Physical computing projects run on electricity. Of course, there are a few purely mechanical devices that do computing (like this cool four-bit calculator made from cardboard and marbles). Most of the time, off-the-shelf hackers need more than gravity to bring our projects to life.
Today, we’ll cover the things to consider when deciding how to power a project.
What, Exactly Are We Powering?
Physical computing projects typically have two distinct requirements for power. I break them into input and output categories. On the one hand, we need to run the processor, sensors and general input circuitry. On the other side, we need to feed potentially power-hungry “output” devices like LEDs, LCD screens, motors, servos, steppers, audio amplifiers and so on. Most practical projects incorporate both. On-module networking (like a Raspberry Pi with WiFi) and some of the newer artificial intelligence/machine learning hardware (like the JeVois machine vision sensor) are a gray area, since they really relate more to the processing/input circuitry side, while their power consumption aligns more with an higher-powered output device.
The distinction might be a bit new to beginning off-the-shelf hackers because a lot of examples and entry-level projects use nothing more than a breadboard, an Arduino, maybe a couple of LEDs and a little battery. Real-world physical computing projects usually control multiple outputs requiring more than a few milliamps at 5 volts. Moving and turning big things on and off is the physical part. That takes power.
We can also break projects down into portable and plug-in power categories. Portable projects likely use some kind of a battery, while the plug-in project connects to regular house current. My old steampunk conference badges are portable projects. I run the Pi-powered one off of an 1100 mAh power brick, tucked away in my pocket. The Arduino badge runs on a little 4 AA cell battery bank.
A wireless light switch enclosed in a little wooden box that sits on a table is an example of a plug-in project. Sometimes we might use batteries when the project can’t be plugged into a wall outlet. The robotic skull Hedley is an example of a project that is on battery at a local pub demo setting and on plug-in power in front of an audience at a conference tech talk. He’s frequently on a mix of power supplies when we’re developing new features and capabilities.
Batteries come in all different sizes and capacities, ranging from tiny little watch battery cells to large banks of rechargeable power bricks. Take a look at the big parts houses like Banggood, Amazon, Element14 and so on.
Years ago, I used to get a chuckle out of futurists predicting the rise of billions and billions of “Internet of Things” and how everything would seamlessly connect to everything else. While connecting my ballpoint pen to the internet was an astonishingly interesting possibility, very few of those soothsayers ever talked about how to power all those Earth-shattering gadgets.
The good news is that we are continually advancing with the tech and have a lot more practical experience powering our physical computing gadgets. A 3.0-volt non-rechargeable lithium battery runs the wireless sensors in my alarm system for months and months. More good news is that that off-the-shelf hackers have a huge variety of battery options these days. Processors keep getting more sophisticated and power-efficient as well.
Hedley the robotic skull uses lots of power. His Raspberry Pi brain draws a little over 1 amp at 5-volts. His JeVois machine vision sensor pulls about 1.5 amps at 5 volts. His 10.1-inch color LCD screen uses about 0.6 amps at 12 volts. Add in a little 5-watt audio amp and a couple of odd Arduinos and their servos for another amp and a half. That’s upwards of 25+ watts of power when everything is running.
The only way to reliably run Hedley at a conference tech talk, demoing all of his capabilities is by plugging him into the wall. I power Hedley using a variety of re-purposed wall warts, collected over the years from retired cell phones and other electronic gadgets.
Like batteries, wall warts are available in a variety of styles and capacities. The picture to the right (or above if you are on a phone) shows a couple of 12-volt models. The one on the left supplies 1.2 amps and the one on the right is rated at 1.0 amps. Notice the difference in size. The one on the left uses older, heavier linear power supply technology, while the other one is a modern “switching-mode” type device. I’ve mostly moved over to the switching type because every ounce of weight counts as I haul Hedley and all his gear through airports and the 6-odd blocks from the train to my hotel. Hedley currently has cell phone wall warts for the Raspberry Pi and steampunk color LCD monitor. There’s also one for the USB hub and another for the audio amp, the Arduinos and their servos.
While the wall warts are super reliable, I can’t say the same about USB cables, that connect the power supply to the electronic device. Hedley’s Raspberry Pi is very sensitive to the USB cable. If it is low quality, he’ll show a low voltage indicator on his display. A short, high quality cable solves the problem.
At some point, it might make sense to consolidate all the wall warts into a single dedicated power supply for brother Hedley. I’ll probably use one of the latest off-the-shelf power supply boards.
Power Supply Boards
Powering projects from off-the-shelf power supply boards has become much easier over the last year. International internet parts houses offer just about any voltage and current combination you can imagine.
For example, the Alexa voice-controlled relay used a little 5 volt, 2 amp module to power the ESP8266 and the relay board, similar to this one, with plenty of capacity to spare. In this case, we didn’t have a big worry about powering the output, since we were just switching the 110-volt lights (the output) on and off with the relay. We only needed a power supply for the computing side.
Other times we would need to think about the power supply boards for a motor, large servos or possibly a big banks of LEDs. Amazon has a great supply of boards.
We’ve just touched the surface of how to power your projects. I’ll get into the specifics of figuring capacity, the different types of batteries and safety with both batteries and plug-in modules in future stories.
Catch Dr. Torq’s Off-The-Shelf Hacker column, each Saturday, only on The New Stack! Contact him directly for consulting, speaking appearances and commissioned projects at firstname.lastname@example.org or 407-718-3274.