#include <avr/sleep.h>

int btnPin = 2;
int count = 0;

void wakeUp(){
}

void sleepNow(){
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  attachInterrupt(0, wakeUp, LOW);
  sleep_mode();
  sleep_disable();
  detachInterrupt(0);
  }


void setup() {
  Serial.begin(9600);
  pinMode(btnPin, INPUT);
  attachInterrupt(0, wakeUp, RISING);
}

void loop() {
  Serial.print("Awake for ");
  Serial.print(count);
  Serial.println("sec");
  count++;
  delay(1000);

  if (Serial.available()) {
    int val = Serial.read();
    if (val == 'S') {
      Serial.println("Serial: Entering Sleep mode");
      delay(100);
      count = 0;
      sleepNow();
    }
  }

  if (count >= 10) {
    Serial.println("Timer: Entering Sleep mode");
    delay(100);
    count = 0;
    sleepNow();
  }

}