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Diy optical pickup for guitar -- is it possible?

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sorveltaja:
Bill, yes the string shadow, and the placement of the sender/receiver pairs needs more testing.

In fact, one of the commercial optical pickup uses aperture to "tighten" the width of the receiver's "eyesight": https://patents.google.com/patent/US20110265635   

Oscilloscope has been on my buying list for years. I have waited for the "justification" to purchase one. Perhaps the time to get one has come closer.

I breadboarded the 555- timer circuit, using a bit different configuration, that I found on the net. So now it gives DC pulses, instead of AC.
But the results are pretty much same. The leds still have their narrow, individual ranges, that produce output.

Pekka, it seems to be very true, that they have plenty of deviation. Actually, what I'm planning to do next, is to somehow feed each of the IR-leds separately, from the same voltage source.
Maybe at first by using separate trimmer pots for each one, to see, how it works.

 

sorveltaja:
I tested the previously mentioned configuration, using trim pots for each sender/receiver pair. Yes, it kind of works, but the receivers' outputs are still at a very narrow ranges.

On those narrow output ranges, it's more like an on/off switch situation, where the outputs are either non-existent, or are overly strong, causing distortion and a lot of noise.

I don't mind the distortion, as all those outputs, when I finally get there, are to be converted to CMOS- level square wave form.

But with that much noise, it seems to be very difficult to achieve with the current mechanical setup:
 

As I see it, is that when the sender/receiver -components stare each other in the eyes at so close distance, the receiving one goes mad, and gives overly saturated output, even if it isn't amplified in any way.

After all, the optoswitches have similar configuration, but are perhaps designed to be nothing more, than on/off -devices.
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I'll put the current concept aside for now, and move on to the next mechanical configuration, which concentrates to the reflection of IR-beam from the vibrating string.

To be honest, I'm not going to re-invent the wheel. Instead I'll use the info from the previously mentioned patent.

As it is for commercial product, there obviously isn't any "specific" details available.

BillTodd:
The photo transistor is just like a normal transistor, its transfer characteristics are determined by base current. Even in a simple amp circuit the base current is carefully controlled , usually by a feedback mechanism (.
e.g. emitter resistor)  .  The photo tranny's base current is supplied indirectly by the impinging photons,( and it doesn't really care what colour they are)  there is no feed back , even with emitter resistors.

You have to either have to provide a very stable light level  (practically impossible given the circumstances ) or some means of feed back  to control the level.

WeldingRod:
You might be able to get a steady string motion signal by using a speaker near the string, or driving a conventional pickup with a function generator.

Sent from my SAMSUNG-SM-G891A using Tapatalk

sorveltaja:
Bill, yes the phototransistors(at least the ones I have tested(PT202B)) seem to be quite tricky to work with, as they all have different, individual operating levels.

WeldingRod, that's a good idea. There is actually a commercial device called "E-bow", which allows the player to electromagnetically "excite" the strings, but only one at the time, to make the string to "sing" as long as the device is applied to it.

Hmm.. after a quick search, there seems to be a plenty of those who had done "diy" -versions of that device, so I guess it shouldn't be that hard to build.
That could be nearly ideal one to produce constant signal from the string(s), which could then be more easily measured as an output.

In the previous post, I mentioned that I'll be moving on to the reflective approach, as mentioned in the patent. But not so fast, as the saying goes.

While taking the current setup apart, which consist pairs of 3mm sender IR-leds(IR204), and 3mm receiving phototransistors(PT202B), I Somehow got to this crazy, 'mad modder' -mood, and tested, how the actual emitting IR-leds(IR204) react, when they are treated, as receivers.

To my surprise, they provide quite a lot of output, when considered them to be emitters. Also they are bit more forgiving, than the optotransistors, what comes to the operating levels.

So I tested a pair of IR204's, one being the sender, and the other being receiver. Didn't seem to work well with plain DC, so I switched to previously used, pulsed source.

Crude circuit, that I'm currently using to amplify the signal:
 

It is powered by split power supply. But so far, there is good output, although with some hum. But that is mostly due to the jungle of wires involved, when testing.

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