Get into the Holiday spirit with this cool, DIY LED Christmas Tree!
When it comes to mad maker skills, soldering is among the most important and most versatile a young do-it-yourselfer can have.
If you or your budding maker aren’t familiar, there are great guides to getting started with a soldering iron though, there’s nothing that quite beats hands-on practice on a small project.
These festively themed kits combine two Christmas tree shaped PCBs along with all of the components needed to create a cool, futuristic sparkling Holiday decoration. The kits are available for as little as $3.50 and come with all the components required to get started.
Step 1: Schematic and Theory of Operation
Each of the 10K resistors and 47uF capacitors form an RC oscillator that periodically pushes the associated transistor on. The three sets of RC oscillators are transistors are connected in a loop to keep them cycling out of phase which makes the blinking appear random around the tree. When the transistor is “on” current passes through a bank of 6 LEDs and their 1K current limiting resistor causing that bank to blink on.
If you’re looking for an adventure, trying adjusting the value of one (or more) of the 10K resistors a bit to change the blink rate of the LEDs.
Step 2: Populating the Resistors
Begin soldering by stuffing the resistors. Resistors are not polarized in any way, which means that you can insert them in either direction.
Use a resistor colour code chart or app to identify the different resistor values and make sure to insert them into the correct holes.
In some of 3D Christmas Tree kits, a couple of the 1K resistors are replaced with 330-ohm resistors. When available, the 330-ohm resistors should be used for R2 instead of the specified 1K resistor. According to the numbering system that we have used, R2 is the current limiting resistor for the green LED bank (D1-D6). Using this lower resistance allows the green LEDs to glow a tiny bit brighter, which can mitigate the fact that green LEDs often appear a little dimmer than the red and yellow LEDs.
In the end, the value of the current limiting resistors (R2, R4, R6, and R7) is somewhat forgiving and can anywhere around 300 ohms to 3K.
The value for R7 is specified on the higher end (at 2K) because R7 is the current limiting resistor for the red LED D19 at the top of the tree. Since D19 does not blink, it may appear much brighter, so the higher 2K resistance balances the brightness a bit with respect to the other LEDs.
Step 3: Transistors
When soldering in the transistors, be sure to align the flat side of the transistor to the flat side of the white outline on the printed circuit board (PCB). This ensures that the transistor is wired in the correct direction.
Step 4: Capacitors
Solder in the electrolytic capacitors. These are definitely polarized. There is usually a “-” marking along one side of the can and also the longer lead is positive while the shorter lead is negative. Be certain that the positive and negative terminals are matched to the indications on the PCB silk screen printing. As a double check, the solder pad for the positive pin is often square, while the negative pad is round. The square pad is sometimes called the “pin one indicator” and this applies to multi-lead packages like DIP integrated circuits as well. Leave enough slack in the leads to be able to bend the capacitor over onto its side once it is soldered into place.
Step 5: LEDs
Diodes (including LEDs) are also polarized. Be certain to observe that the long lead is positive and the short is negative. Again observe the silk screen printing on the PCB or that the positive solder pad is square. When soldering the LEDs, be sure they keep the same colours grouped together with a common resistor and transistor as shown in the schematic and parts list. If you attempt to drive mixed colour LEDs with the same current limiting resistor and switching transistor, you will likely find that one colour glows brighter and the other colour doesn’t light up at all or only very dimly.
When soldering the LEDs into place, leave slack in the leads so that the LED can be bent off to the side once it is attached. Note that we have not yet soldered in the D19 LED at the very tip of the tree.
Step 6: Test each PCB
Once each of the Tree PCBs is fully populated (except for the D19 LED at the tip), they can be tested by placing about 5VDC onto the “+” and “-” pads at the very bottom of the tree.
For example, you can place some AA batteries into the battery housing and touch the wires to the correct pads on the PCB.
The LEDs should blink and cycle with colourful holiday goodness. If they do not, check the polarities (directions) of the power wires, the LEDs, the caps, and the transistors. If you were careful with all of the polarities while soldering, there should be no problems.
Step 7: Base PCB
Solder the power button and the power terminal onto the Base PCB. When inserting the power button, the notched side of the button should face the nearest edge of the PCB as shown. A piece of resistor lead that was trimmed off earlier may be wrapped around the power terminal and soldered to the PCB as a stain relief to make the connector more robust while inserting the power plug.
The battery pack can be bolted into the base PCB as shown. The wires from the battery pack can be fed up into the PCB trimmed and soldered to the power pads.
Step 8: Final Assembly
Slide the two tree halves into one another being careful to bend any of the components (such as the transistors) our of the way if they catch onto one another. Once the sides are aligned, solder the pads together where the halves touch.
Now the top LED (D19) can be attached and trimmed.
Lastly, insert the tree into the base PCB being careful to observe the “+” and “-” designations on all three PCBs. Solder the tree to the base PCB.
Your 3D LED Tree can be powered from the battery pack OR the power terminal USB adapter. When the power terminal is inserted, the batteries are out of the circuit, so it is fine to leave the batteries installed while using the USB power adapter.