Radar light on an Arduino Uno

// Radar light: an RCWL-0516 microwave motion sensor lights the Uno's own LED
// while something moves, and prints when the room goes busy and quiet.
//
//   RCWL-0516  VIN -> 5V, GND -> GND, OUT -> pin 2
//
// The board sends out a 3.2 GHz signal and listens for the echo. Anything
// moving shifts the echo's frequency a little, and the chip on the board
// turns that into a plain digital output: OUT goes high, to 3.3 V, as soon as
// it sees movement and stays high for two seconds after the movement stops.
// 3.3 V is well over the Uno's input threshold, so it needs no level shifter.
//
// Unlike a PIR, it sees through plastic and thin walls and it has no dead
// time after a pulse, so movement right after OUT drops is seen at once.
//
// Things to try. Click the sensor to start something moving and click again
// to stop it, and count the two seconds. Then click it twice quickly.

const int SENSOR_PIN = 2;
const int LAMP_PIN = 13;

bool busy = false;
unsigned long since = 0;

void setup() {
  Serial.begin(9600);
  pinMode(SENSOR_PIN, INPUT);
  pinMode(LAMP_PIN, OUTPUT);
  Serial.println("Radar ready");
}

void loop() {
  bool seen = digitalRead(SENSOR_PIN) == HIGH;
  digitalWrite(LAMP_PIN, seen ? HIGH : LOW);
  if (seen != busy) {
    busy = seen;
    unsigned long now = millis();
    if (busy) {
      Serial.println("Movement");
    } else {
      Serial.print("Clear after ");
      Serial.print(now - since);
      Serial.println(" ms");
    }
    since = now;
  }
  delay(20);
}