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Arduino – How to save power (and control 5v+)

For the first time ever, I get to play with Arduino at work! We're doing a little bit of work on tracking and the goal is to acquire as much data as possible and report it to a webserver. Therefore I've got sensors, GPS and GSM. As one can imagine, this consumes a lot of power, and as one can also imagine: the goal will be to obtain as long of battery life as possible.

Each of the modules requires either 3.3v or 5v and these are usually hardwired into the power supply. They also require anywhere from 50mA to 2A (GSM when sending data or searching for signals) and therefore can't be controlled directly from pins on the Arduino. Due to this I'll explain a proper mechanism to switch their power supplies on and off. Also, we'll then want to dig into power saving on the Arduino itself, so stick around.

Controlling modules from Arduino digital pins

Digital pins on the Arduino are only good for 30mA maximum. In fact, you shouldn't even be going anywhere near this. Hence, using one to provide +5v or GND to a module will cause that module's current needs to flow through the pin. For example, the GPS module I bought from Jaycar needs anywhere from 0-70mA. Although low, this is still too high for a digital pin.

The goal therefore is to use an electronic switch. You might be thinking 'relay', but you'll also need to note that they require current which can peak above the digital pin tolerance! You'll find that relay modules don't connect the relay actuator to the input.

The answer here is to use a transistor to allow current to flow to your modules. Depending on your current requirement, I can either recommend a 2N2222 (for anything up to 600mA) or a TIP31 (good for 3A.) I also want to switch my GSM module, and that needs 2A, so I bought a handful of TIP31s.

Using them is very straight-forward. Connect your VCCs up to whatever power source is needed and embed the transistors in the ground path. From here, switching the digital pin high will let current flow and complete the circuit for the module. Make sure you set your digital pinMode to OUTPUT.

Ideas for saving power via code

The basic idea is to put the Arduino to sleep whenever there's nothing to do. My device only needed to report every 15 minutes, so in the meantime I tried to make it snooze. It turns out that, based on internal timers and interrupts, you can only get a maximum of 8 seconds power down out-of-the-box. This wasn't quite the 15 minutes I was looking for, but numerous sources online said to just keep putting the unit to sleep for 8 seconds over-and-over.

There's some great reading here by Nick Gammon on power saving. Anything from lower frequencies to shutting down certain parts of your project. You'll also find a blog post here at the Arduino Playground describing other options. Just make sure you read that entire second article as other contributors have made corrections or other important points. Tis great to see the community involved.

...I'll come back to power saving once the project gets started up again. The proof-of-concept worked and now we need to convince the powers-that-be to let us continue.