Arduino Power

Projects involving the Arduino family of microcontroller boards typically don't need any special power management circuitry. You connect a power source, the device boots, does it's job and then stops when the power is disconnected. Unlike a Raspberry Pi system, there is no risk of corrupting an SD memory card.

But for some applications you may want to monitor power status and respond in a more managed way, instead of just shutting down.

Consider a remote sensing device that logs temperature data, for example. If the battery runs out while it is unattended the logs simply stop. There is no way to tell immediately that low battery was the problem. It could have been software or hardware failure.

Likewise, for a portable system, it would be helpful to know how much battery life remains and when it needs recharging.

This project presents several approaches to Arduino power management with circuits, software and descriptions for each of them. Starting with a simple power-on/power-off switch all the way to a data logging system that includes battery status which is intended for unattended environment monitoring.

You can pick and choose which of the subsystems you are interested in. I'll describe all of them in increasing complexity but you can comment out those parts you are not interested in.

The project is built on the ideas in the LiPoPi and Pi Power projects, which provides similar functionality for Raspberry Pi systems.

Overview

The system provides several functions:

• Power On/Power Off using a push button

• Monitor battery voltage using the Arduino ADC

• Shutdown the system on low voltage

• Record the battery voltage and a shutdown message to file

• Display battery status using a Red/Green LED

I will introduce each of these in turn, but the common thread to all of them is combination of the Adafruit PowerBoost and the Arduino.

With each of these you add and customize two functions, arduinoPowerSetup() and arduinoPowerMonitor(), to the standard setup() and loop() functions in your code.

Adafruit PowerBoost Shield

The system uses a rechargeable LiPoly Battery and an Adafruit PowerBoost Charger Shield, which can both charge the battery and boost its output to 5V.

Adafruit makes three versions of the Power Boost - two breakout boards and an Arduino Shield. Most of the work described here uses the Power Shield, but I'll talk about using the breakout boards later on.

Please read this page on how to configure the PowerBoost Shield. To prepare the Shield kit you need to:

• Solder on the stacking header strips

• But do not attach the switch

• Solder on a length of wire (4 inches, 22AWG solid core) to the Enable Pin

• Bridge the pads for Analog pin A0 on the underside of the shield

Power On/Power Off Circuit

This basic circuit uses the press of a momentary push button switch to power up the system and, when running, to power it down -similar to the way my phone works.

When you press and hold on a powered down system, the PowerBoost is turned on and supplies power to the Arduino which then boots up and runs the program that you have loaded onto it.

When you press the button again, and release it, the Arduino disables the PowerBoost and, because it is no longer supplying power, the Arduino shuts down.

Please read this page for the details. Here is the schematic:

Note: You do not need this circuit to use the rest of this project. You can use the optional switch provided with the PowerBoost shield. The primary difference is that the basic switch just shuts the system down with no way to record a log message.

Monitor Battery Voltage

The Arduino has 6 Analog to Digital Converters (ADCs) on analog pins 0-5, which makes voltage measurement straightforward. The PowerBoost shield allows you to connect the Battery voltage (Vbat) to one of these by bridging the appropriate solder pads as described on the PowerBoost Shield page. In this example I have linked it to Analog pin 0.

The voltage of LiPoly batteries varies from around 4.25V when fully charged to around 3.7V when discharged. The ADC converts the voltage range of 0V to 5V into the integer range of 0 to 1023, and we access this value by calling analogRead().

This code snippet shows how to do this as well as calculating the relative battery state using the min and max battery voltages.

 float maxVoltage = 4.25;
  float minVoltage = 3.75;
  arduinoPowerVoltage = float(analogRead(arduinoPowerVoltagePin)) / 1024.0 * 5.0;
  float fractionalVoltage = (arduinoPowerVoltage - minVoltage) / (maxVoltage - minVoltage);

LiPoly batteries do not lose voltage linearly as they discharge, but it's close enough that we can estimate the remaining battery life using the fractional voltage.

