Well, the whole mbira is put together now- and it went basically exactly according to plan. Except for one major caveat: the foam that I used for the pressure sensors basically just doesn’t work for this application. On a tabletop, it works great- going from 2M to 2K resistance by pushing down on it. However, in the mbira, the wrapping between the bridge and the tine keeps the foam at between 80-200k. The real problem though, is that foam is very slow to react, and stiff- so if you push down on it it takes a lot of force to get a big change in resistance – so in a voltage divider circuit with a 120k pull-up resistor, there is about a 10% change in voltage. (Which is exacerbated by the lever mechanics of the design.) And when you let go of a tine, the tine rebounds to a much higher value than its original value, and then slowly returns to the middle.
At this point, I am going to stick with this design for a little while and see if the foam ‘breaks in’ and gets less stiff and more responsive. However, I’m not expecting much. I think I will also search for some other conductive fabric and foam and see if I can’t find something more flexible. As a last resort, I could get small FSR’s from sparkfun, but I’d like to stick to this design as a proof of concept.
The mbira looks great, though- and the CdS photocells on the tips work great – I get around 90% range on most of them. The interface with max/msp is also coming along- I’ve got max reading all 30 tine inputs. I’ll talk more about the max/ firmata interface later.
Here are the wires connecting the pressure sensor to the PCB. I soldered small pieces of copper tape to the wires- the copper tape is then stuck to the foam, and the whole assemblage placed in the heatshrink with the tine.
The finished mbira! I haven’t shrunk the heatshrink since the sensors aren’t working so well. I don’t have too much hope that sealing the heatshrink will improve the performance of the foam so I’m holding off on it until it breaks in or I get better conductive foam or fabric.
The photocells are the top 15 graphs, laid out in the same order as the tines. The pressure sensors are on the bottom. You can see how the pressure sensors are a lot less sensitive than the photocells.
