I had hoped to have the whole mbira put together today- and I am close- but I ran into a problem I hadn’t solved yet. Let me explain how the sensors on the mbira work.
Each tine has two sensors- a cds photocell on the tip and a pressure sensitive sensor by the bridge. The photocell is fairly straightforward, although the wiring to connect it to the circuitboard is messy, but the pressure sensitive sensor was tricky. I was using a standard pressure sensor from sparkfun, but at $10 a pop 15 sensors was getting pricey, and I had read about an alternative solution using conductive foam (Adrian Freed from cinmat at UC Berkeley also made a kalimba using pressure sensitive fabric). Basically, you take a piece of conductive foam, like the kind IC’s are shipped with, and sandwich it between two metal conductors. The resistance of the foam changes according to how much the foam is squeezed. It’s actually quite sensitive- on a tabletop you can go from 2 megaohm to 2 kohm just by squeezing.
So the mbira has 15 small pieces of foam, one for each tine, sandwiched between conductive copper tape on the tine and a wire leading to the pcb. The metal tape on the tine leads to a strip of metal tape connected to ground, and the wire is connected to a voltage divider.
The problem is finding a way to mount 15 of these systems on a bridge 6 inches wide in a way that will make a good electrical connection, not let there be any crosstalk between tines, and will be dependable. I’d kind of been hoping the pressure of the tine would keep the foam in place, but I had a feeling I needed a better solution. Sure enough when I started putting the tines on, it became a nightmare keeping things aligned. As much as I hated to do it I was forced to use electrical tape to keep each pressure sensor intact.
Electrical tape, by the way, sucks in ways I am sure everyone understands. Also, after spending all of this time making beautiful laminated wooden tines I don’t want ugly electrical tape glaring at me everytime I pick up the mbira.
Thankfully, after putting together the first five tines I came up with the perfect solution- 1/2″ heatshrink. It will still go over the top of the tine but will look a million times better and also be more stable and dependable.
So, still have to finish the tines but at least have a gameplan for how to approach this nagging problem.Here is a picture of the tines with tape. Notice the black and white wires sticking up on top of the aluminum bridge- those are what connect the conductive foam to the circuit board. It looks like wider spacing than it actually is because the photo is so close- it’s actually about 3/8″ from wire to wire.
