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Diy optical pickup for guitar -- is it possible?
sorveltaja:
Although the square wave generator pulses the IR-leds, which are now in series, there seems to be a lot of deviation between their output ranges.
At the very beginning, I tested those IR-leds in parallel configuration, just to find the same differences.
So today I tested them with an adjustable DC source, still in series. All of them have different voltages, or "sweet spots", where they give the most output for the receivers.
Bit confusing, but I guess they(IR204's) weren't made to be that accurate, after all, what comes to the operating voltage and current.
One thing, that I might test with them, is to use potentiometers to "tune in" each led. If that doesn't work, it's just an end of part one, as I have already few other types of IR-leds on the shelf.
Bill, I took a look of the shannon sampling theory. Somehow it makes sense, but I haven't found yet a way, of how to implement it in practice.
I'm not quite sure yet about the optical layout. For example, Opik pickups by Light4Sound use reflective technique, which is one option.
Current setup focuses on disturbing of the IR-beam, instead of cutting it, as it could require microscopic precision.
Using the optoswitches is also one option. My local supplier has some Omron optical switches available, which I guess, could have better tolerances, than the discrete IR-components.
BillTodd:
how are you measuring ir output?
The ir laser in a cd pickup uses a second monitor diode to detect the laser level, that is fed back to the laser current to keep the laser output level constant with temperature , aging etc.
It seems more likely to me that the variance you are seeing is due to optical path differences.
sorveltaja:
I think the IP-receivers' outputs could be measured with multimeter, but the readings come and go so fast, that it's a bit hard to follow.
When plucking a guitar string, there is first the highest peak, then the signal slowly fades away, so it hasn't that much of constant values.
Without having a storage oscilloscope, I judge the results just by listening. That way it's easier to observe, if the signal is too weak, or is it too strong, causing distortion.
I have tested the IR receivers one at a time, and the signals from them goes first through the unity-gain op-amp(9V battery powered), then to the laptop's mic input.
Optical path differences, yes they likely have an effect. By using variable DC-supply(30V/3A), that displays both voltage and current, I got some results.
From left to right, E1 is the thickest string, while the other E is thinnest.
When using plain DC, the "window", where the leds gave any usable output was really narrow(numbers above the letters)
One (or at least me) could expect more output, if more voltage is used. But no, outside of the above voltage ranges, there was no output at all.
I must be missing something essential here. Perhaps the receivers have something to do with those narrow windows? I Can't tell.
Then comes the test, using pulsed square wave at ~120KHz.
Generator schematic:
As can be seen, there is 7812-regulator, meaning, that it provides regulated +12V voltage for the 555- timer IC.
Results are very confusing. Again, numbers above letters present voltages fed to the circuit, to find out the sweet spots of the IR-leds outputs:
For some reason, E1 has the broadest response, as it gives good output, when supplied from 11 to over 20 volts.
All the others have very narrow output ranges, that works only below the actual, regulated voltage.
BillTodd:
at a guess the larger string is obscuring the most light and allowing the transistor to operate in its linear ( non-saturated) region.
ATM you seem to have the string just wobbling about infront of the transistor, you really need to get only the edge of the shadow to fall on the transistor die . Sketch out a scale diagram of light source , string and receivers , to workout where the best placement for each should be. you may need to add apertures to source and receivers to get a crisp shadow.
You are making it hard for yourself if you don't have the right tools. Without an oscilloscope you are blind . You don't need anything expensive, the little pocket devices sold ln eBay would be more than good enough for your purposes.
BTW the 33u in the output that drives the leds , is making them see an ac drive . It is not necessary .
PekkaNF:
To my experience pretty much all optocomponenets are pretty unlinear and they have large variance between componenets even in same manufacturing batch.
I would start checking iif all those optodiodes will need individual current or voltage source. If they are laser diodes, they normally have their own drivers and they behave very diiferently from normal LEDs. My point is that each emitter needs certain driver and you might need a separate driver for each channel.
Also receivers are most likely unlinear and strings/optical baths are diferent -> you need some processing for each channel. I can't estimate if with one opam you can bias, linearize and amplify signal. They must be tuned individually. No idea how to do that.....different frequences and plucking the string might not be exactly constant------probably to keep un plucking until statistically representatice smaple cloud has been reached.
Pekka
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