temperature display unit. Now I want to remove the 3 digit display and connect it via darlington (arrays or 2003a's pending cathode or anode), to three large 10" single digital displays. I can't work out the logic of the re-wiring.

    I know the single digits have a,b,c,d,e,f,g, a dp connection and either common cathode or anode connections (10 pins total).

    From what I see, the 3 digits units also have "a" to "g" (pins 11, 7, 4, 2, 1, 10 & 5) and pin 3 for DP, and pin 8, 9 & 12 for what I assume is the common anode or cathode connection. But how to I re-wire them?

    (PS. Also, do both pins 3 and 8 of a single digit led display have to be connected to either power or ground (pending cathode or anode), or can you connect 1 only?)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Generally the reason displays are multiplexed is to save on parts, and there is also some saving in power used. Conversion is easy enough, you will need 3 latch and hold ICs. Since the display is already decoded you could capture the decoded output, hold it, and use it to drive the digits.

    A schematic is really needed to define the circuitry though. You will need access to the clock and the driver/decoder circuitry, as well as to the LEDs themselves. I am assuming the circuitry has already been built, and you are modifying it.
     
  3. chrischrischris

    Thread Starter Member

    Feb 18, 2012
    283
    10
    Hi Bill.

    The 3 digit display unit is part of a temperature guage unit. This unit is only about 5cm x 2cm x 2cm. There is a 20 pin IC chip driving the unit, along with some capacitors, resistors and a diode.

    The 3 digit display has 6 pins each side. Looking at the back of the 3 digit display, pin "1?" (on the left) goes to R3, pin 2 goes no where, 3 no where, 4 to R2, 5 to R3, 6 to no where. At the base, the pin 7? (on the left) goes to no where, pin 8 to R8, pin 9 to R7, pin 10 to R6 (which actually doesn't have a resistor!), pin 11 to R5 and pin 12 to R4. With the pin numbers above, I may have them back to front. There may however be links on the PCB between the digit displays and the board???

    By applying power to each resistor I just worked out:
    R4 = "e"
    R5 = "d"
    R6 = "nil"
    R7 = "c"
    R8 = "g"
    R1 = "a"
    R2 = "f"
    R3 = "b"
    It lite up the 2 right digit segments only (as the temperature is less than double digits). Does this help unravel my mystery. If not I'll draw up a diagram of what I can see.
     
  4. Wendy

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    Mar 24, 2008
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    The real problem is the clock. If you can find that it becomes easy.
     
  5. chrischrischris

    Thread Starter Member

    Feb 18, 2012
    283
    10
    I may be completely off track, but can I convert my 3 lots of single seven segment led displays to a multiplex 3 digit unit somehow, then just link corresponding pins:

    Multiplex setup.gif

    I googled an example of digital thermometer (see below) and it seems all the segments of each display are linked. Can I do this somehow?

    Example circuit off the net.gif

    Photos of the unit I have are below. I'm really hopeing there's a way:

    board 1 of 4.JPG board 2 of 4.JPG board 3 of 4.JPG board 4 of 4.JPG

    Alternatively, can I reverse engineer it by noting which pins light up at 1, 2, 3, .... 45. (I don't expect the temperature to get above 45 celcius in Austalia though it does sometimes get close! [​IMG])
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Yes, that would be the easy way all right. Just wire the 7 segment displays the way your multiplexed display is wired.

    You could boost the transistors current (new transistors) to allow full brightness for the new display units.

    I'll be back, I have something I want to show you, after I take some pictures.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    OK, I don't know how big you want your display to be, but this is a great example of what I was talking about...

    [​IMG]

    This board is an example of what I was talking about. It cost me around $1.60 from Tanner Electronics (a mom/pop shop in Carrolton).

    It could be done much larger with individual LEDs. I have some drawing in my stadium drawing of 1' (2'?) displays.

    I bought this to disassemble, the price was too good to ignore.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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