Actually, the 2N2222 datasheet specifies that Ib = Ic/10 when used as a switch.
Cool, so 1K is a good point to start with!
Not necessarily. It depends on the situation. For example..... if you have a 5V source, and the voltage difference between the source and the transistor base is say 4.4 V (after taking account of a diode voltage drop), then a 1 kilo-Ohm resistor would give you maybe 4.4 milliAmp of base current. But just say you need 20 milliAmp base current....... a 1 kilo-Ohm resistor won't allow it. The resistor would need to be smaller... like 220 Ohm.If you want to use a 1 kilo-Ohm resistor, then the voltage source would need to output a voltage much higher than 5V if 20 milliAmp base current is needed........ but since you're trying to drive the transistor with maybe 5V from an arduino output..... then better to stick with 5V (arduino digital pin output) and a small resistor.... like 220 Ohm.
Determine the DC voltage (Vemitter) that you are expecting at the emitter (for however many LEDS you plan to use, and knowing the operating current of the LEDs).Then know decide on what voltage you're going to apply (on the source side of the base resistor)... eg, Vinput.Look up the specs for 2N2222. If the continuous base current information says don't go over 20 mA (or whatever the value will be), then Vinput = Ibase*Rbase + 0.6 + Vemitter.So Rbase = (Vinput - Vemitter - 0.6)/Ibase; where Ibase could be 20 mA.
Sorry for the sarcasm, but it's really very confusing for some is a newby (like me)!