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Help with 7 Seg display

Discussion in 'General Electronics Discussion' started by shaheen, Aug 18, 2010.

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  1. shaheen

    shaheen

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    Nov 1, 2009
    I want to operate a common anode 7 segment display (which operate at 12V), with a micro controller's output (which is 5V). The thing which i need is to convert 5V into 12V.
    I think there are two ways to do that,
    1: I use relays, but this is very expensive.
    2: I use transistor, but I don't know how to use transistor?

    Is there any other simple and less expensive method to convert 5V in to 12V?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Better make this a new thread

    What sort of 7 segment display is it. It's unusual that they wouldn't operate from 5V.

    The normal way is with a transistor. The common anode is tied to +v, each cathode connects via a resistor to the collector of an NPN transistor which has the emitter grounded. The base is connected (via a resistor) to the PIC (or other 5V logic) output.
     
  3. shaheen

    shaheen

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    Nov 1, 2009
    It is a big 7 segment display (9cmx12cm).
    What are the values of collector and base resistors?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    OK, I see. I realise that some monster 7 seg displays have multiple LEDs in series and require a higher voltage.

    The collector resistor is just what you would use to limit the current to a safe/desired value for the supply voltage (technically supply voltage less Vce(sat) od the transistor).

    The base resistor should be calculated to give sufficient base current for the transistor, collector current, and switching voltage. In your case, it would be (4 * HFE)/Ic. The transistor specs generally give a range of HFE, use the smallest in this calculation.

    You need to pick a transistor too. That largely depends on the current required for the display.
     
  5. shaheen

    shaheen

    28
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    Nov 1, 2009
    I found an IC ULN2803. please see the data sheet and tell me is this IC done the above job?
     

    Attached Files:

  6. shaheen

    shaheen

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    Nov 1, 2009
    The schematic of my circuit is here. Is it correct?
    7-segment display is common anode.
     

    Attached Files:

    • ckt.GIF
      ckt.GIF
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  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    To me it looks like pin 9 is not connected and pin 10 is connected to *exactly* the wrong supply rail.

    You haven't mentioned the current required for these devices, so I can't comment on the choice of device, but it is likely to be OK.

    Make sure that the saturation voltage of this device doesn't cause you problems.

    edit:if I make the text really small maybe you can't see my error
     
    Last edited: Aug 20, 2010
  8. shaheen

    shaheen

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    Nov 1, 2009
    Pin 9 is connected to ground(which is not shown).
    I uploaded another circuit, from which I got the idea to use this IC. please see that circuit and tell me can I use this IC?
     

    Attached Files:

    • ir.GIF
      ir.GIF
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  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Sorry,I got pin 9 and 10 around the wrong way I think. They're OK now.

    You haven't mentioned the current required for these devices, so I can't comment on the choice of device, but it is likely to be OK.

    Make sure that the saturation voltage of this device doesn't cause you problems.
     
  10. shaheen

    shaheen

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    0
    Nov 1, 2009
    What you mean by "device"? Is it 7 segment display?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Yes. The 7 segment display. Presumably the current required is small enough and the voltage drop across the driver won't affect it.

    What I'm saying is that in the absence of information from you, I think it will probably work.
     
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