Also, my first thought when reading Lefty's reply was that I MUST put a resistor between the output and the base not to burn the output pin on the Uno, and this was of course confirmed by Grumpy_Mike. However, I am not quite sure of the rest... Do you mean that I should wire a "suitable" resistor to the collector, and then wire the led to the resistor, and the wire the led to the 5V power source on the Uno (yes, why should I use a external 9V battery when the on board 5V is enough, silly me...)? Further, I guess that a "suitable" resistor in this case would be at least (5-1,5)/0,150= 23ohm (maybe 30 ohm just to be safe...)?
Yes, the resistor from the arduino pin to the base will protect the output pin, but if there is not resistor on the collector circuit then there is nothing to limit the current flowing through the led and collector/emitter circuit to ground and damaging both the transistor and led, so it too needs a resistor to set the current flow to 150ma or whatever amount the LED requires. The choice of using the arduino 5V pin or an external 9 volt source is up to you but it will effect the calculation for proper sizing of the collector resistor.
Lefty