• 1
    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 adveture, trying adjusting the value of one (or more) of the 10K resistors a bit to change the blink rate of the LEDs.

  • 2
    Step 2

    Populating the Resistors

    Begin soldering by stuffing the resistors. Resistors are not polarized in anyway, which means that you can insert them in either direction.

    Use a resistor color 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.

    Are you new to soldering? There are a lot of great guides and videos online about soldering. Here is an example:


    If you feel that you need additional assistance with soldering, try to find a local maker group or hacker space in your area. Also, amateur radio clubs are always excellent sources of electronics experience.

  • 3
    Step 3


    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.