64 LED matrix with 16 pins

Discussion in 'The Projects Forum' started by Jerry Donaldson, Jan 25, 2009.

  1. Jerry Donaldson

    Thread Starter New Member

    Jan 25, 2009
    1
    0
    I am trying to figure out how to drive 64 LEDS with 16 microprocessor
    I/Os.

    I understand I can tie all the cathodes tied together for each row and
    then drive each seperate column with transistor. The transistors
    can be turned on and off by the microprocessor I/O pins sourcing (20ma max).

    But an I/O pin can sink only 20 ma max and only 30ma max for each group of
    8. Since 8 LEDS may be on at any one time that can be 160+ ma.

    What would I use to supply the ground for the common cathode row lines?

    Software guy trying to do hardware.

    Thanks for any help.
    Jerry Donaldson
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You never drive devices directly with a uC I/O pin. Anything that fails means the uC gets fried, too. Use some device on the other side of a resistor like a logic level FET or a Darlington transistor to handle the current.

    How are the LED's to be arranged? And are multiple ones going to be on?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    2N7000 are N-channel MOSFETs that can work at logic level. They're available in a TO-92 case, just like transistors. You can drive the gates from your uC using 220 Ohm resistors, ground the source, and use the drain to sink current from the LEDs.

    You might also investigate Charlieplexing.
     
  4. Dan2

    Member

    Oct 16, 2008
    19
    0
    you dont have to have all the LED's running at the same time - make each one flash fast enough so you cant see them flashing.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
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