I'm advising a friend on building an 8x8x8 monochrome LED cube. He found this popular design, but I feel like there's a lot of room for improvement.
One area in particular is this array of NPN BJTs which are connected to the VCC lines that power each layer of LEDs. (Note that VCC is at the bottom of the schematic and the layers are fed via connector at top-right.)
The tutorial describes these transistors being turned on to enable each layer:
By only turning on the transistor for one layer, current from the anode columns can only flow through that layer. The transistors for the other layers are off, and the image outputted on the 64 anode wires are only shown on the selected layer.
But I contend that they are effectively driving each layer's supply to ground (via R20..R28) when on, presumably in saturation, which is just a waste of power. (Thus inverting the functionality, meaning that the layer of interest to receive power should have its transistors turned off.)
Is that a correct assessment? Also, it is my understanding that using NPNs in parallel like this is not good design. Am I misinterpreting this or missing something obvious? (Was this done just to use common parts or be inexpensive?)