Notes

//the time we give the sensor to calibrate (10-60 secs according to the datasheet)
int calibrationTime = 30;

//the time when the sensor outputs a low impulse
long unsigned int lowIn;

//the amount of milliseconds the sensor has to be low
//before we assume all motion has stopped
long unsigned int pause = 5000;

boolean lockLow = true;
boolean takeLowTime;

int pirPin = 3; //the digital pin connected to the PIR sensor's output
int ledPin = 13;


/////////////////////////////
//SETUP
void setup(){
Serial.begin(9600);
pinMode(pirPin, INPUT);
pinMode(ledPin, OUTPUT);
digitalWrite(pirPin, LOW);

//give the sensor some time to calibrate
Serial.print("calibrating sensor ");
for(int i = 0; i < calibrationTime; i++){
Serial.print(".");
delay(1000);
}
Serial.println(" done");
Serial.println("SENSOR ACTIVE");
delay(50);
}

////////////////////////////
//LOOP
void loop(){

if(digitalRead(pirPin) == HIGH){
digitalWrite(ledPin, HIGH); //the led visualizes the sensors output pin state
if(lockLow){
//makes sure we wait for a transition to LOW before any further output is made:
lockLow = false;
Serial.println("---");
Serial.print("motion detected at ");
Serial.print(millis()/1000);
Serial.println(" sec");
delay(50);
}
takeLowTime = true;
}

if(digitalRead(pirPin) == LOW){
digitalWrite(ledPin, LOW); //the led visualizes the sensors output pin state

if(takeLowTime){
lowIn = millis(); //save the time of the transition from high to LOW
takeLowTime = false; //make sure this is only done at the start of a LOW phase
}
//if the sensor is low for more than the given pause,
//we assume that no more motion is going to happen
if(!lockLow && millis() - lowIn > pause){
//makes sure this block of code is only executed again after
//a new motion sequence has been detected
lockLow = true;
Serial.print("motion ended at "); //output
Serial.print((millis() - pause)/1000);
Serial.println(" sec");
delay(50);
}
}
}