I’m having a bit of trouble determining if this is safe. I want connect three red LED’s, each with their own 330ohm resistor, in parallel all to the same I/O pin on an UNO so they are all powered by the I/O pin and all turn on at the same time. I know the forward voltage of the LED should be around 2v, 20mA current, and the I/O pin max current on the UNO should be lower than 40mA. Is this setup within safe limits or do I risk damage to the UNO drawing too much current from one I/O pin? Also, for future reference, how do I figure this out mathematically? Thanks!
So you have (5-2.2) / 330 = 8.4 mA per led. Times 3 leds equals 25.2 mA. Slightly high as the spec is 20 mA; 40 mA is the absolute maximum. I would not design for 25 mA continuous current; rather add a driver.
There are other limitations (100 mA per port and 200 mA through Vcc or GND);
Thank you!
One last thing, is there any difference in specs between the 5mm and 3mm LED's? Obviously the 3mm are a bit smaller and dimmer, but is the voltage drop / current the same? I can't seem to find a direct answer to this.
One more question, with an LED's specs, how would you determine the minimum current required to actually illuminate (albeit dimly) an LED?
For example regarding my original post if I were to use, say 660 ohms instead of 330 to drop the current load from the I/O pin, how can I predict if the LED's will be visibly illuminated?
One last thing, is there any difference in specs between the 5mm and 3mm LED's?
You'd have to check the manufacturer's specs. The brightness of regular little LEDs is rated as mcd (millicandelas) at a specified current. High power LEDs and LED "light bulbs" are rated usually rated in lumens.
...If you buy from a legitimate supplier they should give you the manufacturer and manufacturer's part number, and possibly a direct link to the datasheet. If you buy from eBay, etc., you may not get the full specs.
Obviously the 3mm are a bit smaller
Obviously. 
and dimmer,
Not necessarily.
but is the voltage drop / current the same?
The voltage drop is usually the same (for all red LEDs, etc.) because as far as I know they all use the same chemistry/construction. The current ratings are also generally similar.
Note that the voltage is "exact" but the current rating is the maximum. It's OK to run a 20mA LED at 5 or 10mA. It won't be as bright, but a "high brightness" LED will be brighter at 10mA than a standard LED at 20mA.
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Thanks for the info. Very much appreciated!
moses1592:
One last thing, is there any difference in specs between the 5mm and 3mm LED's?
Which specs? The quoted brightness may differ because for a LED using the same die, the (same amount of) light will be emitted slightly differently from the two packages.
moses1592:
Obviously the 3mm are a bit smaller and dimmer, but is the voltage drop / current the same? I can't seem to find a direct answer to this.
Equally obviously, this will depend entirely on whether they are from the same manufacturer. One manufacturer will be using the same die (chip) for most or all of the same colour LEDs; the only difference will be the amount of plastic injected into the mould for the different size packages (and possibly, the colour of the plastic). There is no reason whatsoever they would use different dies with different characteristics. Note also that the leadframe is the same for the two packages.
And in fact, the opposite will be true - the 3 mm will appear substantially brighter to the eye than the 5 mm because the same amount of light is coming from a smaller space. SMD LEDs - smaller again - appear quite uncomfortably bright.
moses1592:
One more question, with an LED's specs, how would you determine the minimum current required to actually illuminate (albeit dimly) an LED?
There is no "spec" for this. For a properly manufactured LED with no faults, the light emitted will be directly proportional to the current supplied. In total darkness, you will likely be able to see some light from a LED powered at nanoAmp currents with multiple Megohm resistors.
moses1592:
For example regarding my original post if I were to use, say 660 ohms instead of 330 to drop the current load from the I/O pin, how can I predict if the LED's will be visibly illuminated?
For a 680 Ohm resistor, I predict you will notice a rather minor difference from a 330 Ohm resistor. Visual sensitivity to light is logarithmic, like sound.
I did at some point, buy some known dodgy (white) LEDs which clearly had resistive leakage and cut out at a current around a milliamp or less. I used a couple (back to back, with resistor) to light up my doorbell push for some years however; they may still be doing it ...
DVDdoug:
The voltage drop is usually the same (for all red LEDs, etc.) because as far as I know they all use the same chemistry/construction.
There are "new" red and green (and probably other color) LEDs with forward drop around 3V. They are considerably brighter than the "old" ones.
The nominal forward voltages in many RGB LEDs are the same (3~3.5V) for green & blue, but the red is always considerably less (~2V) in all the ones I've seen. @Smajdalf Can you give a link to a ~3V red led?
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Product Name： 5050 High Bright Red LED
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Color : Ultra Bright Standard White
Hmmm...
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Forward Voltage VF 2.8 --- 3.4 V IF=60mA
I suspect they forgot to update the forward voltage as well as the colour!
PaulRB: you are right! I measured the LED linked before and it truly has V_f < 2V. And it is hard to find a red LED with 3V forward drop. I found one datasheet but I don't own this LED so I cannot be sure.
I have LED chains (the same as here) which is 20 LEDs parallel. One has 4 colors: blue, green, red and yellow. All the LEDs have about the same forward voltage. In fact the red LEDs have slightly higher - about 0.1 V more than the blue. I have noticed interesting thing: the blue and green LED have transparent "lens" of the package. The red and yellow LEDs have red and yellow colored "lens". Maybe they are only white LEDs with filter?
ASMC-PRB9-TV005.pdf (985 KB)
It's a good theory. Either white with a coloured lens, or UV with a phosphor which glows red/yellow. (White LEDs are also UV with a phosphor that glows white, of course.)
Smajdalf:
Maybe they are only white LEDs with filter?
Since the white LEDs have a phosphor specifically to generate white light that would be an absurdly impractical way to construct red or yellow LEDs.
Do people actually not recall just a few years Back when it was common for LEDs to be encapsulated in plastic of the corresponding colour? 
And frequently still are!
Paul__B:
Do people actually not recall just a few years Back when it was common for LEDs to be encapsulated in plastic of the corresponding colour?
Yes, of course I remember. But that raises the question: why was the plastic coloured? Was it just for easy identification without having to power the led? Was it to cut down on stray light of the wrong colour appearing to illuminate the led?
I think it was for the obvious reason that it makes it easier for the end user to know what color it is .
Yeah, that's what I thought most likely. If a user manual for a product built using the leds says "if the green led is on, do this, otherwise do that" and there are several unmarked leds and they are all clear and none of them were on, the end use would be somewhat confused!
I wish they would go back to that line of leds.
PaulRB:
Yes, of course I remember. But that raises the question: why was the plastic coloured? ... Was it to cut down on stray light of the wrong colour appearing to illuminate the led?
I believe it was, just as you use coloured bezels over displays to improve contrast. Where the display is white, you use a grey bezel.
With the efficient LEDs we now have (I avoid the over-hyped term "high brightness"; it is just that what were originally available were "very low brightness"), this is simply less of a concern and of course, now we have RGB LEDs where you no longer refer to "if the green led is on". 
raschemmel:
I wish they would go back to that line of leds.
Why? What I cited is on eBay right now. 
Paul__B:
now we have RGB LEDs where you no longer refer to "if the green led is on".
Colour-blind people love them
I have a friend who has serious issues with a single indicator that shows different states with steady colours. E.g. green indicating 'standby' and red indicating 'running'.