Servo fader on an Arduino Uno

// Servo fader: push the slider, the horn follows.
//
//   A0     the slide potentiometer's wiper (its ends on 5 V and GND)
//   pin 9  the servo's signal wire (its red wire on 5 V, brown on GND)
//
// A servo does not take PWM the way an LED does. It takes a pulse width: a
// pulse of 544 us once every 20 ms puts the horn at one end of its travel,
// 2400 us puts it at the other (the Arduino Servo library's numbers), and
// anything between is an angle between. The servo holds whatever it was last
// told, and it walks there at its own speed, about a tenth of a second for
// sixty degrees.
//
// So the sketch reads the fader, turns 0..1023 into 0..180 degrees and sends
// the matching pulse, fifty times a second. mokxi_servo.h makes the pulse with
// delayMicroseconds, since the Uno's analogWrite runs far too fast for a servo
// to read.
//
// The reading is averaged over a few samples so the horn does not twitch on
// the last count of the ADC, and the angle is printed only when it changes.

#include "mokxi_servo.h"

const int FADER = A0;
const int SIGNAL = 9;

Servo horn(SIGNAL);
int shown = -1;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Uno: a fader on A0 drives a servo on pin 9");
  horn.begin();
  horn.write(90);
}

void loop() {
  long sum = 0;
  for (int i = 0; i < 4; i++) {
    sum += analogRead(FADER);
  }
  int reading = (int)(sum / 4);
  int angle = (int)map(reading, 0, 1023, 0, 180);

  horn.write(angle);
  if (angle != shown) {
    shown = angle;
    Serial.print("fader ");
    Serial.print(reading);
    Serial.print(" -> ");
    Serial.print(angle);
    Serial.println(" degrees");
  }
  delay(20);
}