Light Emitting Diodes (LEDs)

Types and Characteristics

• LEDs come in a wide assortment of colors (wavelengths).

• The current passing through an LED must be limited to avoid burning it out.

• An LED will only pass current in a specific direction, in particular when its longer lead (the anode) is connected to the positive voltage side of a circuit and the shorter lead (the cathode) is connected to the ground side of a circuit.
  

• There is a fixed voltage drop across an LED, referred to as its forward voltage.

• Operating voltages and currents vary depending on LED color and type.

• The normal operating current for most LEDs is about 20 mA.

• The voltage drop across an LED, referred to as its forward voltage, depends on the color: Red - 2V; Blue - 3.3V; Yellow - 2.2V; White - 3V; Green - 2.5V

Interfacing LEDs to the Arduino

• The easiest way to control an LED is to connect it directly to a digital output pin (with a resistor to limit the current) as follows (see also ELEGOO Lesson 3):
• The necessary resistance to obtain the desired current can be determined as R = (V_s - V_f) / I, where V_s is the source voltage, V_f is the forward voltage of the LED and I is the desired current (normally around 20 mA).

• A digital output pin can be programmed to have either a HIGH (+5V) level or a LOW (0V) level. Thus, if we set the digital output pin to a HIGH level, there will be sufficient voltage to light an LED.

  int ledPin = 12;                // LED connected to digital pin 12
  
  void setup()
  {
    pinMode(ledPin, OUTPUT);      // sets the digital pin as output
  }
  
  void loop()
  {
    digitalWrite(ledPin, HIGH);   // sets the LED on
    delay(1000);                  // waits for a second
    digitalWrite(ledPin, LOW);    // sets the LED off
    delay(1000);                  // waits for a second
  }
  

• Another option is to use pulse wave modulation (PWM) to fade an LED on and/or off. Note that only certain digital pins support PWM (see Elegoo Lesson 4 for more information).

  int value = 0;                            // variable to keep the actual value
  int ledPin = 9;                           // LED connected to digital pin 9
  
  void setup() 
  {
    pinMode(ledPin, OUTPUT);      // sets the digital pin as output
  } 
  
  void loop() 
  { 
    for ( value=0; value<=255; value+=5 ) {   // fade in (from min to max)
      analogWrite( ledPin, value );           // sets the value (range from 0 to 255)
      delay( 30 );                            // waits for 30 milli seconds to see the dimming effect
    } 
    for ( value=255; value>=0; value-=5 ) {   // fade out (from max to min)
      analogWrite( ledPin, value ); 
      delay( 30 ); 
    }  
  }