Exercising the Little Grey Cells
and the Battery Cells
Like the Scarecrow off to see the Wizard, all our bots
longed for now was a brain. For the Bogie, we had the
perfect fit. We pilfered the board from the DFRobotShop
Rover, which last made an appearance in our column back
in March 2011. The DFRobotShop Rover includes a board
based on the Arduino Duemilanove, but what we were
most excited about were the built-in motor drivers.
Unfortunately, there were only two motor drivers while the
Bogie had six wheels.
This conundrum, however, wouldn’t require the likes of
Sherlock and Watson to figure out. We resolved to use one
motor driver per side, so all we would have to do is splice
all of the motor wires together to a common power lead
and a common ground lead. Since the end of the Runt
Rover wires are pins instead of simply wire, we were able to
use PWM connectors to gather them together.
We cut the other end off of the PWM cables and
simply spliced the three PWM leads together and tinned the
tip. We used black or red heat shrink on the end of the
wire to keep track of whether it was the common power or
common ground for the side. We always prefer a solution
like this that involves connectors instead of the slightly more
permanent and messy solution of soldering motors. In the
future, we might want to wire up the Bogie with a board
that has six motor drivers — one for each motor. When that
time comes, all we’ll have to do is disconnect our modified
PWM cables and put the motor pins directly into the
With just that minimum amount of soldering, we were
ready to fire up the Bogie. Upon plugging it into wall
power, the LEDs lit up with vibrant blue hues.
The Sprout is ostensibly even easier to wire up with just
two motors. A board with only two motor drivers would be
perfect. Here is where we come to perhaps the biggest
caveat of the Runt Rovers — you need to make sure you
have motor drivers to make them work, and the basic
Arduino board like an Uno is not going to cut it. The Uno
has plenty of analog and digital sensor ports, but no motor
drivers. If the Runt Rovers were driven by servos, you could
just wire them into the power, ground, and PWM ports on
the Uno. However, the motors have nowhere to go, like
Jem and the rest of the Holograms after a concert is
It is theoretically possible to run some DC motors
straight from an Arduino Uno board without motor drivers.
If you hook both motor pins to PWM ports on the Uno, you
can achieve simple motor control but this technique is risky.
The maximum current rating on the I/O pins is 40 mA.
You’ll have to be using a tiny motor indeed for that to be
The DC motors that come with the Runt Rovers draw
190 mA at no load and 250 mA at max load — far more
than the Uno I/O pins can handle. So, by risky we mean
trying to use PWM ports on the Arduino Uno to act as
substitute motor drivers is a surefire way to destroy your
SERVO 08.2015 73
A WEE BIT OF PROGRAMMING.
READY TO TACKLE TERRAIN.