Monitor the scattered light from two lasers. Use the intensity of light, as read by the two photo sensors, to determine the position of the finger. Each sensor is behind a tube to limit ambient light influence. The tube's diameter and depth would be tailored to favor laser scattered light over ambient light. Perhaps a third photo sensor could monitor ambient light and contribute corrections to the calculation.
With experimentation one could arrive at a "Scattered Light Intensity/Finger Position" profile. Perhaps this could lead to a mathematical solution OR perhaps some sort of look-up table -- perhaps along with extrapolation?
I would make the inside of the tubes as black as possible, to reduce reflections. That way, it can be designed to "see" only direct scatter from the finger.
Also, to deal with the variability of ambient light, the lasers could be modulated and the photo sensors would, when the laser is off, then be used to measure the ambient light, and this could be subtracted.
Variations in "finger reflectivity" could be an issue. But, perhaps the information from the two lasers could be used to balance this out. For instance, the profile could only involve the ratio of intensity from the two sensors. OR, the musician could be provided with a special glove, to unify the reflectivity. But, finger size might, also, be a factor -- but not as much, and looking at ratios should remove that variability.
MorganS:
If $17 is too expensive then I suspect that two lasers will be out of the budget too. That sensor is also not flexible.
Yeah, but it was fun to conceive of.
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And who says it isn't flexible Maybe the base bar bends slightly, causing a different sort of change in scattered reflection -- a dip in brightness on both sides that could be registered as different from a side-to-side slide.
You could use an array of micro lenses in front of an array of micro-photo sensors like a plenoptic system, similar to the way Lytro cameras work. That way the distance would be cooked in to a single sensor.
For a project, I have to find the position of a finger on a line. Like a touchscreen but on only one dimension. For that I thought to use a resistive wire, then to read the voltage with an analog pin. But the wire must measure between 25 and 75 cm with at least 1k ohm.m to limit current and I've look on many websites and I can't find this kind of wire (or plate, this will also work). Does anyone know something like this? (I also try carbon on paper, it works but not really reliable and linear).
Thank you.
I don't think you can usefully measure finger position by voltage like this without using a conductive glove,
voltages on the body are strongly influenced by mains pickup. Once you use a conductive glove there's no
restriction on the resistance and ordinary resistance wire would be feasible - some voltage amplification
would be needed due to the low impedances involved.
Could use three parallel wires/tracks, ~1mm apart, with the center track the output and the outer tracks the resistor.
But that doubles the resistor value problem.
Leo..
Maybe the base bar bends slightly, causing a different sort of change in scattered reflection -- a dip in brightness on both sides that could be registered as different from a side-to-side slide.