On and off desk lamp: button on 7, relay on 2, on an Arduino Uno

// An on and off desk lamp: one button toggles a relay, the relay the lamp.
//
//   relay module S -> pin 2   + -> 5V   - -> GND
//   relay COM -> 5V; relay NO -> 220 ohm -> LED -> GND (the "lamp")
//   button: one side -> pin 7, the other -> GND (internal pull-up)
//
// A relay is a switch worked by a magnet: pin 2 HIGH pulls the contact from
// NC across to NO, and NO feeds the lamp. Here the lamp is an LED; in a real
// lamp it would be mains, which is why the board never touches it directly.
//
// Holding a button down is one long LOW, and loop() sees it thousands of
// times. To toggle once per press, the sketch remembers what the button read
// last time and acts only when it changes from HIGH to LOW: the edge. It
// checks again 20 ms later, so a bounce of the contact is not taken for a
// press, then waits for the button to come back up.

const int RELAY = 2;
const int BUTTON = 7;

int lastButton = HIGH;
bool lampOn = false;

void setup() {
  pinMode(RELAY, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: press the button on pin 7 to switch the lamp");
}

void loop() {
  int now = digitalRead(BUTTON);
  if (lastButton == HIGH && now == LOW) {
    delay(20);
    if (digitalRead(BUTTON) == LOW) {
      lampOn = !lampOn;
      digitalWrite(RELAY, lampOn ? HIGH : LOW);
      Serial.println(lampOn ? "relay on: lamp on" : "relay off: lamp off");
      while (digitalRead(BUTTON) == LOW) {
      }
      delay(50);
      now = HIGH;
    }
  }
  lastButton = now;
}