#include <avr/sleep.h>
const byte LED = 13;
int sensorPin = A0;    // select the input pin for the potentiometer
int sensorValue = 0;  // variable to store the value coming from the sensor
// keep the state of register ADCSRA
byte keep_ADCSRA;
void wake ()
{
  // cancel sleep as a precaution
  sleep_disable();
  // must do this as the pin will probably stay low for a while
  detachInterrupt (0);
  ADCSRA = keep_ADCSRA;
}  // end of wake

void setup ()
{
  digitalWrite (2, HIGH);  // enable pull-up
}  // end of setup

void loop ()
{

  pinMode (LED, OUTPUT);
  // three second LED to confirm start of process:
  digitalWrite (LED, HIGH);
  delay (3000);
  digitalWrite (LED, LOW);
  delay (50);
  // read the value from the sensor:
  sensorValue = analogRead(sensorPin);   
  // turn the ledPin on
  digitalWrite(LED, HIGH); 
  // stop the program for <sensorValue> milliseconds:
  delay(sensorValue);         
  // turn the ledPin off:       
  digitalWrite(LED, LOW);   
  // stop the program for for <sensorValue> milliseconds:
  delay(sensorValue);

  pinMode (LED, INPUT);
  keep_ADCSRA = ADCSRA;
  // disable ADC
  ADCSRA = 0; 

  set_sleep_mode (SLEEP_MODE_PWR_DOWN); 
  sleep_enable();

  // Do not interrupt before we go to sleep, or the
  // ISR will detach interrupts and we won't wake.
  noInterrupts ();

  // will be called when pin D2 goes low 
  attachInterrupt (0, wake, LOW);

  // turn off brown-out enable in software
  // BODS must be set to one and BODSE must be set to zero within four clock cycles
  MCUCR = bit (BODS) | bit (BODSE);
  // The BODS bit is automatically cleared after three clock cycles
  MCUCR = bit (BODS);

  // We are guaranteed that the sleep_cpu call will be done
  // as the processor executes the next instruction after
  // interrupts are turned on.
  interrupts ();  // one cycle
  sleep_cpu ();   // one cycle

} // end of loop