This code is lifted from the arduino.cc site here, the code I used is also attached.

/* * ////////////////////////////////////////////////// * //making sense of the Parallax PIR sensor's output * ////////////////////////////////////////////////// * * Switches a LED according to the state of the sensors output pin. * Determines the beginning and end of continuous motion sequences. * * @author: Kristian Gohlke / krigoo (_) gmail (_) com / http://krx.at * @date: 3. September 2006 * * kr1 (cleft) 2006 * released under a creative commons "Attribution-NonCommercial-ShareAlike 2.0" license * http://creativecommons.org/licenses/by-nc-sa/2.0/de/ * * * The Parallax PIR Sensor is an easy to use digital infrared motion sensor module. * (http://www.parallax.com/detail.asp?product_id=555-28027) * * The sensor's output pin goes to HIGH if motion is present. * However, even if motion is present it goes to LOW from time to time, * which might give the impression no motion is present. * This program deals with this issue by ignoring LOW-phases shorter than a given time, * assuming continuous motion is present during these phases. * */ ///////////////////////////// //VARS //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 = 7; //the digital pin connected to the PIR sensor's output int ledPin = 8; ///////////////////////////// //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); } } }

You can see from the code, the sensor first calibrates itself and then watches for movement. When it detects movement, the blue light goes on. You can watch the serial monitor to see how long the movement lasts.