I was wondering what circuit was best. I choose B but I'm not sure if you want more voltage or more current through the diodes. I feel C is redundant as diodes are the same. This question continues with solving the chosen circuits resistors for Ic = 20+-5 mA, using E12 series resistors. I have done this for B getting 220ohms(210 calculated but not E12) and 4.8k(5k calculated).

I have a couple more questions:
-Is the voltage drop across a blue LED around 1.8V like other LEDs?
-Are 220ohm and 4.8k resistors E12 series? I've never heard of E12 series before but I assumed it meant the standard types we ues in labs.

 

Blue leds are usually rated at a Vf of 3.6v to 4.8v. If at 20ma to 50ma

E12 resistors is simply the multiple values of the resistor. there are others like E6, E24, E48 & E96. E12 comes in 10, 12, 15,18, 22, 27,33, 39, 47, 56, 68 etc.

now that you know the Vf & If ranges and the transistor beta, I think you should now reconsider the values you have arrived and the choice of circuit.

moz

 

Ok, thank you very much. Ill give it a go.