Arduino: LEDs and resistors

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Breadboard Layout, Arduino Lesson 2. LEDs

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Sensors and leds. This need resistors? and how and where? - General Electronics - Arduino Forum

L1: Turning on an LED - Physical Computing

L1: Turning on an LED - Physical Computing

Noob] Why do I need 1 resistor for each LED ? Can't I use 1 resistor for all the the LEDs ? : r/arduino

LED Resistor Calculator – RC Models, DIY Hobby Elektronics, Arduino projects, RC Airplanes

RGB led: can I put only 1 resistor at ground? - General Electronics - Arduino Forum