LEDs and Resistors

[kryshe]

Hey! I have a question about LEDs and Resistors. In lots of schematics for Arduino or the Axoloti I see that an LED is always connected with a resistor to ground. Is that actually really necessary? For example I added a LED to a Sequencer by sending out the Square LFO via a digital out. The digital out is connected to the LED and then directly to ground. I used a resistor but the light was to low so I just removed it and there is no problem. So whats the thing with these resistors???

[floodcomics]

AFAIK it's just to protect the LED from burning out and extend the its lifetime. I also skip the resistor on weaker LEDs in order to up the brightness and have never had any problems.

[janvantomme]

These are current limiting resistors. LEDs have a current rating in mA, e.g. the amount of current needed for the LED to work properly. If you don't have a resistor, the LED will try to use as much current as it can, and burn. The value of the resistor can be calculated with this online LED calculator for instance.

LED Calculator - Current limiting resistor calculator for LED arrays

This video form learnelectronics explains everything you need to know about LEDs + the math needed to calculate the resistors you need.

[DiegoLiedo]

I would definitely use a resistor, not only to extended de led's life. GPIO pins have a current limit and if it's exceded either the pin or some other stuff inside the microcontroller can burn.
If you're using normal 3 or 5 mm. LEDs a 10-20 ohms resistor may achieve full "datasheet" brightness. If it's power LEDs you're driving more components should be used.
Not only because it works once means it'll work forever, and resistor cost and difficulty to connect is close to nothing.