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  1. #1
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    7-segment multiplexing or not?

    Dear Colleagues!

    I'm planing to build my MCP now and I'm concerned with the way to interface 7 segment displays. I have two different options:

    1. Using mupltiplexing device, such as MAX7219 I have already ordered to play with. It seems that multiplexing is used in most cockpit devices, such as cpflight or opencockpits MCP (as seen on youtube). MAX7219 allow to set brightness (actually, is it implemented in real Collins 737 MCP?). But how realy is it percieved by eye? Is 800hz enough for 8 digits? Another think I like there is that MAX7219 required only one resistor used to set current.

    2. Using CD4511 driver (1 per digit) and shift registers to reduce outputs used (I'm going to use arduino). I presume, this way excludes any blinking of the digits, but it, as I understand, does not allow to set brightnes and soldering work will be realy great.

    So, which way to prefer? Wich way did you prefer? I would appreciate any opionions and suggestions!

  2. #2
    75+ Posting Member skalarki's Avatar
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    Re: 7-segment multiplexing or not?

    Hi

    I am using in my panels MAX7221 - functionally same as 7219. Definitely this is right choice, 16 wires for 8 digits - no mess in wiring.
    Do not worry about frequency - these drivers are designed for displays and MAXIM engineers knows what are they doing. Human eye can't spot flickering with freq. higher then 50Hz (see normal lighting bulb is 50Hz).
    - Indeed digits can be digitally dimmed in 16 steps - no need for extra hardware.
    - One resistor for current limiting
    - Any other method is not worth wasting time, even if can be bit cheaper, like 8 transistors and microcontroller.


    Regards, Marcin
    SKALARKI electronics - everything for your new A320 cockpit!
    http://www.skalarki-electronics.eu

  3. #3
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    Re: 7-segment multiplexing or not?

    Thanks, Marcin. I also think, that multiplexing is better. BTW, what is the difference between 7219 and 7221? As I understand, basicaly 7221 supports more protocols. But is there any crucial difference else?

  4. #4
    75+ Posting Member skalarki's Avatar
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    Re: 7-segment multiplexing or not?

    See here for details:
    MAX7219, MAX7221 Serially Interfaced, 8-Digit, LED Display Drivers - Overview
    Only protocol difference, noting else. Very easy to use, but I had some problems on the beginning.

    Regards, Marcin
    SKALARKI electronics - everything for your new A320 cockpit!
    http://www.skalarki-electronics.eu