7 segment led very dim

Discussion in 'The Projects Forum' started by djstar, Mar 13, 2011.

  1. djstar

    Thread Starter Active Member

    Jan 26, 2008
    37
    0
    Hi i am making an electronic dice and ive got a 555 astable, with its output connected to a 4510 and this is connected to a 4511 bcd-7 seg. This is then connected to a standard 7 seg led (http://www.datasheetcatalog.org/datasheet/hp/HDSP-5507.pdf). The cicuit is ran off a 9v battery.

    I built the circuit and it functions correctly. However i wanted to use a larger led display so i brought a (http://docs-europe.electrocomponents.com/webdocs/020c/0900766b8020c616.pdf). However when i connect the output of the 4511 to the display it will only display numbers 1-5, the display has no segments lit up every now and again. im guessing this is when it is spose to display the number 6. when it does display the numbers 1-5 the diplay is very dim and them it takes about 5 seconds for segments to increase from being very dull to being full on.

    is this because the ic can not deliver enough current to drive the display?

    any help would be appreciated because im sctraching my head at the moment.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,054
    Check your battery voltage while everything is operating. Good chance it's not enough. You might try a 5v supply such as a phone charger or USB supply, something that can deliver more current than a 9v battery.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    9v batteries (alkaline) are only rated for around 500mAh-550mAh. NiCD and NiMH are rated for much less. They go dead pretty quickly.

    Without a schematic, it'll be hard to give you more advice. Posting a schematic helps us help you.
     
  4. djstar

    Thread Starter Active Member

    Jan 26, 2008
    37
    0
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    We need a detailed schematic between the 7447 and the LED. That is where the problem likely is. There should be one resistor per segment, and none between the common lead of the display and the power supply.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    There is a detailed schematic of the 4511 to 7-segment LED at the end of the PDF.

    The original 7-segment display likely had only one LED per segment.
    The replacement 7-segment display has four LEDs per segment.
    If=20mA, Vf=...
    2.0v-2.5v - High efficiency red
    2.2v-2.5v - green
    2.1v-2.5v - yellow
    1.8v-2.5v - Super bright red
    So, multiply those Vf numbers by 4. I don't know which color you purchased. I suspect green.
    4x2.2v = 8.8v, which is more than most 9v batteries output under a light load.

    Looks to me like you'll need a 12v supply. I'll suggest using eight AA batteries in a holder. The existing 330 Ohm resistors will give around 12mA current through each segment, and should be plenty bright enough to see.
     
Loading...