Motor pinwheel on an Arduino Uno

// Motor pinwheel: hold the button and a MOSFET runs a DC motor from a 9 V
// battery.
//
//   pin 2    the button: one leg to 5V, the other to pin 2 and through a 10k
//            resistor to GND
//   pin 9    the MOSFET's gate
//   MOSFET   source -> GND, drain -> the motor's other lead
//   motor    one lead to the battery's +9 V, the other to the drain
//   diode    across the motor, its stripe to +9 V
//   battery  its - joined to the Uno's GND
//
// A pin gives a few milliamps at 5 V and a motor wants hundreds at 9. The
// MOSFET is the switch between them: a voltage on its gate (and almost no
// current) lets the battery's current through the motor. The diode catches the
// spike the motor's coil throws back when it is switched off, which would
// otherwise hit the MOSFET.
//
// The grounds have to be joined. The gate voltage only means something
// measured against the source, which sits on the battery's minus.

const int BUTTON = 2;
const int MOTOR = 9;

int last = LOW;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(MOTOR, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: motor pinwheel");
}

void loop() {
  int pressed = digitalRead(BUTTON);
  digitalWrite(MOTOR, pressed);
  if (pressed != last) {
    last = pressed;
    Serial.println(pressed == HIGH ? "motor on" : "motor off");
  }
}