The eViolin has two main interactions between the musician and the instrument. Specifically, the left hand is on the fingerboard and the right hand is on the bow. Both need to be solved and their information integrated. Let's start with the fingerboard.
There are several methods that can be used to measure the distance on the fingerboard. My first idea was to use resistive fabric on the board; however, it seemed to be non-linear. The other idea was to use a pre-packaged sensor from SpectraSymbol. The SpectraSymbol sensor is a good linear sensor that can be used for measuring distance. Pictured below are the three sizes that I have (50mm, 100mm and 300mm).
The smaller ones are for taking apart and for examination and testing (see picture below).
However, as you can see from the picture below, the sensor is too wide to fit on the fingerboard and allow four sensors to be positioned on the fingerboard right next to each other. Since the sensor is too wide to fit, my plan is to remove the top of the sensor, which is a conductor, and leave the bottom, which is the actual resistive sensor.
In order to tell what note is being played, we need to have a way to measure the distance to the end of the fingerboard, away from the nut (pictured above on the left side before the tuning pegs). And we need to know what string is being played.
Without any strings, we can put a grounding tip on each finger. This will achieve the desired note location, but it will be hard to tell what string the fingers are on, as they may spill over from one string to the next. If the finger touched the adjacent string, it would register as having a finger pressing both strings, which might be unintended.
An alternative is to use strings as part of the sensors. By grounding the strings and grounding the end of the resistive sensor, we can measure open strings and any distance away from the nut. Pressing the string down onto the bottom layer of the sensor and having it grounded will give both tactile feel and accurate finger measurement. As each finger gets pressed higher and further from the nut, a lower resistance will be measured. This method eliminates cross-talk between strings. Even if the fingers press two strings down intentionally, it would still produce the desired effect.
SpectraSymbol makes these sensors in lengths of 300mm at 10 Kohms and the standard violin fingerboard is 280mm long, so this should work. I will need to modify the end near the nut as the fingerboard is too narrow for four strings worth of sensors. However, trimming them just a bit at the nut end should suffice. This may change the resistance characteristics, resulting in some non-linearity. In the end, this will need to be taken into consideration.
The circuit looks like this:
