Christmas Tree - Mini With LEDs & USB

I purchased a 6" Christmas tree years ago with plans to light it up with leds that were powered by the USB from my laptop. Well, I found the tree and decided to make it happen.

The results are nice. It has 4 different colors and 8 leds total. It is a little dim in bright light, since it is meant to be used in a darkened room.

You can use a website like this one: http://led.linear1.org/led.wiz to calculate the optimal resistant and branches to power the circuit.

I knew that too many leds on one branch of a parallel circuit would be too much of a voltage drop. I decided that I wanted 3-4 branches with 2-4 leds on each. With that as the basis, I then laid out several combination on a breadboard. I tried several resistors with resistance from 100-1000 ohms. When I got the brightness that looked good I tested the next branch. I ended up with 220 ohms for everything but green. Those used a 100 ohm.

I found that similar voltage drop leds (this is largely based on color, red have the smallest drop, and blue/green the highest) worked the best in series. I went with reds on one branch, a orange and yellow in two branches, and green in one branch.

Each branch of the circuit had two leds and a resistor. I used the smallest green and black wire I could find.

First I soldered the positive lead (I used green wire to hide it in the branches) to the wire, then a jumper from the negative side to the positive of the next light. I left the second negative lead bare. Then I soldered all of the leds this way. I tested each set back on the breadboard to verify that everything was correct and still working after solder.

Leds are sensitive to heat. When you solder, use a hot iron and be quick. Otherwise stop and let it cool before trying again.

When all were done, it was time to add a black wire as ground. Each set had a 3-4" black wire added to denote the ground side. All of these went to a single pigtail. I soldered them together, but you could also use a wire-nut. The connection was covered in black electrical tape so it would not be noticeable in the tree.

All of these wires needed to go to resistors, and then to the USB cord. I decided to make a quick custom board. The power came in one end. Then a resistor went to each branch. For the green leds I used a smaller ohm resistor. For the yellow/orange, I tied them all to the same resistor.

I used nail polish to sketch out the pattern on a board. Then it was etched and drilled.

This was overkill. You could have used perf board, or even soldered the resistors directly to the wires and then used a wire-nut to connect the power to the USB cord.

My USB cord came from a broken keyboard, but you could buy one and cut the end off that you don't need.

The internet says that red should be +5 and black should be ground, green and white are for data. My first attempt I tried that and nothing worked. Since I had been testing the circuit every step of the way, I knew the problem was with the cord. Eventually I figured out that the white was ground. It helps to have a continuity tester or voltmeter.

I didn't want to plug this creation into my laptop untested, so I used a bench top power supply I'd built. It is handy to have a 5 volt power supply with a USB plug sitting around.

Otherwise the recommendation would be to use a powered USB hub. That would give you a little protection to your more expensive laptop.