W E R cabling job for a thinner, tailor-made, and overall more premium-looking cord to compliment the design. This
audience member was still not impressed.
“I bet you can’t wait until power is wireless!” she quipped.
After the welcomed segue, I was able to jump back into
my pitch, but something about what she said stuck with
me. On the drive back, I thought about whether this could
be transformed into a truly wireless design. With the advent
of wireless charging stations and standards such as Qi,
consumers see wireless charging as a common feature and
expect designers to bake it into every one of their gadgets.
The square cube law limits our ability to cheaply and safely
transmit power over useful distances wirelessly at the
consumer level. But there is one form of energy present
in the same environments you’re likely to find people: light
energy. Capturing it is pretty easy, and there is a time-tested solution to draw on for inspiration inside every solar-powered office calculator.
Making something completely wireless that doesn’t rely
on changing or charging batteries shouldn’t be difficult,
evidenced by the vast array of solar Io T devices that
advertise regular and reliable wireless communication with
no maintenance necessary. However, these devices are
usually simple sensors with small duty cycles; the typical
use-case for wireless sensor solutions is a small sub-
kilobyte payload over a proprietary low-power wireless link
lasting a few seconds, with potentially hours in between to
allow for a solar cell to recharge the onboard supercapacitor
that ultimately powers the device. This means the average
current consumption of these devices is usually in the
microamp range, so that even the smallest solar panel can
easily keep these devices’ small supercaps topped off.
In its current form, MyForecast consumed upwards of half
an amp with all the LEDs on full brightness. I clearly needed
to rethink my entire design if I wanted this to work. One
way to radically redesign a gadget is to look back to the
Core Function, or “What the most abstract way to describe
the design’s intention?” I settled on: give the consumer a
way to quickly, intuitively, and cheaply consume a relevent
weather forecast. This project was then broken down into
three separate problems: the display/interface that tells the
user the information they need to know, the WiFi system that
downloads the information, and the power capture/supply
solution that keeps the whole thing running.
I decided to keep the WiFi system from the original
MyForecast design largely unchanged. The ESP8266 can
be put into a deep sleep mode, where average current
consumption reaches 75 microamps. This was well within
the limits of my design goals, so I moved on to the trickiest
part of my design: the display/interface.
To initially set up the device, the consumer has to input
their WiFi hotspot credentials so MyForecast can connect
to openweather and download the weather forecast for their
area. A previous project I worked on used the WiFi system
to act as an access point during setup, which allowed
the consumer to connect to it using a smartphone or PC
and complete the setup using their browser. I reused this
approach and added a few new touches like Captive Portal.
Once this was tested and I could see a successful forecast
downloading over my debug serial link, I started to look at
how I could display this parsed forecast to the consumer.
I did not have any immediate ideas pop into my head while
looking through my parts bins for a sub-milliamp
display. I considered low-power LCD screens
and segmented displays, but I held out for a more
interesting approach. A few days later I stumbled
across someone else’s e-paper project while
trawling Hackaday, a site filled with advice and
open-source hardware plans for engineers and
hobbyists. I tried to see if I could get an example kit
or demo board from an e-paper manufacturer. After
much searching, I found one company, Pervasive
Displays, who agreed to work with me and guide
me through some of the intricacies of e-paper.
With their help I created an open-source reference
design for one of their latest screen types: www.
crowdsupply.com/soniktech/e-paper-shield-kit . To
interface with the display, I wrote a library based
on Adafruit’s open-source graphic effects library,
which saved me lots of development time.
Now that I had the WiFi system and display
parts of the design hooked up and working,
I moved onto the last part of the design: the
power. Keeping a gadget untethered requires a
little math to make sure you don’t spend money
where you don’t need to, or worse, spend money