sparkfun Simon Tilts Series Assembly Manual page 18

Page 18 of 20
There was one problem with using a breakout board: the sensor itself is not
a PTH part. Although we could include a footprint on the PCB to mate with
the accelerometer's pinout, we were hesitant to use this approach. Adding
one more "black box" to the project can increase the barrier to
understanding a technology for a newbie.
Ultimately, we opted to create our own unique custom molded part:
It uses an IR emmitter and detector in order to know the position of the BB
ball inside the cylinder. The IR emitter and detector are positioned so that
the IR beam "shoots" through the hole in the bottom of the cylinder. When
the BB is resting at the bottom of the cylander, then it is blocking the IR
beam. When the BB is resting at the upper end of the tilt arm, then the
"look-through" hole is unobstructed, and the IR detector can sense the
emitter.
In order to make using this part easier, we created a device in our Eagle
Library:
The package includes the correct positioning of the emittor, detector and
dimension route out area to mate with the tilt arm part. An important thing to
note is that when choosing a pull-up resistor for the detector line, we found
that 330 ohms was the best value. This value is used in the kit. We first
tried using the internal pullups on the ATmega328, but the voltage swing
from the detector was very minimal which made it hard to get accurate
readings.
We'd like to send a huge thank you to Modrobotics, because they printed
three rounds of prototypes for us!
We'd also like to thank Mountain Molding for all their help with design,
tooling, and manufacturing the part!