learning the ropes

The following is a record of an electronic surgery performed on 4/18/2007 in the Physical Computing Laboratory at ITP, wherein the patient “Dust” was brought to “life” in the manner of Frankenstein.

We added more details to the state machine which defines Dust’s behavior and started writing code to implement it.

We want to be able to sense how far people are pushing the poles in our installation. I thought we could do this by measuring how much force the PVC poles are exerting on the ring they’re sitting in.

Since force sensing resistors from Interlink Electronics are expensive ($5-6/each) and also because I couldn’t see how the fragile FSRs would fit into the holes we planned to use, I wanted to find a better solution.

I discovered it was possible to create FSRs out of wire and plastic wrap. Others have used conductive foam and wire mesh. Reading about linear position sensors also gave some insights.

I took 22 gauge wire from the physcomp lab, stripped it, and bent it back and forth to mimic the “fingers” on the FSRs I purchased from Interlink. After making two wire finger pieces, I wrapped one in seven layers of plastic wrap. I place the second set of wire fingers on the outside of the package and wrapped it into the existing package. My first few tests seemed very promising. When no pressure was applied to the package, the resistance was infinite. When I squashed the package, the resistance dropped down to about 10K.

The next trick was to try to duplicate this behavior on the end of a PVC pipe. We first tried applying the plastic wrap/wire packages around the end of the PVC pipe. The results were less encouraging than my initial experiments.

The homemade sensors were unreliable: either the sensor package was too tightly squashed between the PVC and the surrounding hole (and gave no resistance) or it was too loose and no amoung of bending the pole caused a reading.

This reminds me of the story of the gingerbread man — you know, he ran as fast as he could… but still ended up in the oven.

We started out with two halves joined together, but then realized that our presentation model should incorporate some of the circuitry. We cut out a door in the back to hold the speaker, LEDS, and vibrating motor.

Polymer clay (in this case SculpeyPremo) bakes for 25 minutes at 275° (or close to it).

After baking, the surface looks more matte.

Taking a look back at the breadboarded circuit always turns up something I’ve neglected on the schematic: in this case, it’s the vibrating motor.

I’m also a little concerned; I haven’t heard the amplified sound from the MP3 player through a circuit we had on another breadboard.

Work continues on the schematic for Dust. I have spent hours in Eagle drawing this thing.

Design Questions:

• Current design will require two USB ports (Arduino + MP3 player). Will it be necessary to include both of them on the PCB?
• How much current does the circuitry require?
• What type of battery will we use to power the circuitry?

I also started creating a PCB design. I want to print it out this afternoon to see the physical size and see if this corresponds with the size we want to make the wearable item.

My friend Mark P. Sullivan always said he wanted to make windchimes with all of the dead hard disk drives he had collected over the years. This came back to me as we struggled to find ways to make music with tops, so I started disassembling all of the broken drives I could find to listen to the sound of the platters

Case Mods
We modified our original protoype using one of the disk platters. Several small screws were placed within a cavity in the top beneath the platter in the hopes of making a sweet ringing sound. This was not successful. The centripetal / centrifugal (I always confuse them) force kept the screws jammed against the walls of the top as it spun so it didn’t make any sound.

This is the best spinning top we have so far. Its proportions are comparable top those given in an article we found about machining tops from aluminum.

Whistling… Not Quite
I tried to drill holes in the platters, thinking it might be possible to get a whistling sound as the top spun. The only sound I produced was a pop as the platter shattered. It appears that the disk platters are not metals as I thought. They shatter like glass.

I constructed another test top out of a metal cone from a ceiling-mounted air vent we found on the junk shelf. I did my best to match the “ideal” top proportions.

This one doesn’t spin very well — and is quite dangerous when launched from the power drill.

The Audio Art class will present a show of our work on the 9th Floor of the Tisch Building on Sunday, April 29th from 5-8pm. Email me for further details.