So we can get the battery voltage and estimate the life remaining, but we need a way to communicate that to the user. We could use an alphanumeric display and in the next section I will show how to log this to a file on a SD card. But those can be overkill for many projects.

A simpler approach is to use an RGB and change the color to reflect battery life. For example:

• Green - more than 50%

• Yellow - 20% to 50%

• Red - less than 20% - time to recharge

To implement that you just add an RGB Led and two 1K resistors to the circuit and add a bit of code to arduinoPowerMonitor().

RGB LEDs typically use a Common Anode so this circuit works with that:

NOTE: In practice, I use the 3.3V supply with my LEDs.

NOTE: As I will show later on, there can be a significant 'oscillation' in the measured voltage, for reasons that I don't understand. If the battery is close to one of the values where the LED color changes, you may see it switch back and forth between the two states - very annoying. A quick fix is to place an 0.1uF capacitor between analog pin A0 and Ground.

This breadboard layout does not show the battery or the Arduino. The sketch arduino_2_voltage_led implements Power On/Power Off and LED voltage display. The code sets the green or red pin to low to turn the led on and high to turn it off.

NOTE: You can check the measured voltages and/or debug by uncommenting the calls to Serial in the code and running the Serial Monitor in the IDE. You will see actual and fractional battery voltage. For some reason the first call is incorrect.

Logging Battery Voltage to a SD Card

The LED indicator of voltage is useful for portable, battery powered Arduino systems where you don't care too much about the details, you just want to know when you need to recharge the battery.

But if you are monitoring sensors in a standalone system, say, then you might want to record the battery voltage along with sensor data to give you a better idea of power consumption over time.

For this part of the project, I'm using the Adafruit Data Logging Shield which can record data in a CSV format file on the attached SD card, along with timestamps. The Adafruit Guide explains how the SD and RTC libraries work and you should understand that before working on this part of the project.

The circuit is unchanged from the previous example - it's worth keeping the LED as it provides immediate feedback on the battery status, but this is not necessary.

Because we are logging data to the card, it is important that we log a shutdown message. That way we can tell if the shutdown was intentional or whether some error happened to the system.

In addition, because we are monitoring the voltage, we can check when the voltage drops below some minimum level and safely shutdown the system. Again, we can log a message about that just before the shutdown.

The sketch for this is in arduino_3_voltage_logging. This includes a few utility routines to generate a properly formatted timestamp string, etc. and includes a hack to flash the red LED if you forget to insert a SD card, which I do all the time.

The whole stack of Arduino Uno, Data Logging Shield, PowerBoost Shield and breadboard looks like this:

Voltage Data

Here is a plot of the Battery voltage over time showing the gradual decline over a period of around 20 hours. The decline is not truly linear but, for the purpose of estimating the remaining power, it is close enough.

As I mentioned earlier, it really helps to add a 0.1uF capacitor between Analog Pin A0 and Ground. In this sample, the purple line shows the impact of the capacitor compared to the original in blue.

Logging Environmental Data to a SD Card

To finish up, here is one more addition to the project. One project that I have in mind is a standalone environmental monitor for my garden - temperature, soil moisture, etc.

I will install the Arduino, etc, in a sealed box that will be left unattended for some period of time. As well as the battery running out, I want to check that no moisture gets inside the case and that it does not experience extreme temperatures that might damage the system. A cheap and easy way to measure temperature and humidity is the DHT22 sensor - check out Adafruit's Tutorial for the details.

In my example code, I use digital pin 5 instead of pin 2, as I am already using that for the Power Off button interrupt.

The code for this is in arduino_4_voltage_temp_humidity_logging. Once I had this working on a breadboard, I wired this up on an Adafruit Proto Shield. The DHT22 sensor is pretty bulky - but it is cheap. Take a look at other breakout boards from Adafruit, Sparkfun, etc for other alternatives.

Power Conservation in Arduino projects

The next stage in my project is to reduce power consumption and extend the battery life.

My starting point for this is excellent Adafruit tutorial on Low Power Datalogging from Tony DiCola.