CODING

THE PI DERBY
Tutorial
Above Move the ball bearing as shown to advance your horse
the finishing line, the winning margin is calculated
and printed using the correct horse racing terms: a
head, short head, length, and so on. Then the horses
are wound back to the start with an accompanying
ratchet sound just like the original, and the race can
start again. At any time, pressing the RETURN key
will restart the race. We found occasionally that a
ball would sit on the contacts but still not make an
electrical connection. In that case, a small vibration
of the box soon made a good connection.
The software
The derby.py code listing is shown overleaf; this and
the accompanying graphics are on our GitHub page
(magpi.cc/1NqJjmV). Like a lot of our projects, it’s
written with the Pygame framework, and follows a
pattern familiar to regular readers. The showPicture
function first draws the track, then the two horses,
and then the front rails; the last bit is the horse’s
name and the distance to the finishing line, along
with the current state of where the ball should go.
You’ll notice that the variables describing some
positions have two or three simple additions, rather
than a single number. This makes it a lot easier to
spot where the individual offsets are applied when
developing the code, but you could pre-add them if it
worries you. The movePhase variable list shows if the
current target for the ball is at the start or the end of
the line. The horse is checked at each movement to
see if it has crossed the winning line, and there’s also
provision for the unlikely event of a dead heat.
Taking it further
You can modify the amount of movement a horse
makes each time, thus requiring more or fewer ball
movements per game. The game could easily be
extended to more players; with each player needing
four GPIO ports, there are ample resources for six
players without resorting to any extra electronics.
You could replace the single horse graphic with a
number of frames to make smoother, more realistic
movement like we did in the Olympic Swimming game
(The MagPi #48); however, the original idea was to
reproduce the mechanical fairground attraction.
>STEP-03
Drill the base
The diagram shows the view into the tray; you might be better off
marking the back and drilling from that side. Wire up the nuts and bolts
to the Raspberry Pi's GPIO lines, using the solder tag washers, and test
the connections are correct before fitting the acrylic cover. We used a
small piece of double-sided sticky foam to fix the ribbon cable to the
back of the box.
>STEP-04
Finishing off
Make sure to clean off the ball bearings with some solvent, as finger
grease can result in the ball failing to make a good contact. You can
paint around the nuts with Bare Conductive paint to make a more
reliable contact if you wish, but allow at least a day for this to dry.
raspberrypi.org/magpi January 2017 53