I'm planning to run a single LED at varying voltages (and presumably light intensities). Is this possible, or are LED's simply on or off (without varying intensities)? Thank you all in advance.
You can vary the brightness of a led by varying the amount of current flow through a forward biased led, not by controlling the voltage applied to an led, as once the applied voltage is higher then the led's Vf rating the voltage will remain constant across the led. A series resistor is the usually method used to control the amount of current flow for standard 20ma type leds. A series resistor/led network can be driven at various brightness levels by wiring it to an arduino pwm output pin and sending different duty cycle values to vary the average brightness displayed.