LEDs and Digital Output

LEDs - Light Emitting Diodes

LEDs

• Light emitting diodes are basically small light bulbs (but use significantly less power)
• LEDs are everywhere!
  • (Nearly) all the blinking lights around us (e.g. router, washing machine, etc.)
  • Smartphone flash
  • TV remotes (infrared LEDs)
  • Energy-efficient light bulbs
  • Some displays

How LEDs Work

How LEDs Work

• Light emitting diode is made of two layers of semiconducting material
• When the voltage at the anode (longer leg) is about 2.2v more than the voltage at the cathode (shorter leg), the LED emits light
  • This positive voltage difference is called forward biasing

LEDs are Directional (aka Have Polarity)

• LEDs allow current to flow in only one direction
• If positive voltage is applied to the anode, light will emit
• If positive voltage is applied to the cathode, light will not emit (aka the polarity is reversed)

LED Precautions

• LEDs have limitation in how much current they can handle
• More current = brighter light …up to a point, and then the LED breaks
• This is why we use a resistor to limit the flow of current
• We call this a current limiting resistor
• The resistor can be before or after the LED in the circuit

Exercise

• What do you think will be the difference between using 10KOhm resistor vs 330 Ohm resistor?

LED Datasheet

• Datasheet
• Forward Current
• Peak Forward Current
• Forward Voltage

Choosing a Current-Limiting Resistor

• Rule of Thumb: 330 Ohms or 220 Ohms are common options
  • For this class, go ahead and use whichever is available
• Why?

Let’s turn on LED without any Code

• We can “hardwire” an LED to turn on by connecting the anode (long side) to a +3.3V and the cathode (short side) to ground
• The LED is always on because the anode is always at +3.3V and the cathode is always at ground

Turning on LEDs in Code

• We need to be able to control the voltage at the anode so it can be either +3.3V (LED turns on), or GND (LED turns off)
• Connected anode to pin D2 on Photon 2
• Connected cathode to GND
• Now using code, we can make the voltage at pin D2 either +3.3V or GND

Wiring

Digital Output

• These are signals that are HIGH or LOW
  • HIGH is 5V (pin VUSB) or 3.3V (pin 3v3)
    • HIGH is considered “true”
  • LOW / false is 0V (ground)
    • LOW is considered “false”

Setting input / output with pinMode

Syntax

pinMode(PIN_NUMBER, MODE);
//MODE: OUTPUT or INPUT

• Before we can use pins on the Photon 2, we need to specify if pins will be used for input or output
• Most pins on the Photon 2 can be configured to SEND output (e.g. to turn on a light) or to RECEIVE input (e.g. a button press)
• When you want to use a pin in your program, you should specify its mode in setup()

Writing Digital Output with digitalWrite

Syntax

digitalWrite(PIN_NUMBER, VALUE);
//VALUE: HIGH or LOW

• You can send an ON (HIGH) or OFF (LOW) signal at output on a pin
• This is writing a digital value
  • Digital values are like boolean values: 0/1, on/off, true/false

Credits

• Afrank99 - Own work, CC BY-SA 2.0, Link
• CC BY-SA 3.0, Link
• Images created with Fritzing