Relay switches a motor: button on 12, relay on 9, on an Arduino Uno

// A relay switches a motor: press the button to start or stop it.
//
//   pin 9 -> 1k -> the NPN transistor's base; emitter -> GND
//   5V -> the relay coil -> the transistor's collector, a diode across the coil
//   two AA cells -> the relay's contact -> the motor (and an LED with 220 ohm)
//   10k from 5V -> pin 12; the push button from pin 12 -> GND
//
// The motor has its own battery; the relay's contact is the only thing that
// joins it to the motor, so the board never carries the motor's current. The
// diode catches the kick the coil gives when it is switched off.
//
// The sketch waits for the button to stay put for 10 ms before it believes a
// change, then flips the relay on each press.

const int RELAY = 9;
const int BUTTON = 12;

int buttonState = HIGH;
int lastReading = HIGH;
int relayState = LOW;
unsigned long lastChange = 0;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, relayState);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: press the button on pin 12 to start or stop the motor");
}

void loop() {
  int reading = digitalRead(BUTTON);
  if (reading != lastReading) lastChange = millis();
  if (millis() - lastChange > 10 && reading != buttonState) {
    buttonState = reading;
    if (buttonState == LOW) {
      relayState = !relayState;
      digitalWrite(RELAY, relayState);
      Serial.println(relayState ? "motor on" : "motor off");
    }
  }
  lastReading = reading;
}