Here is my simplest Arduino-based gadget:
It consists of an AtTiny85, one LED, one resistor and one CR2032 battery.
The objective was to have a "flashing LED" that could be used as a burglar deterrent, with a minimal power consumption.
Sketch below:
// ATtiny85 sleep mode, wake on pin change interrupt or watchdog timer
// Author: Nick Gammon
// Date: 12 October 2013
// ATMEL ATTINY 25/45/85 / ARDUINO
//
// +-\/-+
// Ain0 (D 5) PB5 1| |8 Vcc
// Ain3 (D 3) PB3 2| |7 PB2 (D 2) Ain1
// Ain2 (D 4) PB4 3| |6 PB1 (D 1) pwm1
// GND 4| |5 PB0 (D 0) pwm0
// +----+
#include <avr/sleep.h> // Sleep Modes
#include <avr/power.h> // Power management
#include <avr/wdt.h> // Watchdog timer
const byte LED = 3; // pin 2
// watchdog interrupt
ISR (WDT_vect)
{
wdt_disable(); // disable watchdog
} // end of WDT_vect
void resetWatchdog ()
{
// clear various "reset" flags
MCUSR = 0;
// allow changes, disable reset, clear existing interrupt
WDTCR = bit (WDCE) | bit (WDE) | bit (WDIF);
// set interrupt mode and an interval (WDE must be changed from 1 to 0 here)
WDTCR = bit (WDIE) | bit (WDP2) | bit (WDP1) | bit (WDP0); // set WDIE, and 2 seconds delay
// pat the dog
wdt_reset();
} // end of resetWatchdog
void setup ()
{
resetWatchdog (); // do this first in case WDT fires
pinMode (LED, OUTPUT);
} // end of setup
void loop ()
{
digitalWrite (LED, HIGH);
delay (1);
digitalWrite (LED, LOW);
goToSleep ();
} // end of loop
void goToSleep ()
{
set_sleep_mode (SLEEP_MODE_PWR_DOWN);
ADCSRA = 0; // turn off ADC
power_all_disable (); // power off ADC, Timer 0 and 1, serial interface
noInterrupts (); // timed sequence coming up
resetWatchdog (); // get watchdog ready
sleep_enable (); // ready to sleep
interrupts (); // interrupts are required now
sleep_cpu (); // sleep
sleep_disable (); // precaution
power_all_enable (); // power everything back on
} // end of goToSleep
It sleeps most of the time (lazy thing!) and wakes every 2 seconds to flash the LED for one millisecond. It should use about 4 µA while asleep and when awake the LED will consume about 2 mA. Since that is only for 1 mS per 2 seconds that should average out at about another 1 µA.
If we assume that the battery has a capacity of 210 mAh then it should last for:
210 / 5 * 1000 / 24 / 365 = 4.79 years
