Monday, October 10, 2005

Rock the Bells

The week 4 lab had us adding a servo motor to our board of tricks. The installation of the servo was quite easy, but the programming was a bit trickier.

Unlike most motors, a servo motor’s rotation is limited to a set amount. In the case of the motor we are using, the motor will rotate just over 180 degrees. I planned on controlling the servo with a potentiometer, converting the analog input into values that could be used to control the motor. This took a little bit of back and forth to dial in; I adjusted the constants in order to get the maximum rotation out of the motor. Once this was accomplished, it was time to try to build something to make use of this new motor.

As I contemplated the back and forth action of the motor, trying to decide what it would be good for, I also started digging around the junk bins in the shop, looking for inspiration. I came across a radio control toy, but just the controller, no car in sight. The antenna caught my attention—supper whippy with a plastic tip, it seemed well suited to the servo’s back and forth motion. I removed the antenna from the controller, and started to play with it. It seemed like it would make a nice percussion instrument, as I snapped it against the table. I decided this would be the first part of my device and with a few bends I attached the antenna to the servo motor.

Now for something to smack! I first considered using two plastic cups, aluminum cans, or chunks of pipe. They all seemed like they might produce a good sound, but I was hoping for something with more charisma. I considered stopping in a toy store to look for something to appropriate, but as my time was limited, I never got the chance. Fortunately, the window display at Astor Place Liquors provided the solution.

After purchasing two crystal wine glasses to act as my bells, it was just a simple matter of assembly. I found some wood and made a base on which to mount the motor. From the base I erected a dowel onto which I fastened a cross bar. The cross bar suspends the wine glasses in an upside down orientation. The device works by rotating the potentiometer, causing the motor to rotate. The wire antenna turns with the motor until it is caught on the dowel. The antenna flexes until it bends far enough to slip past the dowel, initiating a forward whip and striking the dome of the wine glass, producing a ringing tone. Turning the potentiometer in the other direction causes the motor to rotate back, reproducing the same effect on the second wine glass.