This driver is able to drive LEDs by applying a high logic level sourced from output pin of a logic IC belonging to common 74xx logic families (HC, HCT, LS, etc.) or CMOS microcontrollers.
The output pin of IC is simulated by a voltage supply and a switch.
General rule of thumb is that the output pin of the IC has to be able to source at least 1 mA of current when it is set to high logic level.
As a real NPN transistor, you can use almost any "jellybean" small-signal one, such as BC54x or 2N3904, which are cheap and widely available.
