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MC14489 - driving higher current displays — Parallax Forums

MC14489 - driving higher current displays

xanatosxanatos Posts: 1,120
edited 2014-03-28 15:19 in General Discussion
Hi, I'm driving some large 7-segment displays that have several LEDs per segment. I'm using MC14489 chips but I will need to use external circuitry to actually drive the displays, since I need at least 8v and around 40ma to light them up. I'm thinking 2N3906 PNP on the 5 common cathode lines and 8 2N3906 npn on the segment lines. Am I missing anything in this scheme?

Thanks!

Dave

Comments

  • kwinnkwinn Posts: 8,697
    edited 2012-11-10 22:42
    The problem will be with the PNP anode drive transistors. With 8V on the emitters and 0V or +5V from the MC14489 on the base they will always be conducting. You will need NPN's to drive the PNP's so the PNP is only conducting when the MC14489 anode pins are high.
  • xanatosxanatos Posts: 1,120
    edited 2012-11-11 17:30
    More problems than just that, apparently... So I went with optoisolators (TLP781s since I had a bunch) on the Anode drive side (the MC14489 requires Common Cathode displays) to get the level translation taken care of, and that seems fine. It's the cathode drive side that's interesting. Testing the MC14489 revealed that the common cathode drivers in the chip are apparently open collector. So I have to use a pull-up on each of the five lines to get a 0v to 5v swing to drive the PNPs. And then, of course, you're right... 5v basically looks like ground to the display at this point, so whether the cathode was driving or off, the segments are lighting a little... ghosting. So I'm going with optoisolators on the cathode lines as well! If it wasn't for the speed that the 14489s can be updated with from the stamp, I'd just have used 595's and the isolators and been done with it! I'll probably figure out a faster, better arangement for all this once the PC board has been created and shipped... :-)

    Thanks for your thoughts,

    Dave
  • kwinnkwinn Posts: 8,697
    edited 2012-11-11 19:23
    No reason that the anode pnp's and cathode npn's cannot both be open collectors, and sometimes it's nice to have that info. Really like those older data sheets that showed "typical output' schematics.
  • Alejo83Alejo83 Posts: 28
    edited 2014-03-25 12:02
    Xanatos, can you please send to me the schematic to drive these high current display because I am doing the same project that you did with the same mc14489 driver.

    Thank you and I'm sorry for bothering!
  • xanatosxanatos Posts: 1,120
    edited 2014-03-25 12:16
    Alejo83 wrote: »
    Xanatos, can you please send to me the schematic to drive these high current display because I am doing the same project that you did with the same mc14489 driver.

    Thank you and I'm sorry for bothering!

    No bother, happy to help - if I can... I have to dig a bit to get that schematic. I'll post when I find it.
  • xanatosxanatos Posts: 1,120
    edited 2014-03-25 12:37
    Here's what I have. I never really created a full schematic, just some parts to help me figure out my PC Board layout :)

    So here's the schematic piece I made, and the PC Board layout. The left-most chip is the BS2xx I used. p13, p14 and p15 are running to the MC14489, and the three remaining "chip pad patterns" are really not chips, but are actually chip sockets stuffed with my TLP781 opto-isolators). The two at the top of the image are driving the segments, the one at the bottom right is driving the commons via a 2N2222 or 2N3904 - any GP switching NPN will do. The column of 8 pins at the upper right are the segment outputs, and the 4 below that are the digit commons.

    If you look at the pinouts for the MC14489 and the TLP781, you should be able to recreate the schematic. You should also be able to see where the discrete resistors go, and the few surface mount items go in between the pads in the BS2 and the MC14489.

    Good luck & have fun!

    Dave
    Board4Parallaxforums.jpg
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    DriverSchematic.jpg
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  • Alejo83Alejo83 Posts: 28
    edited 2014-03-25 14:52
    thank you so much! You really save me a lot of time and I going to be able to finish on time.... I really appreciate all your help.

    Have a great day!
  • Alejo83Alejo83 Posts: 28
    edited 2014-03-28 15:19
    I am sorry I know that you have helped me a lot and I would appreciate if you could answer to me the last two questions?

    1- is the 2n3906 transistor that is driving the common anodes a PNP?

    2- if I want to drive 4 single large segments with only one MC14489 do I just wire together all the segments of the four displays?

    Thank you so much and I swear this is the last time asking too much questions.

    Have a great day!
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