Newbie - Issues with lowpower + SD card logging


I'm trying to get my Pro Mini to log PIR movement and go into sleep after the PIR has been activated.

I can get it to boot up just fine and await the first reading from the PIR, it logs the event on the SD card but it doesn't go into sleep afterwards (and awaits the next movement-reading from the PIR). Instead it will just flip to the Sleeping state upon activation and then to Awake on the next activation.

Im using a HC-SR501 PIR and this is my (extremely) messy code:

// PIR motion detector sensor with Arduino nano in ultimate power saving mode 
//(reducing power consumption almost by 70% (4.7mA) of normal operation consumption (13mA)) 
// Current code by Electromania 24th Feb 2016 //
// Thanks to:-  for information about sleep.h library and modes
             // for useful explanation of  "Low Power Arduino! Deep Sleep Tutorial" for bare arduino uno chip
             //  // datasheet

#include <avr/interrupt.h>        // Library to use interrupt
#include <avr/sleep.h>            // Library for putting our arduino into sleep modes
#include <SdFat.h> 
SdFat SD;

const int PIRsensorInterrupt = 0; //interrupt 0 at arduino nano pin D2
const int LedPin = 9;            // external LED or relay connected to pin 13
const int chipSelect = 10;

volatile int lastPIRsensorState = 1;  // previous sensor state
volatile int PIRsensorState = 0;   // current state of the button

void wakeUpNow(){                  // Interrupt service routine or ISR  
  PIRsensorState = !lastPIRsensorState;    // we negate previous state and assign to current state

void setup() {
    pinMode(LedPin, OUTPUT);    // initialize pin 13 as an output pin for LED or relay etc.
 Serial.begin(115200);     // initialize serial communication only for debugging purpose
  Serial.println("Warming up... wait for a min...");


 // delay execution of sketch for a min, to allow PIR sensor get stabilized
 for( int i = 1; i <= 120; i++){  // LED at pin 13 blinks until PIR sensor is stabilized
    digitalWrite(LedPin, HIGH); 
    digitalWrite(LedPin, LOW); 
  Serial.println("Ready");     // enable only for debugging purpose
  pinMode(PIRsensorInterrupt, INPUT);        // define interrupt pin D2 as input to read interrupt received by PIR sensor

void Hibernate()         // here arduino is put to sleep/hibernation
 set_sleep_mode(SLEEP_MODE_PWR_DOWN);  // lowest power consumption mode 

 //"The Power-down mode saves the register contents but freezes the Oscillator, disabling all other chip functions 
 // until the next interrupt or hardware reset."  text from ATMEGA328P datasheet

 ADCSRA &= ~(1 << 7);   // Disable ADC - don't forget to flip back after waking up if you need ADC in your application ADCSRA |= (1 << 7);  (From Kevin's sketch)

    noInterrupts ();          // make sure we don't get interrupted before we sleep
 sleep_enable();                       // enable the sleep mode function
 sleep_bod_disable();                  //to disable the Brown Out Detector (BOD) before going to sleep. 

  attachInterrupt (PIRsensorInterrupt, wakeUpNow, CHANGE);  // wake up on low level

 //attachInterrupt(PIRsensorInterrupt,wakeUpNow, CHANGE);   // Attach interrupt at pin D2  (int 0 is at pin D2  for nano, UNO)
  // here since PIR sensor has inbuilt timer to swtich its state from OFF to ON, we are detecting its CHANGE IN STATE to control our LED/relay at pin 13. 
   // therefore, we will not need to use arduino delay timer to Set "ON time" of our LED/relay, it can be adjusted physically using potentiometer provided on PIR sensor board.
   // This further helps in using SLEEP_MODE_PWR_DOWN which is ultimate lowest power consumption mode for ATMEGA8328P chip  
   //(please note - because of onboard power regulators of arduino boards, power consumption cannot be reduced to predicted few microAmps level of bare chips. 
   //To achieve further reduction in current consumption, we will need bare ATMEGA328P chip)

  interrupts ();           // interrupts allowed now, next instruction WILL be executed
  sleep_cpu ();            // here the device is put to sleep

   for (int i = 0; i < 3; i++) {
   if(i != 9)//  because the LED/Relay is connected to digital pin 13
    pinMode(i, INPUT);
 sleep_mode();                // calls function to put arduino in sleep mode
 sleep_disable();            // when interrupt is received, sleep mode is disabled and program execution resumes from here
 detachInterrupt(PIRsensorInterrupt);   // we detach interrupt from pin D2, to avoid further interrupts until our ISR is finished

void loop() {
 interrupts();    // enable interrupts for Due and Nano V3

 if (PIRsensorState != lastPIRsensorState){

  if (PIRsensorState == 0) {
     digitalWrite(LedPin, LOW);
     Serial.print("Sleeping-");            // enable for debugging
    Serial.println(PIRsensorState);   // read status of interrupt pin


  else {   
     digitalWrite(LedPin, HIGH); 
  String dataString = "";

     dataString += "movement detected at ";


  //iniot SD card
  if (!SD.begin(chipSelect))
    Serial.println("Card failed, or not present");
  Serial.println("card initialized.");


    // open the file.
  File dataFile ="data.txt", FILE_WRITE);
  // if the file is available, write to it:
  if (dataFile)



  // if the file isn’t open

     Serial.print("Awake-");    // enable for debugging
       Serial.println(PIRsensorState);  // read status of interrupt pin   enable for debugging


   lastPIRsensorState = PIRsensorState;    // reset lastinterrupt state
  Hibernate();   // go to sleep - calling sleeping function

It seems so simple, yet I've spent the last 3 hours trying to fix it - without success....

Have you tried to log what the PIR sensor does? The sensor may "bounce" and cause the reading to fluctuate when the state changes.