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Diy optical pickup for guitar -- is it possible?
sorveltaja:
A new version of the pickup housing. Couldn't find any 'neat' way, that could be easily done, so why not go for quick and dirty. Well, maybe not so quick, but at least it's dirty.
The idea is loosely same as before, but this time, instead of using the drilled holes on the guitar to put the wires through, wires go through the hole on the side of the pickup housing.
The skeleton version of it:
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And with the parts:
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I printed that out, and although bony, it has some rigidity in it. If that isn't rigid enough in practise, I just have to add some wall thickness.
That kind of 'open' construction means, that the optocomponents should stay tightly in place through all the wild wiring hassle.
With right tolerances, they could be pushed in place seemingly tight, but that isn't enough. The trick is to apply small drop of lacquer or paint to the neck of the led/phototransistor.
When it's then pushed in the place, and let dry overnight, it should be more than enough to keep the bugger in the place.
Quicker way, using super glue might seem tempting, but the fumes of it will ruin the hemispherical 'seeing' area of the component, as they are usually made of some sort of plastic.
sorveltaja:
First tests with a new mechanical setup:
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Wires of two pickups in place. But that housing might get too crowded, when all the wires are stuffed there, especially when using a 3mm thick shielded cable for the receivers.
But that's what I have at the moment.
One shortcut could be to mod the housing, so that the wires would come out of the side of it, instead of only at the end, as it is now.
Now, that the two pickups are in place, I tested shortly the 'E' -labeled one, which is for the thickest string.
The screw-adjustment for the height seems to work, as expected.
10Khz square wave was then fed to the sender/ir-led, and to my surprise, there was... well, I don't exactly know what it is, but perhaps a reaction, when adjusting the pickup up or down.
When it was in one position, there were bumps between the square waves:
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After some height adjustment, the bumps were gradually gone. So the difference, of what the receiver/phototransistor 'sees', whether it's string shadow, or whatever involved factor, can be visualised using scope. During that, the string stood still, and wasn't plucked, or otherwise excited.
Obviously that effect is greatly reduced, when testing with thinnest strings, but should be visible, though.
sorveltaja:
Now, that the amplifier pcb is almost finished, I got curious of how the optical pickup sounds like. I tested it only on the thickest string. I'm going to order four missing ic's, once the holidays are over:
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Output from the 386 amp is certainly very strong, as it's used in x200 gain configuration. The output is distorted in rather ugly way, but that doesn't matter, when the signal is to be conditioned for the cmos devices.
So far, there are no gain adjustment pots. Maybe they could be fitted afterwards, don't know yet. If not, then it's just time for a new pcb.
But to test how the string actually sounds, I tuned the overall output down by lowering the sender/ir-led voltage to the point, where there wasn't distortion.
Yes, it is possible to get a 'clean' output of this setup. Although I wasn't too much interested about that, it might well change.
The sound of the E (thickest string) is very bassy, not in a muddy way, but like a real bass. Didn't even know, that guitar can produce such low frequencies, without octave-down -effects.
Not only that, but when I increased the voltage of sender/ir-led, the overall output was again very high, I noticed, that plucking the other strings(whose pickup wasn't connected),
the E string ringed, responding to some of their plucked note harmonics.
sorveltaja:
This one might be boring one, but as it is essential for the project(or at least with current mechanical setup), I'll give it a go anyways.
It's about height adjustment of the pickups. Today I tested, how to get the pickup(sender ir-led/phototransistor pair) to the position, where the string is at the center of the ir-beam.
That could of course be done by listening the output, and guessing the sweet spot by ear.
But guessing games aside. To see, if I can find anything useful for the matter, I fed all kinds of signals to the ir-led, and looked the results on the scope, while moving thin, string-like wire across the ir-beam.
The pickup used for this test is separate one, but similar to the ones, that are attached to the guitar.
Method, which I mentioned earlier, that feeding square wave to the ir-led, and judging the result by the looks of receiver output, when moving an object across the ir-beam, doesn't seem to be reliable.
There are simply too many things to be adjusted, before one can see the results(if any).
While frustrated about turning all the knobs of scope and function generator, there was one thing, that I hadn't noticed before.
When I slowly moved thin(and thicker ones also) string across the ir-beam, the current readout of the psu went temporarily from 23 mA to a few mA lower, as the string was moved up and down.
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After repeating that procedure few times, there was constant reaction of the current readings of the psu.
The circuit, that was used to amplify the phototransistor output, is same as in previous post.
The thing seems to be to feed that circuit through the ir-led, with a signal, that makes the receiver/phototransistor amplifier 'busy', while 'receiving'.
Even a small amount of deviation in the ir-beam seems to give the amp time to take a breath, so the current consumption decreases shortly.
In the end, I think that the deviations of current consumed by receivers amp could be more reliable way to measure the best position of the string, to get maximum output.
sorveltaja:
While the current mechanical setup is in the guitar, and the amplifier pcb is fastened to the guitar also, I decided to have a look of another kind of mechanical/optical setup, which I tried very shortly in the beginning.
It was left aside, as I didn't have an amplifier circuit to provide enough boost back then. But now that I have, I thought that re-visiting that design could be interesting.
Don't mind the bigger setup, as the one, that I'm babbling about, is the small one on the upper side of the picture. It's glued in place:
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The sender(ir-led), and phototransistor have 50 degrees angle between them. No any particular reason for that angle, other than to see, does the concept work:
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That test housing has a "backrest" to keep the sender/receiver combo from rotating around, when adjusting the height. Seems to work quite well.
As can be seen on the first picture, that thing was tested on the thinnest string(~0,2mm). If that works, any string thicker than that should give even more usable output.
To my surprise, there was stronger output, than was expected, after adjusting the pickup closer to the string.
So far, I have only used the scope to see the results, but it's the resulting sound that counts after all. I haven't got that far yet, as projects like this tend to expand(explode) in form of countless possibilities involved.
But yes, I think the mentioned sort of setup could very well be more appealing to the most of experimenters, that are willing to get their feet wet.
Quick summary of the two lately tested concepts so far, but take it with pint of salt:
One with sender/receiver pair in opposite(horisontally) sides of the string:
+ immune to string bending(which is also a horisontal movement)
+ the "sweet spot"(max. output) could perhaps be found by measuring the current consumption of the amplifier circuit
+ overall output level
- not a very practical to use, as the sender/receiver pairs (and their housings) protrude above the strings, so it's not possible to dampen the strings in natural palm-assisted way.
Then the one, that relies on reflecting the ir-beam from the string surface from the below:
+ remains under the strings
+ allows free playing, and string dampening at will
- rather sensitive to string bending(maybe not so, if multiple pairs of sender/receivers were used)
- can't think of reliable way to judge the "sweet spot" yet
But as always, all that needs a lot of testing.
To expand the subject even more, I'm thinking of ordering some hall-sensors, to see, if they could be abused used for this purpose. But that's beyond of this thread, though.
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