Multiplexing number of led-displays

Discussion in 'The Projects Forum' started by sureshparanjape, Sep 10, 2014.

  1. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
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    I wish to do a project which would multiplex 16 displays for a digital clock with date,day,month,temperature and time in hour and minutes.I was successful in a similar project using 16 x 2 LCD display and atmega328 micro controller. I have done search on internet and is acquainted(somewhat?!) with the basics of charlieplexing, multiplexing and 4 to 16 decoder. I have references which give circuits for a digital clock using fewer 7 segment displays. I have removed the requirement of display of seconds from this project to bring down the requirement to using 16 displays(not all of them 7 segment). I aware of the requirements of pin currents and total current for a micro controller. If these requirements are met for each of sixteen display that would be in the project, my level of knowledge fail to see any other difficulty that I WILL encounter in the project. Any suggestion/comment, from too ambitious/foolish to a helpful to undertake the project?
    Thanking you for your comment( adverse or helpful) in advance.
    Suresh Paranjape
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Sounds ambitious; good luck, and let us know how it comes out, or even better, let us know if you run into any problems.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    1) For a multiplexing has 4 to 16 decoder that it's as 74LS154, the output is low, then you should using 16 pnp to drive as "米" rice fonts leds. (Common Anode Type)

    2 rice font leds - 14 segments LED.
    2ricefontleds.jpg

    4 rice font leds- 14 segments LED.
    4ricefontleds.jpg

    2) For a multiplexing has 4 to 10 decoder that it's as CD4028, it's need two piece, the output is high, then you can using ULN2803x2 to drive as "米" rice fonts leds. (Common Cathode Type)
    [​IMG]
     
    Last edited: Sep 13, 2014
  4. MrChips

    Moderator

    Oct 2, 2009
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    As the number of multiplexed displays increase you encounter flicker and lower brightness.
    You can overcome these limitations by dividing the displays into groups and driving each group with its own dedicated microcontroller.
     
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You can use CD4511 segment drivers, they have a built in latch, so just load the digit and set the latch high, eliminates flicker, uses common cathode displays.
     
    Last edited: Sep 13, 2014
  6. SgtWookie

    Expert

    Jul 17, 2007
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    If you use a 14-segment LED display, you won't be able to reproduce all of the characters you have shown. The "B" would not be distinguishable from an "8" nor a "D" from an "O" or "0" you would have to use lower case "d", "b", and "o". The letters "K", "N", "Q", "R", 'V, and "X" would all appear somewhat different than shown.
     
  7. lightingman

    Senior Member

    Apr 19, 2007
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    Indeed, the CD4511 is the way to go. Avoid going down the TTL route as CMOS is so much more forgiving and on PSU issues.
    Just use one of these for each digit, tie all the data inputs together IE. A to A, B to B, C to C and D to D. Use a 1 to 16 line decoder to select the latch enable for each digit. You then have four data lines and four digit latch lines. The program will then be so easy to write.

    An alternative option would be to use one 4511 and tie all the segments together. You would then use a four to 16 line decoder and 16 transistors or a couple of 8 channel drivers to drive each digit. Your program will be more complicated, your displays will be a little dimmer and may flicker. You would have to send the data and relevant digit drive at the same time and in sequence for the whole 16 digits. You would have to do this in a loop somewhere in your program and at intervals that will give the least delay to reduce the flicker.

    Daniel.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    Thanks, I knew that.

    On the fonts table also had some other errors as D and 0 were double error, K, N, Q, R, V, X were segment errors, the B and D can be shifting two left segments to the middle, then B and 8 will be different, and D and 0 also will be different.

    The fonts table was linked from other website for reference, although I knew those have some errors for 14 segments, but I still made the link, because I was designed a program to display the fonts for preview, but I didn't copy all the fonts, if the op has interested in 14 segments then I will upload the file to the forum.

    I saw some 16 segments led long time ago, and the fonts type were look better, but I haven't buy it.
     
  9. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
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  10. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
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    Beautiful; many thanks for your suggestions. I hope that I can find rice fonts leds locally. Thanks again.

