IEEE Spectrum November, 2016 - 22

needed was integration, configuration, and
some glue software.
First, I needed to build an infrared control link, which is basically two infrared
LEDs operated via the Raspberry Pi's general purpose input/output (GPIO) connector and Lego's receiver. I used schematics
and instructions by Alex Bain to build the
hardware. For the software, I downloaded
and installed LIRC, a package that has support for decoding and transmitting signals
used by over 2,500 different infrared remote controls.
Getting the LIRC package to work with
my home-brew infrared link was a simple
matter of editing some configuration files
and specifying which GPIO pins I had wired
up for input and output.
Now I needed to get LIRC to send valid
Lego command signals. This means specifying the waveform-a pattern of infrared
pulses-that must be sent for each Lego
command. Fortunately, Lego has released a
document specifying the protocol and format of all commands (for example, a binary
value of 1 is transmitted by six pulses of IR
light at a frequency of 38 kilohertz, followed
by a pause of 553 microseconds). The Lego
Power Functions system supports up to
four receivers working on different channels, and each receiver has a red side and a
blue side, each of which can independently
control a motor.
Building on this information, Conor Cary
created lego-lirc, a Java program that generates command waveforms, complete
with the correct checksums, in a format that
LIRC understands. I downloaded lego-lirc
and, with the Lego documentation in hand,
created additional waveforms that allow
the transmission of PWM (pulse-width
modulation) commands. These commands
allow precise speed adjustment of Power
Function motors without requiring timing
loops in the application software. (To avoid
the hassle of running lego-lirc, you can just
download my file of generated LIRC waveforms directly from my GitHub repository
under the username of dspinellis.) To configure LIRC to use the Lego commands, I
copied the waveform to the LIRC configuration directory. I could then send Lego com22

NOV 2016

NORTh AMERICAN

BloCK BY BloCK

lego power Functions allow motors
to be controlled with infrared signals
[top]. signals can be generated by
connecting infrared leDs to a pi
[middle images]. A lego enclosure
holds the components [bottom].

SPECTRUM.IEEE.ORG

mands from the Pi's command line through
LIRC's irsend program.
The final step was to issue the LIRC commands from the Scratch environment. I
enabled "remote sensor connections" in
Scratch. This makes Scratch behave like a
local server running on the TCP port 42001.
Client software can connect to Scratch using this port and listen for messages from
Scratch programs. (It's also possible to
have the client software and Scratch environment run on separate machines, so you
could have the Raspberry Pi-based infrared interface controlled by a Scratch program running on a desktop computer, for
example.) I then installed Phillip Quiza's excellent scratchpy library, which allows you
to write Scratch clients in the Python programming language.
Finally, I wrote a Python script that receives Scratch broadcast messages specifying Lego remote commands, and runs
the LIRC command-line client to send
them (this is also available from my legopower-scratch GitHub repository). To run
the script, run the control.py program in a
separate terminal window and launch the
Scratch environment. While control.py is
running, it will display on its standard output the remote control messages it sends
or the errors it detects on the incoming
Scratch messages.
In Scratch, programs are constructed
by chaining together graphical blocks on
screen. Blocks perform functions such as
program-flow control and graphics manipulation. To send a message to a Lego
Power Functions receiver, a "broadcast"
block is used, with a simple text string of
the form "Lego
." So, for example, the message "Lego 2 blue -7" will send a signal by
way of the Python client and my transmitter to turn the motor connected to the blue
side of the receiver on channel 2 at full
speed, backward.
How does the system work in practice
with its intended audience? I tried it out with
a young budding engineer-who quickly
wrote a Scratch program to control Lego's
Volvo Wheel Loader kit with a computer's
arrow keys. -DiomiDis spinellis

DiomiDis spinellis (4)

RESOURCES_hANDS ON

http://www.control.py http://www.control.py http://SPECTRUM.IEEE.ORG

Table of Contents for the Digital Edition of IEEE Spectrum November, 2016