I wanted to do something special for my friends and family this Christmas; something that I could make for them using the skills I have been developing in my Electrical Engineering studies, but that would also present an opportunity for some creative expression. I settled on making Christmas ornaments.
I began by first designing the circuit itself. I wanted the final device to have some level of intractability, and for the circuit to be able to last for at least a week on a Christmas tree. After exploring several ideas, I settled on a circuit that would blink two sets of LEDs (red and green) with independent bistable multivibrator opamp circuits. Each circuit would have a switch to change the resistance in the RC section of the circuit, giving the option to independently control the speed of each blinking circuit.
To manage battery life, I did some experiments to determine the lowest current at which each color LED could be driven with while still producing light, then selected current-limiting resistors to produce those values. Finally, I built the entire circuit on a breadboard to make sure everything worked as expected.
On the artistic side, I decided to make a Christmas tree shape, planning to use the circuit components as “ornaments” on the tree, silkscreen for a layer of snow on the branches, and the gold ENIG coating for the star, garland, and foot of the tree. I designed the shape in Inkscape before importing the artwork into KiCad, where I laid out the circuit onto it. Using 4 layers made it easier to place the components in an aesthetically pleasing manner. For cost reasons, I decided to use mostly surface mount components, opting for the affordable (yet still hand-solderable) 0804 package size. The exceptions to this were, of course, the LEDs themselves, which were 3mm through-hole devices for aesthetic reasons. The plastic casing tends to make for a nicer festive “glow” than the bright pinpoint of light offered by surface mount LEDs. A quick round of design rule checking to ensure the circuit was faithfully translated, and the board were ready for manufacture! After counting out my many family members, I decided to make 30 boards.
IMAGE: Inkscape and KiCad screenshots.
A week later, with the boards in hand, I set to work assembling them. I decided to forgo a stencil for solder paste, and instead used a syringe to apply paste to each pad individually. (If I do this again, I probably will go back to stencils for any job of more than 10 boards or so – hand applying was a monotonous process.) After pasting, I placed each component in batches with tweezers. I placed all components of a single type on all the boards in a row before moving on to the next to help make sure I didn’t get any of the similar-looking resistors mixed up.
Once all the SMT components were placed, I used a heat gun to solder each board. This again worked alright, but if I do any more large projects like this I will almost certainly try to find or build an oven to use, rather than holding the gun over each individual component until the paste melted and fused.
Next came the through-hole components: The LEDs, power transistors, switches, and battery holders. With the end in sight, these went by quickly. Each board was then washed in IPA to remove any lingering solder paste, then fitted with a battery and tested. Any faulty boards were reworked, and in the end, I was the proud owner of 30 beautiful blinky boards!
If you would like to use my design for your own projects, I have releacsed it under a Creative Commons Attribution Share-Alike license. All the design files can be found at https://github.com/dakotawinslow/Ornament2025/. Happy Building!