The final block is, of course, the LEDs. At first glance, the PCB looks like it contains 32 LEDs. In reality, there are 64 LEDs as each is a bi-colour LED capable of glowing red or green. As you may expect, the LEDs are multiplexed. To control so many LEDs with so few MCU pins, we connect the LEDs in a matrix of 4 columns with 8 LEDs in each and use 12 pins.
Multiplexing is where each column of LEDs is activated for a short time followed by the next column.
If each time slot is short enough, our eyes don't see any flicker. This is a practical minimum duty-cycle as the brightness reduces. Resistors R6 to R13 set the peak LED current to about 30mA. Because there can only be 8 LEDs on at any time, the total maximum current drawn by the Tree is about 400mA. Any 9Vdc plug pack rated at 500mA or more should be suitable. So each pin is buffered by an emitter follower. So, where a more conventional LED matrix would have 4 high current source drivers and 8 lower current sink drivers, this design has drivers that can source and sink.