Resistor Values to switch a 100ma led with a on a 5v power supply with 2n3094a 3.5v base
It's pretty simple. For minimum power loss in the transistor, you want it to be in saturation. In this condition, the rule of thumb is that a gain of 10 to 20 is reasonable. So, for an LED current of 80 mA, you want a base current in the vicinity of 5 to 10 mA. Assume a 0.7 volt base emitter drop and you should be able to figure out the base resistor.