PCB word clock

charlie-plexed 11x10 LED matrix, perfect fit for a word clock. Also LEDs soldered on upside down.

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Charlieplexing LEDs in a 11 x 10 matrix is a perfect match for a word clock! You don't have that many LEDs to turn on. The hassle with a good case stopped me from building one for the longest time. I couldn't decline this challenge!

General Idea

@Radomir Dopieralski mentioned in #Hacker Channel that one could use LEDs backwards to shine through the PCB and at this point I'm not sure if he mentioned charlie-plexing and word clock as well, but let's just say I was inspired to design a board like this. At this point I already had a board prepared, a 10x11 LED matrix with 5mm charlieplexed LEDs. Charlieplexing can get very confusing, but I found a trick online [research link], that helps a lot to layout a matrix. Instead of controlling it with 10+11 pins this will take only 11 pins.

LED footprint

From there I've edited an LED footprint in EAGLE so that I was able to just change the LEDs without rewiring the schematics. The blue circle is on the layer "42 bRestrict" - this will stop the polygon on the bottom layer from putting copper there. At the same time the copper will frame the letter. Since I'm an expert in soldering LEDs the wrong way by accident, I will use standard 0603 LEDs, instead of special "bottom mount" LEDs. On the top Layer of the LED footprint is a smaller, rectangular shaped Object on "41 tRestrict" that's supposed to let the light through as well. The letters where placed on the bottom layer.

What's Charlieplexing

It's always good to try to explain stuff in your own words, legally speaking as well. Blinking LEDs is always awesome, because it's wysiwyg. Connect the anode to 5V, the cathode to GROUND and it explodes. Add a resistor and everything is great, as long as it's the right value. The other way around they do cool stuff too, but not important for this project. Here it's important that they're not glowing.
The cool thing about ATMEGA chips is, that they can source/drain enough power to make LEDs blink and that they're on Arduino boards. You can even put two opposing LEDs on the data pins and and make them blink depending on the OUTPUT of the pins.

Given this schematic and you only have ground and power, then this would be your truth table. Adding more LEDs would only add more LEDs that shine simultaneously.


Luckily the ATMEGA has a third state, a high impedance state - also called Z state. You can achieve that by turning the pin to an INPUT pin. The following table only shows how to activate the LEDs.



Pros and cons

So why all this? It saves pins and doesn't take much power! But I will have to worry about brightness and programming it, once the boards are here.

brd - 117.67 kB - 01/12/2017 at 15:18


sch - 164.66 kB - 01/12/2017 at 15:18

See BOM Download

sch - 164.66 kB - 01/12/2017 at 15:18

See BOM Download

brd - 116.40 kB - 01/12/2017 at 15:18


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  • some documentation added

    davedarko01/15/2017 at 13:46 0 comments

    Since someone on twitter pointed out that the project documentation is an epicfail, I felt compelled to edit the details and add informations on charlieplexing and the general idea. I first was a bit annoyed, but mostly because the entity was right :) I tend to not document my projects very well, mostly because it's easier to write about it afterwards and laziness. Now it feels more complete though.

  • boards are ordered

    davedarko01/12/2017 at 15:19 2 comments

    So for science I've also added the german layout and ordered this one on 0.8mm boards.

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Enjoy this project?



Ted Yapo wrote 01/12/2017 at 19:09 point

I like the idea of using "normal" LEDs for this!  They used to make larger SMT LEDs specifically for inverse mounting:

It looks like they're being discontinued, though.  I used them to backlight a logo on some boards before:

But, they were big and more expensive than standard LEDs.

I found that the fiberglass diffused light very well - possibly enough to cause bleed-though to neighboring symbols in a tight array.  Maybe a ring (or grid) of plated-though vias around each symbol would block some of the light if this becomes an issue.

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davedarko wrote 01/12/2017 at 20:13 point

that looks nice! I was wondering what color I will use and green was on the top of my list.

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K.C. Lee wrote 01/12/2017 at 21:24 point

There are 7,529 search results (300+ pages) on digikey if you search for 

Mounting Type: Surface Mount Bottom Entry

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davedarko wrote 01/12/2017 at 21:51 point

any in 0603, that don't look like normal LEDs?

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K.C. Lee wrote 01/12/2017 at 23:56 point

0603 is a small package, so likely are the LED dies are bonded to a substrate with an epoxy lens cover and cut down to the size.  i.e. the all going to look similar may be slightly different lens shape/thickness.

Not sure what exactly you are looking for.  Best bet is to go to a particular vendor's catalog to choose a style.

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davedarko wrote 01/13/2017 at 10:42 point

No no, I'm fine with normal 0603 SMD LEDs from eBay, I think. I felt snappy because you've placed your search results so "dry". I'm not really looking for something, I'll just put them the other way around and hope for the best, since searching with your recommended terms for LEDs in the size of my project, I noticed that they all looked like good old LEDs to me :)  

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Sean Hodgins wrote 01/12/2017 at 18:39 point

how will you make the LEDs bridge the small gap to the board(from the lens gap? Or do you know something I don't know? Haha can't wait to see how this works! 

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davedarko wrote 01/12/2017 at 18:50 point

a lot of solder :) in the case of 0603 LEDs it can't be much, I hope...

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Radomir Dopieralski wrote 01/12/2017 at 16:46 point

Can't wait to see it in action.

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al1 wrote 01/12/2017 at 16:43 point

This is really nice looking. 

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davedarko wrote 01/12/2017 at 18:18 point

yay, nice words from an LED matrix master :)

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oshpark wrote 01/11/2017 at 23:50 point

Very intresting!  We look forward to seeing more

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davedarko wrote 01/12/2017 at 09:39 point

Thanks :) @Radomir Dopieralski was talking about stuff like that over on the #Hacker Channel and I just had to try it!

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