Dealing with false triggers from a PIR sensor 

The PIR sensor is very sensitive and is known to be creating false triggers, i.e. it will occasionally go LOW to indicate that there is no movement, even though there is movement.

There are two things you can do to create a more reliable detection system (and this can deliver the functionality you are asking for).

1. Calibrate the sensor.

Notice that there are two orange knobs on one side of the sensor. Have a look at this picture where I am marking the role of each knob.

The one on the left is for distance. Turn it clockwise to make it possible to detect movement up to 7 meters away from the sensor. I tend to keep this turned almost all the way anticlockwise since all my experiment are short distance.

The one on the right is for the dwell time. Turn it clockwise to decrease this time, meaning that for the device to trigger, a longer movement is required. I keep this turned all the way anticlock-wise, producing a very sensitive signal, and manage erroneous trigger in the sketch.

There is also a jumper that controls the trigger mode. Make sure you have this in H-mode (see picture). In H-mode, the sensor will trigger repeatedly when it “thinks” that there is movement.

2. Add some logic to the sketch to help deal with the imperfections of the sensor.

In this example (heavily borrowing from a sketch I found in the Arduino playground), I define a cutoff time. Any LOW triggers from the sensor within this cut-off time are ignored as false and the Arduino continues to think that there is movement. Have a look at it (but get a copy from Github if you want to try it out):

/*  Infrred sensor demo sketch
 * 
 * This sketch demonstrates the use of the PIR infrared motion sensor.
 * 
 * The sketch adds to the bare minimum version by allowing extra time 
 * after the sensor goes LOW for the LED to remain HIGH. This could be 
 * useful in a scenario where you want a light to go on in a room when 
 * someone enters it, but to turn of a few minutes after they leave it.
 * 
 * This sketch was adapted from the original that comes with the 
 * Adafruit library for Arduino Step by Step by Peter Dalmaris.
 * The location of the original is at https://github.com/futureshocked/ArduinoSbS2017/blob/master/_0500_-_Infrared_Sensor_2/_0500_-_Infrared_Sensor_2.ino
 * 
* Components
 * ----------
 *  - Arduino Uno
 *  - Infrared motion sensor
 *  - An LED
 *  - A 220 Ohm resistor for the LEd
 *  
 *  Libraries
 *  ---------
 *  - None
 *
 * Connections
 * -----------
 *  Break out    |    Arduino Uno
 *  -----------------------------
 *      VCC      |      5V
 *      GND      |      GND
 *      OUT      |      4
 *      
 * Also connect an LED to simulate a controlled device. 
 *           220 Ohm
 *    3 ----/\/\/\/\----(LED |)----GND
 
 * 
 * Other information
 * -----------------
 * For information on PIR sensors: https://en.wikipedia.org/wiki/Passive_infrared_sensor
 * Datasheet: https://www.mpja.com/download/31227sc.pdf
 *  
 *  Created on October 14 2016 by Peter Dalmaris
 * 
 */
 
int ledPin   = 4;             // choose the pin for the LED
int inputPin = 3;             // choose the input pin (for PIR sensor)
int pirState = true;          // we start, assuming no motion detected
int val      = 0;             // variable for reading the pin status
int minSecAfterPIRLow = 5000; // If the sensor reports low 
                              // remain HIGH for this many milliseconds
long unsigned int timeTurnedLow;                  
boolean takeLowTime;  //This variable is used to record the event where the sensor turns off

//the time we give the sensor to calibrate (10-60 secs according to the datasheet)
int calibrationTime = 30;  
 
void setup() {
  pinMode(ledPin, OUTPUT);      // declare LED as output
  pinMode(inputPin, INPUT);     // declare sensor as input

  takeLowTime = LOW;  
 
  Serial.begin(9600);
  
  Serial.println("Waiting for the sensor to warm up.");
  delay(calibrationTime * 1000); // Convert the time from seconds to milliseconds.
  Serial.println("SENSOR ACTIVE");
}
 
void loop(){
  val = digitalRead(inputPin);  // read input value

  if (val == HIGH) {            // check if the input is HIGH
    digitalWrite(ledPin, HIGH);  // turn LED ON
    if (pirState == LOW) {
      // we have just turned on
      Serial.println("Motion detected!");
      // We only want to print on the output change, not state
      pirState = HIGH;
    }
  } else {
    
    if (pirState == HIGH){
      // we have just turned off
      Serial.println("Motion ended!");
      // We only want to print on the output change, not state
      pirState = LOW;
      timeTurnedLow = millis(); //take the millis at the moment the sensor went LOW
    }
  }
   
  //This block checks to see if enough time has passed after the PRI went LOW.
  //If yes, and assuming that the PIR sensor did not go HIGH again, turn off the LED
  if(!pirState && (millis() - timeTurnedLow) > minSecAfterPIRLow){
     Serial.println("Extended time HIGH ended!");   
     digitalWrite(ledPin, LOW); // turn LED OFF  
    }

  delay(1000);
}