Hm, ok, thanks, that helped. 
I have sort of a followup question, hope this can go into the same thread. So if I know the dc current gain (designated as beta i think), could i potentially use a transistor to limit current that goes on through the emitter to the device? I mean, by placing a correctly valued resistor in series with the base of transistor, to control the saturation current?
For example, take the pic If I control the value of Rb to control the current the transistor lets through when saturated, could i omit the 62 Ohm resistor wired in series with the LED?
Maybe a stupid question, sorry 
Cheers, Val