// Bit bang low frequency PWM.
//
// Parameters:
// outputPin (byte)       - Pin number to output PWM.
// dutyCycle (byte)       - PWM period from 0 - 100.
void slowPWM(byte outputPin, byte dutyCycle, unsigned long period)
{
  pinMode(outputPin, OUTPUT);
  static byte outputState = LOW;
  static unsigned long lastSwitchTime = 0;

  unsigned long onTime = (dutyCycle * period) / 100;
  unsigned long offTime = period - onTime;

  unsigned long currentTime = millis();

  if (outputState == HIGH && (currentTime - lastSwitchTime >= onTime))
  {
    lastSwitchTime = currentTime;
    outputState = LOW;
  }
  if (outputState == LOW && (currentTime - lastSwitchTime >= offTime))
  {
    lastSwitchTime = currentTime;
    outputState = HIGH;
  }
  digitalWrite(outputPin, outputState);
}