    Suresh Paranjape
     
  11. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
    2
    Thanks. We have a saying in my mother tongue- You try to make a flute using a carrot- if it works, good; otherwise eat it up! Here I can't eat the carrot but I would have enjoyed working on it!
    sueshparanjape
     
  12. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
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    Thanks. It would encounter flicker, as I have read on internet.I am toying and working out mathematics about the maximum current requirement for say group of four or solder 5 x 7 led matrix using leds available on a strip, which has 10 mA requirement for three. I will have more fun in writing sketch for these !Using more micro controllers would increase the cost.
    sureshparanjape
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    ScottWang, my post wasn't really supposed to be critical of you; it was more of a caution for "newbies" that it wouldn't exactly be as "easy" as replicating the fonts presented. There were drivers for 14-segment LED displays, but I can't remember the part numbers offhand, and they would have been obsolete shortly after LCD displays hit the market.
     
  14. ScottWang

    Moderator

    Aug 23, 2012
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    SgtWookie, I didn't blame you at all and I also thank you for your mentioned and complement about the fonts errors, whatever I knew or not, the true was that somethings wrong over there.

    I did the thing as you very often, so I also hope when my post had anything wrong, someone could point out, thank you.

    The fonts as below are matched the 14 segments led dispaly.(From Wikipedia)
    [​IMG]
     
    Last edited: Sep 14, 2014
    SgtWookie likes this.
  15. ScottWang

    Moderator

    Aug 23, 2012
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    If you can find the 16 segments led display, that is better than 14 segments.
    alpha-numeric-led-digit.gif
    - source

    Sixteen-segments led display.
     
  16. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
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    Many,Many thanks for all the helpful and constructive comments. These would keep me busy for quite some time.
    sureshparanjape
     
  17. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    why not use a 16f59 as driver for 4 digits. You need 16+4=20 I/O lines.
    8x multiplex is doable at 5 volts, with 24 IO.

    I use 8x of these chips to drive a 768 LEDs matrix.

    If you use CMOS or TTL its quite a hassle. And yes you need digital MOSFETs for the sinks (or the Anodes), if you drive them directly you get brightness variation.
     
  18. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
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    This forum is extremely useful, particularly for persons like me, who is totally outsider to electronics and no other avenue open for interaction with knowledgeable people being a 76 years old. I thank sincerely all those who have commented on my (silly) project. My hobby of thinking continues in this direction only because of my uneducated friend, who has experience in hardware electronics, acquired by getting shocks(!), helps me to build the circuit.
    He has soldered 7 x 5 led matrix, using led that come on strips. I have succeeded in arduino sketch to display all alpha numeric numbers on it, without using any shift register. The measured current used by one led is 0.7 ma. I have multiplexed rows, so that at most 5 led are lighted at a given instant. Therefore the maximum load of this is less than 5 x .7=3.5 ma.
    Assuming that the above is correct, Can I use such 16 displays since the total load of current would be less than 16 x 3.5=56 ma, considering all 35 x 16=560 led are in parallel?
    If the answer is yes then the next hurdle of connecting these so that it would require less circuitry. When one does Charleplexing 7 segements displays, it makes use of the fact that all 7 leds are in parallel, if it is anode type(also for cathode but parallel in different sense). Can one consider a row ( or column) as similar to one 5 led( or 7 led) display and Charlieplex all rows (or columns) of all displays? Does my drawing similarity have a logical fault?
    Secondly I have read about Coder and shift register. I have somewhat got the idea of the difference between them. My thinking is that I would be be better of by using a decoder at anode or cathode end to reduce the requirement of pins on atmega328.
    Would it not be possible to get display on these sixteen simply by managing sketch and not using shift register?- another question
    Please tolerate my ignorance about the subject that I is apparent in my writing.My use of terms may be so wrong that they mislead readers. I do appreciate the comments made by experts- unfortunately my knowledge base is so small that I fail to get the benefit out of these.My sole aim is to enjoy the new subject by having a project.Please help me , if you can.
     
  19. ScottWang

    Moderator

    Aug 23, 2012
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    You said one led is 0.7 mA, how was the brightness, is that clear enough to see?
    Do you have the circuit now?
     
  20. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
    2
    Our aim is to have display to be viewed in a small room of dim 12 feet x 15 feet. We could nicely view alphabets from a distance of ten feet.However my friend said that he uses 4.7 readily available current reducing resistance. I did the similar calculations for the maximum current load of 16 displays. It comes to be 85.10 ma(=((5 x 5 x 16/4700)*1000 and it is volt x max.no of led per display x no of displays) and naturally it has brighten up the display and within acceptable limit.
    As regards to the circuit, I am getting at it step by step. My first step at it is as the file I have attached. Can you please check whether it is a legit circuit? !
    Thanks for the patience to read my thread.
    sureshparanjape
     
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