Motor on a transistor, on an Arduino Uno

// Motor ramp: a motor through a transistor, sped up and slowed down.
//
//   pin 9  through 1 k into the transistor's base
//   the transistor's emitter to GND, its collector to one motor terminal
//   the other motor terminal to 5 V, and a diode across the motor with its
//   band (cathode) on the 5 V side
//
// A board pin can give about 20 mA. A small motor wants ten times that
// standing still, so the pin drives a transistor and the transistor drives the
// motor: a little base current through the 1 k lets a much larger one through
// from collector to emitter. The PWM on pin 9 switches that current on and
// off 490 times a second, and the motor's own inertia averages it into a
// speed.
//
// # The diode
//
// A motor is a coil. The instant its current is cut, the coil tries to keep it
// flowing and the voltage on the collector flies up until something gives,
// which is usually the transistor. The diode across the motor gives the
// current a way round instead: it is reverse biased while the motor runs and
// only conducts on that kick. Leave it out on the bench and the transistor
// lasts an afternoon.
//
// The duty walks from 0 to 255 and back, five counts every 40 ms, so a whole
// cycle takes about four seconds.

const int DRIVE = 9;
const int STEP = 5;
const int STEP_MS = 40;

int duty = 0;
int direction = STEP;

void setup() {
  pinMode(DRIVE, OUTPUT);
  analogWrite(DRIVE, 0);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: motor on a transistor, pin 9 PWM");
}

void loop() {
  analogWrite(DRIVE, duty);
  if (duty % 50 == 0) {
    Serial.print("duty ");
    Serial.print(duty);
    Serial.println(" / 255");
  }
  duty += direction;
  if (duty >= 255) {
    duty = 255;
    direction = -STEP;
  } else if (duty <= 0) {
    duty = 0;
    direction = STEP;
  }
  delay(STEP_MS);
}