FIGURE 2A. Note the color scheme and
addition of the protractor markings.
provided in Part 1) and using a custom
color scheme. As you can see, I chose
gold foil paper for mine in keeping with
the movie. Glue the compass rose
pattern you have selected to the bottom
of the hat box lid. In addition, you can
line the inner and outer lid rim to form a
decorative golden ring.
The completed compass showing the
four cardinal compass points (N, E, S, W)
along with the protractor scale indicating
angles between 0 and 360 degrees
(where true north is 0 degrees) is shown
in Figure 2B. As you can see, this 14”
diameter compass is much larger than
the standard magnetic compass shown
next to it.
Although I have not done so yet, the
top of the compass can be covered with
a clear lid made from a circular cutout of
plastic or glass for both weatherproofing and to keep hands
away from the pointer during operation.
Refer back to the schematic in Part 1 to make the final
electrical connections using the four thin wires that you
routed through the 1/4” hole in the
center of the compass base to the
VEX microcontroller underneath for
SCL, SDA, +5V, and GND. The
optional pushbutton switch can be
used while calibrating the compass
with the quadrature optical encoder.
The final step is to clean up the
assembly by removing excess glue
and covering up any screws that are
visible. This completes the
construction phase of the EM
I2C FIRMWARE FOR THE ELECTRO- MECHANICAL COMPASS
In Part 1, we described the I2C
serial two-wire protocol used to
communicate with digital sensors such
our compass, leaving the firmware
details for this part. So, let’s dig into the firmware needed
to make the microcontroller work with our compass.
FIGURE 2B. The completed EM compass with the pointer and the four cardinal compass
points, along with a protractor scale indicating angles between 0 and 360 degrees.
SERVO 02.2013 69