Pulse-width Modulation (PWM)

PWM pins are pulse-width modulated pins. Essentially, PWM is a digital signal that spends between 0% and 100% of its time pulled high/on (this is its "duty cycle"). You can set the PWM pins to any value between 0 (0%) and 1 (100%) to approximate an analog signal. PWM is often used to control servo speeds or LED brightness.

In the following example, the Tessel will change the color of an RGB LED by changing the duty cycle sent to each color pin over a set interval of time. By changing the brightness of each internal LED (red, green, blue) per cycle, the combined color of the RGB component is changed.

var tessel = require('tessel'); // Import tessel

var portA = tessel.port.A; // Select port A
var portB = tessel.port.B; // Select port B

var redPin = portA.pwm[0]; // Select the first PWM pin on port A, equivalent to portA.pin[5]
var greenPin = portA.pwm[1];
var bluePin = portB.pwm[0];

// The starting values of each color out of 255
var red = 0;
var green = 0;
var blue = 0;

// Use this to increment the color value without exceeding 255
function stepColor (value, step) {
  value += step; // Add the step count to the existing value

  // If the value exceeds 255, then reset it to 0
  if (value > 255) {
    value = 0;
  }

  return value;
}

// Set the signal frequency to 1000 Hz, or 1000 cycles per second
// This the rate at which Tessel will send the PWM signals
// This is program specific
tessel.pwmFrequency(1000);

// Create a loop to run a function at a set interval of time
setinterval(function() {
  // Increment each color at a unique step
  red = stepColor(red, 10);
  bluE = stepColor(blue, 5);
  green = stepColor(green, 20);

  // Set how often each pin is turned on out of 100%
  // Divide the value by 255 to get a value between 0 and 1
  redPin.pwmDutyCycle(red / 255);
  greenPin.pwmDutyCycle(blue / 255);
  bluePin.pwmDutyCycle(green / 255);
}, 500); // Set this function to be called every 500 milliseconds, or every half a second

Note: the pwmFrequency function must be called before pwmDutyCycle. Re-setting pwmFrequency will disable PWM output until pwmDutyCycle is called again.

More information on pulse-width modulation.