7 segment LED help please: Values of Resistors and Pins.

Discussion in 'General Electronics Chat' started by icegongju, Apr 26, 2011.

  1. icegongju

    Thread Starter New Member

    Mar 6, 2011
    Hello I have to create a mutliplexed 7 segment led display and right now I am completely lost on the values of which I should use for my resistors. Please teach me how I can do so.

    My current schematics is as shown in the attached picture. I used a latch to control which LED to be lighted up, then a BJT to pull the voltage up.

    What values should I use for R1-8 and what should I use for R9-16?

    I do not have any information of my 7 segment LED except for the fact that it is a common anode and its pin configuration.

    Also, I noticed there were 2 +5V pins on the 7 segment LED itself. Does that mean I have to connect both pins or do I choose one?

    Please help. Thank you very much! :)
  2. Wendy


    Mar 24, 2008
    Your schematic is seriously flawed. One set of transistors goes to ground, should be NPN, needs resistors, and is used for segment selection. The other set goes to Vcc (+ power supply), needs no resistors to the display (the other set of transistors takes care of that), and used for multiplexing. If you don't know what multiplexing is, you aren't quite ready for that circuit. Multiplexing allows you to only use one decoder for a set of displays, and it also saves current.

    Since the transistors are simple switches simple LED math will suffice. Read chapter one of the LED tutorial.

    LEDs, 555s, Flashers, and Light Chasers

    I have an idea how to fix the Vcc drivers. I'll post is when I get the chance.
    Last edited: Apr 26, 2011
  3. Wendy


    Mar 24, 2008
    OK, you need to replace the area circled in red with this driver.



    You have to remember, a display is nothing more than 8 diodes with one common pin, in this case common anode. Figure out the dropping voltage of the LED and the rest of the design is easy.

    Your original schematic did not have a connection between Vcc (which I assume is 5VDC, it was not labeled) and ground (which also was not labeled, and I am not sure was ground).