Dimmer on an Arduino Uno

// Dimmer: a potentiometer sets how bright an LED is.
//
//   A0     the pot's wiper (pin 2); its other two legs go to 5 V and GND
//   pin 9  the LED, through 220 ohm to GND
//
// The smallest complete analogue circuit there is: read a voltage, write a
// brightness. Turn the knob while it runs.
//
// # Ten bits in, eight bits out
//
// analogRead gives 0 to 1023 and analogWrite takes 0 to 255, so the reading is
// shifted right by two rather than divided: 1023 >> 2 is 255, and a shift on an
// AVR is one instruction where a divide is a call.
//
// # Why the reading is smoothed
//
// A pot's wiper is a sliding contact and the ADC's last bit is noise, so a
// steady knob still reads a count or two either way. Left alone, that makes the
// LED flicker at the bottom of its range where the eye is most sensitive. The
// filter here is the cheapest one that works: three quarters of the old value
// plus a quarter of the new, once every five milliseconds. It settles in about
// thirty milliseconds, which is faster than a hand can turn a knob.
//
// The serial line says the percentage, but only when it has moved by a percent,
// so turning the knob prints a sentence rather than a waterfall.

const int KNOB = A0;
const int LAMP = 9;

int smoothed = 0;
int said = -1;

void setup() {
  pinMode(LAMP, OUTPUT);
  analogWrite(LAMP, 0);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: dimmer");
  Serial.println("turn the knob to set the brightness");
  smoothed = analogRead(KNOB);
}

void loop() {
  int raw = analogRead(KNOB);
  // A quarter of the new reading, three quarters of what we had.
  smoothed += (raw - smoothed) / 4;

  int level = smoothed >> 2;
  if (level > 255) {
    level = 255;
  }
  analogWrite(LAMP, level);

  int percent = (int)(((long)smoothed * 100L) / 1023L);
  if (percent != said) {
    said = percent;
    Serial.print("knob ");
    Serial.print(percent);
    Serial.print("% -> pwm ");
    Serial.println(level);
  }

  delay(5);
}