> In order to make an array of LED's to glow, should I use BC 547
> transistors?

You could use transistor buffers, or perhaps an octal buffer IC like a
74xx244 (+/- 16mA sink/source). Depends on the current required for
each LED. Some are bright at only a few mA and the port could drive
them directly. If brightness is not important, use a limiting resistor on
each LED so the port can handle them all

For example if the port can sink/source 100mA max, that's 12.5mA
per LED max. If Vcc is 5V and the LED drops 1.7V (if red) that
leaves 3.3V. 3300/12.5 = 264 ohms. I'm sure that the LED would still
be bright enough with 470 ohms and put less strain on the port. If the
brightness isn't enough then you'll have to buffer
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