Choosing LEDs/resistors to drive directly from 3v3 HC CMOS logic
With 3.3V systems it is often a good idea to supply bells and whistles like LEDs with their own power rail and drive them with open-collector outputs or discrete transistor drivers.







simulate this circuit &ndash; Schematic created using CircuitLab

There are a number of benefits to this method including.


Ability to use larger voltage LEDs.
Reduced demand on the 3.3V supply. Possibly allowing a much smaller regulator.
Increased resistor size provides more uniformity in LED current over LED forward voltage variations.
Gives you the option to turn off ALL the bells and whistles when in sleep mode.
In battery operated circuits, LEDS get dimmer as the battery begins to die giving you a free bonus feature.