Twin Tweaks ...
to use the position controller to ramp up and down
THE SCRIBBLER RETURNS FROM A HACKING HIATUS.
The documentation accompanying the position
controller also gives a nice physics lesson about the
quadrature wave actually created by the device. While not
quite as accessibly written as the Scribbler documentation
from Parallax (which truly makes good on the claim that the
Scribbler is for ages eight and up), the info on the position
controller would be an excellent introduction to spec sheets
and some basic BASIC syntax. For intrepid tinkerers with an
HB- 25 motor controller, the documentation also shows how
GETTING REACQUAINTED WITH THE HACKER PORT.
After becoming acquainted with the specifications
for the position controller, we were ready to implement
the device with our old pal, the Scribbler. Our Scribbler
made its last appearance in the March ‘06 issue, when
we outfitted it with a multimedia palette to take its
artistry to the next level of awesome. To implement
our mechanical palette, we used the Scribbler’s
convenient “Hacker Port” which we thought would be
nicely suited to the position controller, as well.
The hacker port is conveniently labeled as such
with large print and an arrow, and it is comprised of six
sockets. The sockets are labeled GND, Batt, +5V, P10,
P09, and P08. The ground and power pins are fairly
self-explanatory, though this could be an opportunity
to offer some sound advice about the danger inherent
in hooking up a delicate electronic device meant for a
five volt source to a 12 volt source.
The P08, P10, and P11 pins are input/output pins.
They are actually the I/O ports used by the three LEDs
on the back of the Scribbler, though they can be hooked up
to any sensor compatible with the TTL logic employed by
the Scribbler. The position controller is a prime example
because the optical interrupter switches simply need to read
highs and lows.
The position controller kit comes with a short cable,
but the fact that it was female on both ends wasn’t helpful
for the hacker port which was also female. Thankfully, we
had some extra cables in our toolbox and we were able to
insert the wires directly into the socket.
Crimping on some nice pins would have
been classier, but this got the job done
at least for initial testing. Because the
P08 pin corresponded to the right side
LED, we decided to use it and attach
our encoder wheel to the right side of
The position controller makes a
prime candidate for a first hack because
it also provides an opportunity to
practice some basic mechanical design
skills. The encoder wheel needs to pass
between the optical interrupter
switches, and while the fit is not terribly
unforgiving it is a fine opportunity for
young members of the SERVO Nation to
learn about tolerancing and precision in
design. First, we had to figure out how
to attach the encoder wheel onto the
Scribbler wheel. The plastic encoder
wheel has a small lip, but unfortunately
the Scribbler wheel was lacking a clear
attachment point. Upon pawing through
72 SERVO 07.2010