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Diy optical pickup for guitar -- is it possible?
sorveltaja:
Continuation for the previous post; I tested that configuration, where the sender/receiver pair was perpendicular to the string.
Results were quite disappointing, as there was a lot of noise, although the ir-led was fed with dc. Also the pickup should have been very(too) close to the string, to get usable output.
So far, that configuration is done and dusted.
Variation of that is kind of similar, but this time the sender/receiver pair was set to be parallel with string:
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Output was bit better, with less noise. Also in that configuration, if the pickup was too close to the string, output decreased. I guess, that the reflection of the ir-beam from the string could be better also.
But the form of the output signal seems strange, as the waves are formed from tiny spikes:
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Maybe the schmitt trigger could translate those spikes to more solid, rectangular shape. Haven't made a pcb for that yet, though.
So far, I have found one major drawback in that concept; the output signal changes, when the ir-beam is reflected from players hand, near the strings.
One commercial manufacturer uses the latter concept, but there the optical components have a lot more distance, apertures, and fancy ir-filters between them.
I'm not going to copy that thing, as it was never advertised to have properties, that I'm after. Perhaps slightly adopt the ideas, if needed.
But the general principals are there for testing purposes, after all.
sorveltaja:
One basic testing version of the 6 x schmitt trigger board:
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For some reason, I insisted on making as small as possible board. Then I had to cram the capacitors in:
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It lacks only two caps, which I'm going to order, besides other components. But when testing it with function generator and scope, it worked as expected so far.
All those clumsy jumper wires, I wish I could replace them by using two-sided pcb. That's a subject, which I haven't explored at all, as aligning the two pcb transfers with each other seems overwhelming, to get usable pcb. Maybe that's one tough side project for the future...
Another version could be a bit simpler, with similar layout, using 40106 hex inverter gate, which has the needed six schmitt trigger inputs/outputs in one ic:
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I'm not sure if that 40106 works as 4093's, that I have used, but maybe I'll order some, to find it out.
PekkaNF:
--- Quote from: sorveltaja on January 02, 2020, 08:25:34 PM ---.....
All those clumsy jumper wires, I wish I could replace them by using two-sided pcb. That's a subject, which I haven't explored at all, as aligning the two pcb transfers with each other seems overwhelming, to get usable pcb. Maybe that's one tough side project for the future...
......
--- End quote ---
Not sure how you make the boards, but could you use the same method with transerfer sheets than is used with UV-method? That is to tape the transfer sheets facing each others into an "envelope" I.E. index them firs to each others and then slide the PCB blank in and then tape it into correct place.
Or you could use something to allingn the PCB into a "pocket" like this:
https://justaddelectrons.com/blogi/double-sided-pcbs-aligning-the-sides-for-exposure/
Or just drill holes on opposie corners of the PCB that has a mating parts on masks and then tape the mask (or in your case transfer) which has the same indexing holes. Hell....you might even 3D print an alligment fixture for mass production:)
sorveltaja:
Pekka, thanks for the info. I was thinking of that envelope method, but the paper, where the transfers are printed, should be plain, instead of the magazine paper that has either text or images in it.
Glossy photopaper for laser printer could be a better candidate for that. Or some pcb-transfer specific product.
By using the uv-method, as the link provided tells, making two-sided pcb's could be a lot easier. Haven't got that far yet, when making pcb's. So indeed it would make a lengthy sideproject.
To get back to the project, yet another variation of the recent setup appeared. Previous one used 50 degrees angle between sender/receiver, to reflect the ir-beam from the string.
Output was there, but it was somewhat rather lacking.
Then I got a plan 9 from outer phase, for increasing the angle, and distance between optical components. That way, the sender(ir-led) could shine its light to a longer amount of string, meaning more string surface to reflect, and hopefully more output.
Now the angle between sender/receiver pair is 90 degrees:
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I printed a new housing, which should be a bit more 'universal', if the distance/angle between optical components needed to be increased even further:
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Result is more responsive, and more 'solid' looking output, than the one with 50 degree setup. The sender is again fed with dc:
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Although I'm looking for hard-clipping, maximum output, there is also a great potential for experimenters, who are possibly looking for a clean output from optical pickup.
Just tune things down enough to get to that territory. In that case, using photodiode instead of phototransistor could be a better one, as it has lower output.
After all, these things always require testing, to find the options, that suit.
JohnHaine:
Interesting thread, haven't had a chance to read it all, but do you not have one problem with the configurations you are using? Essentially the string when it is stationary obstructs the beam, so there is no output from the photodetector. When the string moves, the output increases whether the string moves one way of the other. So it will double the frequency of the vibration, and be veru distorted. When plucked, the string moves essentially side to side doesn't it? So to pick that up you need an LED shining down on the string and two phototransistors below it, equally illuminated when the string is centralised, while being differentially illuminated when it is moving. Actually then you could use just a single LED and a "light guide" to distribute its light to each string, or one of those multi-LED strips, possibly with a diffuser above it.
Also in some of the early posts you seem to have a strange hybrid of constant current and constant voltage drive to each LED - really you want constant current so that each LED gets enough forward volts to work. You could use a significantly higher voltage and a bigger resistance, or better use an inductor as a "dropper" using your chopper, with a buck diode to allow the current to flywheel when you switch the voltage off - this would be much more efficient, and the light output would be constant rather than "sampling" the string.
I hope these comments are useful, and apologies if I am covering old ground.
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