Why isn't my transistor outputting enough voltage when switched?
From your description, you have the transistor connected as an emitter follower, like so:







simulate this circuit &ndash; Schematic created using CircuitLab

With an emitter follower the emitter vol;tage is always about 0.7 V below the base voltage, so you can't get more than 2.6 volts across the LED.

You should instead put the LED between the collector and positive supply, and ground the emitter.  This way, when the transistor is saturated it will only have about 0.2 V across it, so you get almost the full supply voltage for the LED.

Preferred circuit:







simulate this circuit

(resistor values are default values from the schematic editor - ignore them)