IEEE Spectrum August, 2015 - 25

DaviD schneiDer (4)

the answer turned out to be already in my
possession: A Kindle touch e-reader. It was
the perfect size-big enough to display a lot
of information at once in a large font and to
show me a decent chunk of a map without
scrolling. Its e Ink display is clear and crisp
even in bright light. And the price was right.
the only problem was that I couldn't hack its
software to make it do what I wanted.
this wasn't just a technical issue; it was a legal one. Modifying such a gadget to operate in
ways other than the manufacturer intended-
even if you own the device-flirts with provisions of the Digital Millennium Copyright Act,
bizarre as that may seem. the U.S. government (for odd reasons, specifically the librarian of Congress) doesn't want you to have full
control of the software on your phone, tablet,
or e-reader-even if it's open source. but after
some pondering, I realized that I could make
the Kindle serve my needs without touching a
single line of code running on it.
the trick was to use the Kindle's "experimental" Web browser, allowing me to put
the brains of the computer elsewhere, in this
case on a US $20 Raspberry Pi Model A. the
Raspberry Pi, being cheap and open, was
perfect for the job. by plugging a suitable
Wi-Fi dongle ($12) into its USb port, I could
make it into a Wi-Fi access point. Run a Web
server on the Pi and it can send htMl of your
choosing to the Kindle to display.
My system consists of three parts: the
Raspberry Pi and its battery (which reside in
the bike's saddlebag); a small PC board of my
own design (which carries a $30 GPS module and plugs into the Pi); and a Kindle touch
(mounted on the handlebars using bits of
foam pipe insulation and a few rubber bands).
two reed switches mounted to the frame
of my bike sense wheel rotation and pedal
cadence by virtue of magnets attached to
a spoke and to the inner chain ring. these
switches are electrically connected to pins
on the Raspberry Pi's GPIO (general purpose
input/output) port, allowing the Pi to know
how fast the wheels and pedals are turning.
the list of software that I needed to install
and configure on the Pi to make all this work
is not short. It includes hostadp, which allowed me to transform the Pi into a wireless

BIKE BYTEs

BuILDINg THE BIKE CoMpuTEr
required making hardware upgrades to a
raspberry Pi, adding gPs and Wi-Fi [top].
With its battery pack, the computer fits
easily into my rear saddlebag [second
from top]. reed switches and magnets
detect how fast the pedals and wheels are
turning [bottom two photos].

access point, thanks to some helpful online
instructions. I also installed Flask, a lightweight framework for writing Web applications in Python (again, following helpful leads
on the Web). but it didn't end there. For example, I needed another program, gpsd, to
parse the GPS data coming in from my little
add-on board. And there were JavaScript
libraries as well to load, including leaflet,
which allowed me to add a nifty mapping
feature to my cycling computer.
eventually, I was able to get my Flask app to
run automatically when the Pi powered up and
have the Pi act as a captive portal, meaning that
it served up my app's output no matter which
URl the Kindle's browser was pointed to.
Next, I had to write a cycling-computer Web
app that would look good on the Kindle's e Ink
display. there were two notable complications.
One arose because of the way e Ink works. It's
very susceptible to ghosting, where pixels don't
properly switch to a new state, so Amazon.
com's engineers arranged for the Kindle's
browser to cycle the entire displayfrom black to
white and back again when a new page loads.
this initially made for awkward flickering, because my initial app sent a new page with updated values every second. the solution was to
use what's known in Web-development circles
as Ajax techniques to update just the necessary
elements without sending a whole new page
and triggering a refresh.
Another twist peculiar to the Kindle is that
when it connects to a new access point, it immediately tries to reach Amazon. If it doesn't
get the right response, it won't treat the access point as valid. the solution was simple:
have the Pi supply the Kindle with the short
response it was expecting from Amazon.
In the end, I managed to pull together a system that shows distance, instantaneous speed,
average speed, cadence, elapsed time, and
clock time all at once, with digits large enough
to read without squinting. Swiping and pressing buttons brings up other screens that offer
such goodies as real-time mapping and logging. even if you have to buy a Kindle for this
project, the cost of the hardware involved is
probably less than half what you'd be paying
for a commercial unit with these capabilities.
-DAvID sCHNEIDEr

SPECTRUM.IEEE.ORG

nORTh aMERICan

aUG 2015

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Table of Contents for the Digital Edition of IEEE Spectrum August, 2015