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Make an LED backlit sign

October 15, 2020

I am a Canadian / American dual-citizen living in Canada, and with the election coming up, I wanted to make a sign to show my allegiances.  

I have always liked backlit LED signs, but for this project I wanted something a bit more special.  So I decided to make a backlit acrylic sign with an American flag behind it.  To do this, I laser cut a wood panel to put behind a laser engraved clear acrylic panel, and I printed off an American flag to put between the layers.  We used 3mm acrylic and 3mm birch wood in this project.

Next I had to figure out how to hold everything together and get the LED's lined up just right.  We recently received a free Printrbot Metal Plus, so I decided to 3D print some channels with holes for LED's.

I have enjoyed using FreeCAD to design 3D printed parts.  FreeCAD is great because, well, it is free, and it is also rather powerful.  I do recommend watching some YouTube videos to get familiar with FreeCAD if you have not used it before.

To make the 3D channels in FreeCAD, I created some blocks, then adjusted the size to my needs.  I then extruded some 5mm holes to insert the bright white LEDs.  

I exported the 3D model as an STL file, then I sliced the file using Cura.  Our Printrbot Metal Plus has a rather large bed, so I was able to print 4 of these at a time.  Many hours later, I had 4 channels that worked great to hold the sign together as well as hold the LED's.
To wire everything up, I used some adhesive-backed copper tape on each side of the 3D printed channels.  One side is positive, one side is negative.  For improved reliability, I soldered the copper tape connections, as they can be a bit finicky. 
The back of the sign is not intended to be seen, so I didn't make too many efforts to make it look pretty.  As you can see, the LED's are all wired in parallel. 
Parallel LED's present a challenge and an opportunity in that 16 LED's end up requiring a bit more current than a typical 1/4 watt resistor can handle.  We found this out the hard way when we used a typical through-hole 1/4 watt started smoking pretty quickly!
Fortunately we have a warehouse full of parts, and I was able to grab a beefy 10 watt power resistor!  
If I had done a wattage calculation ahead of time, I would have realized that this setup draws just over 1W of power as you can see here.  
Instead of using the 10W resistor, we could have wired one resistor per LED, but I kind of like using one beefy power resistor instead.
Finally, we added a handy female DC power adapter and some jumper wires, and we were ready to test it out!
Regardless of your political affiliations, I hope you agree that it turned out well!
Here is a link to the STL file for the 3D printed channels

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