Pulse Width Modulation

Pulse Width Modulation

Analog Output

• Goal
  • Produce analog outputs (e.g. make an LED half as bright, rather than off or on)
• Problem
  • Photon 2 does not have analog outputs (and neither do many microcontrollers)
• Solution
  • We will fake it!

Pulse Width Modulation (PWM)

• PWM is a technique we use to simulate analog outputs
• Basically, we switch a digital ouput on/off very quickly (modulate)
• Also specify how long the output is on, and how long it is off (pulse width)
• The result is the “effective” output voltage can be varied (since the signal is switching between high and low)

Key Terms

• Square wave: a digital output that switches repeatedly from high (3.3v) to low (0v)
• Pulse width: how long the signal is high (usually in milliseconds)
• Duty cycle: percentage of one “cycle” that a signal is at 3.3v
• Period: time for one on/off cycle to complete (usually in milliseconds)
• Frequency: how many times per second the on/off cycle repeats (Hz) (default Photon 2 PWM frequency is 500 Hz or 2 ms)

Key Terms Illustrated

Questions

• What is the output voltage of an Photon 2 pin?
• What is the effective output voltage using PWM with 50% duty cycle?
• What is the effective output voltage using PWM with 25% duty cycle?
• What is the effective output voltage using PWM with 0% duty cycle?

Why does this work?

Using PWM

• Only certain pins support PWM
  • D1 (SCL or A4)*
  • A2
  • A5
  • MISO (D16)*
  • MOSI (D15)*
• PWM pins can have different PWM values (duty cycles), but must share the same frequency and resolution.

*You can use any of these pins labels.

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Writing Analog Output with analogWrite

Syntax

analogWrite(PIN_NUMBER, VALUE);
//VALUE: 0-255

• You can control the pulse width with VALUE
• We discussed duty cycle as 0%-100%, but VALUE is 0-255
  • Why?

Example

analogWrite(LED_PIN, 127);	//50% duty cycle, or 1.67v

analogWrite(LED_PIN, 192);	//75% duty cycle, or 2.45v

analogWrite(LED_PIN, 0);	//0% duty cycle, or 0v

Wiring Diagram

Credits

• The Arduino.cc Team