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This emitter follower driver is able to drive LEDs by applying a high logic level sourced from output of a logic IC belonging to common 74xx logic families (LS, HC, HCT, etc.) or microcontrollers.
The output pin of logic IC is simulated by a voltage supply and a switch.
This circuit could be beneficial in some situations because you don't have to provide a second resistor to the base of the transistor as for more well-known arrangement where transistor has its emitter grounded.
Currently, it mimics output properties of classic bipolar (74xx) IC families (the original one, F, LS, etc.), which provide slightly less voltage (approx. 3.5 V) at high logic level. If you want to adjust the circuit for CMOS IC families (HC, HCT, etc.) providing almost the supply voltage level at its output pins, you have to change the value of the LED series resistor to 120 ohms.
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.
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