PIR motion light on an Arduino Uno

// Motion sensor: an HC-SR501 PIR lights an LED while something moves.
//
//   PIR VCC -> 5V    PIR OUT -> pin 7    PIR GND -> GND
//   pin 13  -> the LED, through 220 ohm to GND
//
// A PIR sensor sees warm things move. Its lens splits the room into zones,
// and when warmth crosses from one zone into the next, the module drives OUT
// high. It holds OUT high for a while after the movement stops (the hold
// time, set by one of the two trimmers on the real module), so the sketch only
// has to copy OUT onto the LED.
//
// A real module also needs up to a minute to settle after power-up before it
// can be trusted.

const int PIR = 7;
const int LED = 13;

int last = LOW;

void setup() {
  pinMode(PIR, INPUT);
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: PIR motion sensor");
}

void loop() {
  int now = digitalRead(PIR);
  digitalWrite(LED, now);
  if (now != last) {
    last = now;
    Serial.println(now == HIGH ? "motion detected" : "all still");
  }
}