Electronic – Best way to run 3.3V circuit for long time using 3 AAA batteries

The Atmega328 provides six power saving modes, ordered from minimal to excellent (estimated current consumptions from this forum post):

• SLEEP_MODE_IDLE: 15 mA
• SLEEP_MODE_ADC: 6.5 mA
• SLEEP_MODE_PWR_SAVE: 1.62 mA
• SLEEP_MODE_EXT_STANDBY: 1.62 mA
• SLEEP_MODE_STANDBY : 0.84 mA
• SLEEP_MODE_PWR_DOWN : 0.36 mA

Quoting the original question:

I figured I could put sleep calls for certain times inside the loop method"

You would need to use sleep_cpu() after setting up the sleep mode you require, from the list above. The Arduino Playground has a useful post about this.

The application needs to be interrupt driven, use the above sleep modes extensively, and wake the processor up on button push, timer overflow and watchdog timer events to actually execute tasks.

Additional power savings can be obtained through the following steps:

• Use the microcontroller's internal oscillator and a low clock rate (8MHz instead of 16) - but ensure that time and timing related code still works as expected. A different version of the bootloader might be needed for this.
• Avoid keeping LEDs on for long if the application uses them. Using a rapid double or triple flash of short duration (0.05 second on, 0.5 second off), with gaps of seconds in between, ensures noticeable indication with minimal power consumption
• Use a switching regulator instead of a linear one, if a regulator is required.
• Run the microcontroller at lower voltage if supported, 3.0 Volts (e.g. CR2032 Lithium cell, no regulator needed) or 3.3 Volts instead of 5 Volts.
• Follow recommendations in the datasheet for unused input and output pin settings for minimum power wastage.

Incorporating these suggestions allows for running microcontroller applications for weeks or months on a single CR2032 coin cell, and years on a LR123 type lithium cell. Of course, your mileage may vary depending on what sensors, outputs and actual processing your application requires.

Some useful references:

• Hush little microprocessor… AVR and Arduino sleep mode basics
• Adventures in Low Power Land - Sparkfun tutorial
• Power saving techniques for microprocessors (forum post)

Coin cells will not work well with your needs. While it varies by brand and size, most coin cells have low capacity, and an internal resistance that limits current output. A CR2032 has a nominal 250ma capacity (and that varies based on current load btw), and an internal resistance (Equivalent Series Resistance or ESR) of 18 to 30ohms, again based on current load). Depending on the speed of the current draw, you are going to get voltage drops, and quickly drain the coin cell. If you can get that much current out of it at a given time. 70mA is pushing it even for high quality CR2032, and 220mA is improbable, frankly, impossible. That is why your CR2032 boosted to 3.3v isn't working. The current draw of the regulator is causing the cell's voltage to drop.

That is, without having multiple in parallel (or using high capacity caps). That's the second part you asked. Batteries in series are a sum of the voltages, with the same amount of current. Batteries in parallel are a sum of the current capacities, with the same voltage.

As for your options, one or two AAA batteries with a boost converter would suit you better. It gives you relatively high capacity while still keeping space to a minimal. The TPS61200 you mention would work, if you can deal with a tiny (3x3mm leadless package. The TPS6107x family would do the same, same minimal components, in a sot6 package. You only need 1 battery for either of these. An AA gives 2400mAh nominal, an AAA gives 1200mAh nominal.