i don't quite get what the schematic,
The two diodes function as a voltage clamp. When drive goes h igh (sufficient for the diode to conduct), the base is held at approximately 1.4v (forward voltage drop for the diodes). So the voltage on the 33r resistor is 1.4v - .7v(the transistor's Vbe) = 0.7v. So the current goes through the transistor and the led must be 0.7v/33 = 20ma. Give or take a few.
Essentially it is using the diodes as a voltage reference. As such, you can replace the diodes with other voltage references, like a zener, or a led, etc.