Servo - February 2013

Part 2

Using Digital

Sensors With VEX

By Daniel Ramirez

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The 2007 movie, The Golden Compass, is an exciting fantasy-adventure film that was based on Northern Lights

— the first novel in Philip Pullman's trilogy, His Dark Materials. It stars Nicole Kidman, Sam Elliot, Eva Green, Dakota Blue Richards, and Daniel Craig. The story depicts the adventures of Lyra Belacqua — an orphan living in a parallel universe on a world that looks much like our own. In Lyra's world, a ruling power called the Magisterium is conspiring to end tolerance and free inquiry. Poor, orphan, and Gyptian children are disappearing at the hands of a group the children call the Gobblers. Lyra discovers that Mrs. Coulter is running the Gobblers. Rescued by the Gyptians, Lyra joins them on a trip to the far north in search of the missing children.

The primary device used by Lyra in this adventure film is a golden compass that always points to the truth. It gave me the idea to build this DIY Electro-Mechanical (EM) compass which I’ll finish describing in this article. While this compass won’t point to the truth, it will point you in the right direction.

In Part 1, I showed you how to build the gearbox with the VEX motor that is part of the EM compass’ motion subsystem. This time — in addition to showing you how to finish the EM compass — we will provide you with the I2C firmware necessary to make the EM compass work, and even more information on other kinds of I2C devices that can be connected using these drivers. You’ll also learn how to integrate electronic compasses into your own VEX robotics applications using the information provided here.

ELECTRO-MECHANICAL COMPASS ASSEMBLY

To finish this compass, you will need to refer to Part 1, so it’s a good idea to have the January 2013 issue handy. Recall that construction details for the gearbox were covered in Part 1. A trip to your local arts and crafts and office supply store is definitely in order if you want to customize the compass using the materials shown in the updated bill of materials in Table 1. The compass can be made as large as is practical using the VEX construction set and gearbox. I chose a 14” diameter and 6” high cylindrical hat box that I bought at a local store, using the lid for the compass and the box for the enclosure.