7 segment display problem with PIC microcontroller

Discussion in 'The Projects Forum' started by djmxyz_1991, Jul 25, 2013.

  1. djmxyz_1991

    Thread Starter New Member

    Jul 25, 2013
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    I have connected 7segment displays to pic microcontroller directly.But 7 segment glows brighter at minimum 10 mA while microcontroller is giving out only 4 mA.Is there any way by which this current gets increase or is there any special 7 segment displays for microcontroller??I dont want to do transistor switching.plz suggest something else ??
     
  2. odinhg

    Active Member

    Jul 22, 2009
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    15
    Could you share with us what microcontroller you are using? Maybe you could use multiple IO-pins for each LED if you really don't want to use external transistors.

    However I do think using transistors to switch your LEDs would be a better solution.
    Or if you only need to display numbers, you could use a BCD to 7-Segment Decoder IC to drive your display(s).

    Like 7448 for example:
    http://www.physics.rutgers.edu/ugrad/327/sn74ls48rev5.pdf
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    The PIC processors I use all have much more current capacity than that--I think it's 25mA. If you're using one with only 4mA available, I suggest switching to another!
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can get the "special displays" you asked about. They cost a bit more but you can buy high brightness (or high intensity) 7 segment LED displays, many types will be very bright running on 4mA per segment.
     
  5. djmxyz_1991

    Thread Starter New Member

    Jul 25, 2013
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    I am using PIC microcontroller with 25mA and 3.3V ratings on i/o pins but whenever I connect 1 i/o pin to Vcc of 7 segment display and other i/o microcontroller pins to all segments,current gets dropped to 4mA and voltage to 1.8V.Although it is glowing properly but not at full brightness.Can you suggest me how to increase this current level or are there any special displays available?which are they?I dont want to use multiple i/o pins for glowing 1 led nor i want to use decoder and also dont want to use transistor switching.Please suggest something.
     
  6. djmxyz_1991

    Thread Starter New Member

    Jul 25, 2013
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    can you suggest me which are those special displays for microcontrollers.give me its model no or company or serial no??
     
  7. djmxyz_1991

    Thread Starter New Member

    Jul 25, 2013
    8
    0
    I am using PIC microcontroller with 25mA and 3.3V ratings on i/o pins but whenever I connect 1 i/o pin to Vcc of 7 segment display and other i/o microcontroller pins to all segments,current gets dropped to 4mA and voltage to 1.8V.Although it is glowing properly but not at full brightness.Can you suggest me how to increase this current level or are there any special displays available?which are they?I dont want to use multiple i/o pins for glowing 1 led nor i want to use decoder and also dont want to use transistor switching.Please suggest something.
    [​IMG]
     
  8. djmxyz_1991

    Thread Starter New Member

    Jul 25, 2013
    8
    0
    I am using PIC microcontroller with 25mA and 3.3V ratings on i/o pins but whenever I connect 1 i/o pin to Vcc of 7 segment display and other i/o microcontroller pins to all segments,current gets dropped to 4mA and voltage to 1.8V.Although it is glowing properly but not at full brightness.Can you suggest me how to increase this current level or are there any special displays available?which are they?I dont want to use multiple i/o pins for glowing 1 led nor i want to use decoder and also dont want to use transistor switching.Please suggest something else
     
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I have to guess from your description what you are doing and it isn't good. Provide a schematic diagram of your circuit configuration.

    I haven't heard any mention of dropping resistors. 1.8Volts leads me to think there are none. That is the voltage drop across your red led segments.

    You are using I/O Pins for anode and cathode control.

    If you are not multiplexing don't bother. Just tie the commons to the power rail for a common anode or the ground for a common cathode type of 7 segment display.

    If you are trying to switch the common through a microcontroller I/O pin then you have just discovered why you need a transistor if that connection is switched.

    With all of those 4mA sections of LED's on, that is 7x4mA = 28mA that is piped through the pin that is controlling the common. It is maxed out. The segments have to share common current and that is why they are starved and can't get the expected 25mA per segment pin. Having the common connection on an IO pin is your bottleneck.
     
    Last edited: Aug 3, 2013
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Potato Pudding made a good point that you need to show us a schematic. :)

    Re the type of display, google for "high intensity" or "high brightness" types. They give a lot more brightness per mA, so can be used as very bright displays, or instead you can run them from reduced mA and still get a decent usable brightness.

    The unit of LED brightness is called a mCd (milli Candella) and when you are comparing displays then one with very high mCd will be a high brightness type. If you try element14 or Digikey where you can see a lot of displays and their specs in a table, sort it by the mCd column which will sort all the displays in order of brightness.
     
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