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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. 

MP

[Reply #1 on:]

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. 

MP

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.

Lefty

[Reply #2 on:]

Have a read of this:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

[Reply #3 on:]

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.

PWM is the better solution. See the example that comes with the Arduino IDE, File > Examples > 01.Basics > Fade or http://arduino.cc/en/Tutorial/Fade

[Reply #4 on:]

Very Simple LED Question

I doubt it. LED questions are never simple...

[Reply #5 on:]

Very Simple LED Question

I doubt it. LED questions are never simple...

Especially if resistors are involved   ... or not!

[Reply #6 on:]

It is a simple enough question, and the answer can be simple enough too, but the simplicity of the question is independent of the answer.

The simple answer is, yes LEDs can vary intensity(brightness), depending on voltage/current.

The more complicated answer:
Hue is not realistically adjustable, but brightness(intensity) is.

Since this forum is about using LEDs with microcontrollers, using PWM is a viable way to control brightness, but it is not necessarily better for every situation.

LEDs have a usable range, where if you dont have enough electricity, it wont light up at all, and it gets to a point where it wont produce any more light, it just causes more damage. In a sense, they can be looked at as simple on or off, but they can also vary brightness to some degree.

[Reply #7 on:]

Is this possible

Yes.

[Reply #8 on:]

Thank you all very much for the help.  As far as I understand, I will be using PWM to dim the light to a certain point. 

I am planning to run a light intensity (irradiance) sensor with it [TSL235R (https://www.sparkfun.com/products/9768)].  Will the pulses of light interrupt the reading of irradiance?  Such as, if I take a 500 ms reading, will the on/off (say 5ms on 5ms off) will it screw up the reading in some way?  It's kind of blowing my mind right now. 

[Reply #9 on:]

What are you trying to measure?
Is the light sensor trying to measure the brightness of the LED or is the LED just interference?