Connect with us

MAX7219 and higher voltage LED displays - how?

Discussion in 'LEDs and Optoelectronics' started by yesyes, Mar 10, 2011.

Scroll to continue with content
  1. Bongo

    Bongo

    9
    0
    Dec 9, 2011
    Yet again, thanks Steve for continuing to offer your help!

    I will have to see if I can get someone in my university's electrical engineering department to allow me to use an oscilloscope.

    I suspect you are right that the problem I am seeing has something to do with the MAX7219 acting as current driver/sink and how that current is interacting with the transistors in the 2803 is the issue here.

    I have also ordered the transistors needed to test the circuits described in the MAX7219 technical note about driving higher voltage & current led arrays: http://www.maxim-ic.com/app-notes/index.mvp/id/1196. Maybe this will solve my ghosting problem, though it means I will be using individual transistors rather than 8 channel ICs for each digit and each segment circuit.
     
    Last edited: Dec 14, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Figure 4 in that document is revealing as it suggests a resistor is required to turn the digit driver off quickly.

    What you're seeing is what I assumed to be an example of the digit drivers not turning off quickly (hence my first suggestion of pull down resistors).

    I do mote that the circuits supplied do not seem to concern themselves with the actual levels, so these may not be an issue (i.e. my second concern about slew rate interaction).

    If you can gain access to a scope, I think it may help greatly.
     
  3. Bongo

    Bongo

    9
    0
    Dec 9, 2011
    I have decided to go the route described in the Max7219 technical note I referenced previously. I have a tech support inquiry into them to be sure I identify the right components for my situation (i.e., led strips running at 12V with integral current limiting resistors; relatively high power requirements, etc.).

    I may end up buying an entry level, portable oscilloscope so I can observe the signals from the Max7219 that will be driving whatever circuit I use.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Let us know how you get on. If you ever manage to diagnose the issue with the other circuit, I'm sure that others will be keen to benefit from your experience.

    (Or send it to me and I'll have a look -- only half kidding :))
     
  5. Bongo

    Bongo

    9
    0
    Dec 9, 2011
    Will do. Thanks Steve for all of your help to this point.
     
  6. paulinho19

    paulinho19

    2
    1
    Jun 1, 2012
    Hi everyone,

    I assembled the same circuit and got the same ghosting effect, after some datasheet reading I found out that the problem is the turn off time of the UND2981A, it's much slower than the ULN2803. If you replace the UND2981A with a Toshiba TD62783AGP the ghosting disappears and everything work fine. =D
     
    Matheus Scotton likes this.
  7. marmitas

    marmitas

    11
    0
    Oct 9, 2012
    Hello, this is my fiest posting.
    I am building a 4 digit seven segment display using a 20 pin PIC (Picaxe), a MAX7219. The design is based on the one posted by YESYES where he uses two ULN2803 to drive the common cathodes. Since I use only 4 digits, I wonder if I can use a single ULN2803 as shown on the attached schematic :confused:.
     

    Attached Files:

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    I don't see any reason why not. The common connections are... common.
     
  9. paulinho19

    paulinho19

    2
    1
    Jun 1, 2012
    Yes I also think it will work.

    Here is a Pic of my 8 digits.
     

    Attached Files:

  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Nicely done.
     
  11. marmitas

    marmitas

    11
    0
    Oct 9, 2012
    Very nice job Paulinho. I have looked around for large size digits (about 8 inches) and found the prices very high. What you did with regular LED is an excellent idea.
     
    Last edited: Oct 10, 2012
  12. eagle22

    eagle22

    9
    0
    Feb 1, 2013
    I do not know if anybody is still following this thread.
    But,I found this thread and i have almost the identical schematic.
    The only difference is 12v external supply and a required segment 60ma with 9.2 v.
    My question is how to calculate the required pullup resistors between the uln2803's.
    I have a general electronics knowledge. Any help would be appreciated.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Assuming that 2k2 is appropriate for 5V, then if you are pulling up to 12V, I would use 4k7.

    The value isn't particularly critical. The ULN2003 pulls down against them,, so just don't make them ridiculously small or you'll be wasting power for nothing.
     
  14. eagle22

    eagle22

    9
    0
    Feb 1, 2013
    thank you steve for your reply,i guess i am looking for a more general knowledge how the uln2803 works. for future projects that do not resemble this one as much. when calculating the required pullup resistance, what (in general) represents the source voltage and what is forward voltage. don't get me wrong, i like answers, but i just learning electronics and would like to learn more.
    thanks again.
     
  15. eagle22

    eagle22

    9
    0
    Feb 1, 2013
    Well, i am back. I have utilized the setup given in this thread. But i am surprised at the lack of voltage and current. I result 7V to a segment, but when i hook up some leds there is a lack of current (30ma with a 1 ohm resistor). i am using ledcontrol library with intensity set at 15. i was testing to determine voltages and current before planning required leds per segment. Why the lack of current for 12v 2A external source?
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    The questions are:

    0) what current do you want?

    1) what is the Vf for each segment? (measure it)

    2) is there anything else limiting current (show us the circuit diagram you're using)?

    I've looked back and there are a number of circuits, I need to know exactly what you're using.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-