Multiplexing for a 7 year old

I have been wanting to make a LED clock for sometime and can’t really believe I never have. So I rummaged through some parts and found some nice 4 segment LED displays. I found some code for a simple clock at nootropicdesign, and it did something pretty neat. It slowed down multiplexing so you could see it, then it would get faster until it appeared “on”. It was a good sketch, but I thought, it needs set buttons, a blinking colon, and a way to trigger the multiplex slow down trick with out restarting. (and resetting the clock!) So I made this little clock and it worked great.

This was a great way to show my 7 year old (who wrote his first Arduino sketch the other night) about multiplexing. A picture is worth 1,000 words right? Well, a hands on demo is worth even more than that. After explaining how it works, he totally understood.

Then I noticed I had, sitting on my bench, some Lexan laser cut, for those little Nokia LCD’s I love so much, that had a 4 segment LED display size hole in them. I made the clock and moved some pins around to free up 3 PWM pins for an RGB LED. Because it’s always good to have a full PWM controlled RGB LED, ya know, just in case. Thinking of programming the LED different colors/brightness according to sun position. I also remembered I have a few Dallas 1307′s.. Maybe tomorrow, it’s late right now, and besides I’m busy porting software for a Touch Shield I got. :)

If your new to multiplexing, it basically combines pins of LED’s and then through switching on only certain pins at a time, you can light only certain LED’s. If you were to apply power to more than 1 cathode and anode, you would get unwanted LED’s lighting. So how can this work? I have to light 14 LED’s just to display 1:00! What makes this work is you light one segment at a time. With a microcontroller, you can do this in an order and at a rate that’s fast enough that the human eye thinks that they are on solid. Most LED 4 digit displays found in microwaves, stoves, appliances, and VCR/DVD players are multiplexed. Its a great way to light a lot of LED’s with less pins than connecting each directly to the microcontroller. Below is a circuit diagram of the LED display I used. The link for the code is below as well, as always, hack away. :) Detailed pinout for Arduino are in the code comments.

Multiplexing!

Get the code here. (Arduino 22 .pde file)

Schematic (Eagle file)

Schematic (png file)

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5 Responses to Multiplexing for a 7 year old

  1. Nice project bro…..This is a perfect teaching tool Thumbs UP!

  2. JamieWho says:

    Wow. Love this. Gives me confidence to reuse a similar clock display that I salvaged from a broken clock radio. I’m significantly older than 7, but it definitely helped me to understand multiplexing with that example.

  3. JanVDS says:

    Hi,

    I’m new to this arduino world (digital electronics in general) and I’m a bit confused about how you made this clock.

    So an Arduino Uno has 14 digital pins as far as I know. But after going through your code you have:
    4 pins for each digit
    7 pins for the seven segments of each digit
    1 pin to blink the colon between hours&minutes
    3 pins for the three buttons
    3 pins for the RGB led

    That’s 18 digital pins in total but an Arduino UNO only has 14?
    You are using the ATMEGA328 in your final solution right?

    I was thinking about making a clock based on this for my children, they can’t tell time yet because they are too young. I would like the RGB led to show a red color when they need to sleep, and when they can get out of bed in the morning I would like to change the color to green.

    Could you post a full picture of your design diagram?

    Best regards,
    Jan

    • Jeremy says:

      Hi Jan, good questions! I realized that I had not made a schematic for that project, so I made one today. It is now in the post above.
      Just a word about Arduino Pins, there are actually 20 pins that you can use. There are digital 0-13 (3, 5, 6, 9, 10, and 11 capable of PWM output), and then Analog In A0-A5. These Analog pins can also be referred to as pins 14-19. Even though they are analog ‘input’ pins, they behave just the same as a digital pins, with the added function of 8 bit ADC on each one. (It can measure voltage in 1,024 steps.)
      Let me know if you have anymore questions, I’ll be glad to help you. Below is a pinout of a ATmega328 the way the Arduino bootloader uses the pins. Have fun with your project and thanks for stopping by!

      http://thecustomgeek.com/images/arduinopins.jpg

  4. Jan Broz says:

    Hi there,
    nice clock :)
    i have similar four digit 7 segment common anode display and i would like to make a clock out of it(hopefully with thermometer – dallas DS1820, but that can wait),
    could you help me out with connecting this to arduino nano through shift registr (74hc595) and i found out that i can use ULN2803A to minimize the pins on arduino board , the board i have is Arduino nano rev 3.0 with ATmega328, as far as i know the nano has 12 digital pins and 8 analog so it should be enough for this project, to make things clear, im just starting with arduino and i have two board the nano and Mega 2560, i want to connect this to nano because i want to make it porable.
    thanks a lot for any help.

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