# Arduino Sleep_Watchdog_Battery

## Watchdog and Sleep functions

This example shows how to make use of the Watchdog and Sleep functions provided by the ATMEGA 168 chip . These functions are useful if you want to build  low power consuming devices operated by battery or solar power.

The reduced power consumption is achieved by through a  intermittent  operation of the system .In case of Arduino your main loop will be  executed once before  the system is  put into the sleep mode. After a few seconds t the watchdog wakes the system up  and the main loop is executed again. The ratio between main loop execution time and watchdog time determines the amount of power that will  be saved.

When we assume that the time to  measure a sensor and making  some decisions will take 10 millisecond and the watchdog is set to 8 seconds the on/off ratio is 800 which extends the battery live time by this factor.

## Battery live time calculation

Now we want to know long we can operate our device with standard alkaline AA Cells. These Cells even the cheap ones provide between 2000 ad 3000 milliamps per hour (mAh). We assume our system takes 20 mA when operating and 0.05 mA when sleeping.

In normal operation with a current of 20mA  the battery will last 2000/20 = 100 hours or about  4 days.
In intermittent operation with a factor of 800 the operating current is reduced to 20mA/800 = 0.025mA plus the current that is drawn in sleep mode 0.05mA.
Now with 2000/(0,025+0,05)  we come to 27000 hours lifetime which is about 3 years.

## Nightingale Example

To demonstrate the watchdog and sleep functions we build a little device that measures the daylight and activates a sound when nigthfalls. We wont use the Arduino hardware here because the USB chip and voltage regulator takes too much power in the sleep mode. So we solder a little circuit around a 28 pole DIL Socket where we insert the atmega chip from the original arduino board after uploading the code .

## Arduino minimal Hardware / Nightingale schematic

DIL 28 Socket upside down

Instead of raping the arduino board its atmega chip we recommend to buy some blank atmega 168 pu-20  chips and burn a bootloader to them. What you need is a AVRIPS-MKII programmer (not so expensive) and a 9 Volt dc power supply to run the arduino board without usb. When you  extract the chips be careful not to bend the IC pins a screwdriver is a helpful tool for this job.

## Source Code

nightingale.zip (1,7 KB)

## Contact

Martin Nawrath, nawrath@khm.de