Twin Tweaks ...
BUSTING A MOVE.
and can only be viewed by looking at them almost straight
on. The LEDs we opted for had a 180 degree viewing angle
which we thought would be plenty for a showman like the
KT-X.
We also considered the power requirements. LEDs are
usually happy with a few milliwatts, but it is still something
to keep in mind. Because we wanted to use several LEDs in
parallel, we didn’t want them to be too power hungry.
Most LEDs that popped up on Digi-Key had very modest
power requirements, so this was not a hugely helpful way
to narrow down the search.
The final consideration that helped us decide was the
physical structure of the LED itself. We wanted the classic
through-hole mount. It would be the easiest to solder and
give us the simplest options for mounting to the robot. We
SCHEMATIC FOR A CURRENT DIVIDER CIRCUIT.
did not want a surface-mount LED because the nearly
nonexistent solder pads and miniscule scale would
make them more trouble than they were worth. We
settled on some through-hole mounted green LEDs
that would nicely match the KT-X’s glowing eyes. For
several reasons that had more to do with timing than
with the hackability of the KT-X, we also decided on
the easy route of an external power source in the
form of a nine-volt battery.
To properly implement the LED suit, we
employed a classic current divider circuit. To ensure
that each LED was the same brightness, we arranged
them in parallel. Each LED was in series with a
resistor to provide the proper input voltage.
Calculating the optimum resistor value was a simple
matter of recognizing that each loop contained the
same current because the equivalent resistance of
each loop was the same. The loop equivalent
resistance was simply the sum of the LED resistance
and the resistor value. The specs on Digi-Key provided
the maximum current for the LEDs which was 20 mA.
Assuming that 20 mA would be the maximum current
in each loop, we were able to determine the
resistance of the LEDs to be 110 ohms. The minimum
resistor value was one simple calculation away after
manipulating the classic formula of V = i*R. We
calculated the minimum resistor value as 340 ohms.
We ended up with 470 ohm resistors which might
dim the LEDs slightly but definitely keep them from
burning out.
To connect the LEDs to the nine-volt battery, we simply
soldered a bundle of wires to the leads off of a battery cap.
Some heat shrink kept things looking nice, and the biggest
challenge was keeping everything untangled. We took a
cue from the KT-X design and used longer wires for the
LEDs destined for the legs and shorter ones designated for
the hands.
A current divider is admittedly a simple circuit, but it is
always a good way to stay current on circuit design. And
while this little circuit may seem a bit frivolous, it is
reminiscent of far more sophisticated projects. Tracking
joint movements with sensors is used in projects ranging
from motion capture for animating movies and gait analysis
for medical applications.
Attaching the LED suit to the KT-X really highlighted
another great design feature bound to be
of interest to tinkerers. While the robot
does have an impressive 17 degrees of
freedom, routing wires around the body is
not nearly as risky business as it might
sound. While the servos retain most of their
180 degree range of motion, the only
trouble areas are near the shoulders and
hips. A little extra length of wire is all that’s
needed, so the addition of any other
external sensors would likely not create an
unmanageable tangle of red and black.
72 SERVO 11.2010
