Dotmatrix Transistors

Discussion in 'General Electronics Chat' started by arthur92710, Jul 15, 2009.

  1. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Hi guys,
    Sorry I haven't been around for a while, I was very busy during most of the school year and had no time for electronics. But now im free.

    I am driving 3 7x5 dot matrix's(21 rows x 5 columns) with 3 4017 counters for the rows and a AVR uc for the columns. The leds become dimmer as more turn on. I knew that this would happen and its was not a problem while testing. Now I am designing a PCB and need some help adding a few transistor. I think that the best place to put some transistors is on the columns, but so far I cant get them to work. I need some advice.
    Circuit http://arthur92710.byethost31.com/elec/757hrmatrix.bmp
    Matrix http://arthur92710.byethost31.com/elec/dotmatrix.bmp
    *EDIT* copy the links into your browser, host is giving a linking error.

    Thanks guys.
     
    Last edited: Jul 15, 2009
  2. sissow2

    Member

    Jul 14, 2009
    16
    0
    looking at the specs on the counter, it doesnt look as if that thing can source a lot a current, so that may be where your dimming problem is coming from.

    Also, i would suggest putting some transistors on the gpio pins that are driving the dotmatrix columns, and possibly some small resistors on the collector of those new transistors to make sure you aren't drawing too much current from the counters upstream.
     
  3. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    I cant replace the 4017's, with higher current ones. How would I add transistors to the column? I have the all the columns (pin 1,3,10,7,8) connected to the other matrix's,so all 1s' are connected to each other, all 3's and so on. I am multiplexing the displays so only one row is on at a time and the columns are individually turned on. I have only 5 columns and 21 rows, so it would be better if the columns have the transistors.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    When I saw the thread title I thought it said "Dominatrix Transistors"!! :eek: I've seen lots of little transistors dressed in black... But never seen one with a whip and high heels! ;)

    Now more seriously, the circuit you posted shows column cathode, so the 5 columns are cathodes and the 7 rows are anodes.

    I suggest you change the multiplexing to 1:5 for better brightness, so you need one 4017 chip and 5 transistors (or a ULN2003 chip which has 7 transistors with resistors built in) to sequence the 5 column drivers.

    Then drive the 3x 7 anode rows with 21 AVR pins, and it's likely you don't have that many so you can use 3 shift register chips like the 74HC595 which only need 2 pins to drive all 21 anodes. In that case you can get rid of the 4017 chip, and use 5 AVR pins to drive the 5 transistors (or ULN chip) and 2 AVR pins to drive the 3 74HC595 shift registers. Total requirement is 5 AVR pins and 4 chips.
     
  5. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    I guess I can do that.
    I have also ran into another problem with scanning the 21 rows. They don't evenly divide by any prescaler.
    Originally I decided on 21 by 5 because thats the only way to use these matrix's on prototyping boards. I need 3 boards If I want to use them as 15x7. Oh well, :rolleyes: I guess Ill get started on that.
    Thanks!
     
  6. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
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  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Arthur,
    Attach your image to your reply. You can do this using the "Manage Attachments" button after clicking the "Go Advanced" button below the reply box.

    Your link to the new circuit doesn't work.
    .PNG format images are preferred over .jpg, as .png format is not "lossy".
     
  8. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Sorry its the host, you have to copy and paste the code into the address bar. Ill start using the .PNG format.

    Also, I was just reading through the data sheet and the rows should have been connected to the drain, and the columns should go to the shift register. This way we only use 2 shift registers.
     
    Last edited: Jul 15, 2009
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    That depends if the LEDs are "column cathode" or "column anode" the column is the 5. Normally that can be told from the part number on the LED display, I think "TC" series are col cath and "TA" are col anode, but that's from memory. The LED datasheet should say.
     
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