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Diy optical pickup for guitar -- is it possible?
picclock:
It seems to me that the signal output you are getting relies on the reduced off axis response of the detector, rather than the deviation of the string.
A possible solution may be to use an aperture in front of a large area detector such that when the string moves the size of the shadow changes. A square or rhombic aperture aligned such that opposing corners are in line with the string, would have minimum light when the string was at rest but it would increase directly in proportion to the deviation.
Just a thought.
Good luck
Best Regards
picclock
sorveltaja:
Picclock, thanks for the suggestion. I'm not sure, how the string actually changes or modulates the IR-beam. But it produces somewhat responding signal from the receiver's output.
As I've noticed, the axis between sender and receivers should be on the same level with string, to get the most output. But that's just one way that seems to work.
Again, I'm not sure if the string is exactly at the same level with the IR-components axis in that case.
One way to get close to the 'sweet spot', where the output is highest, is to feed strong enough square wave to the sender, so that it can be seen at the receivers output.
Although not necessarily square wave anymore after that, but while adjusting the pickups height up and down, that wave changes also, as the sender/receiver pair passes the string.
What comes to using apertures, I haven't got that far yet. With current mechanical configuration, the space between strings is rather limiting, but we'll see.
-------------------------------------
Newbie alert:
I spent plenty of time today to find out the reason, why the recently working circuits didn't give any output anymore, after a good start.
Solutions:
- try different configurations
- if none of them work, check the continuity of the wires
- blame the non-existent quality of your breadboard
- start pulling hair out of your head
- never suspect the 9V batteries, that you are using on a project, that include current-hogs like IR-leds.
After all that, I tested one of the circuits with regulated PSU, and voila, the output was there again.
sorveltaja:
After testing different photodiode(as receiver) configurations, the overall output still wasn't at the level, that I expected.
I guess it would need a lot of amplification to get it to the level, which could then be usable for cmos(0-5V) devices. I used single op-amp for that, and the output should have been multiplied hundreds of times, but that didn't happen, so there is much more in it, than just adding input and feedback resistors.
Same goes with zero crossing detector(for converting sine to square wave) circuit, whose sensitivity should have been 10mV pp.
I tend to see the circuits that I've found as universal building blocks. But no, if they work for a certain application, that certainly doesn't make them universal.
So I'm heading back to square one, using again phototransistors as receivers. Although noisier, at least they provide more output as they are, even without amplification.
sorveltaja:
Again, I was looking ways to amplify the receivers output. Can't tell, why the op-amps didn't work for that purpose in any way :scratch:.
Anyways, I remember seeing this project, that uses an audio amplifier instead of op-amp: https://makezine.com/projects/infrared-string-bass/
After a bit of testing, the 386-based amplifier actually seems to provide a lot of output. I skipped the datasheet's x50 gain version, and went for the x200 one:
Not the most sophisticated way, but if it works, the project could go ahead, after weeks of stuck in the rut.
PekkaNF:
I haven't been experimenting with photo receiver diodes in ages, therefore my memory is sketchy. You probably said which diode you used and read the data sheet. In my youth the photo diodes (as a component, maybe nowadays they have integral circuitry) were actually very unlinear current sources. To make any use of their output you had to treat them as a unlinear current source. I always resorted to datasheet and application notes basic circuits and tested them.
Phototransistor and specially pair mounted on same housing was my favorite, usually they were manufactured for automotive or industrial use and their application notes were made very easy to follow.
None of the photocomponents are very linear and stable, but you are not making here very linear system and your aim is just to get good signal and ennough head-room for the signal to keep circuit reliable.
Most of the times operational amplifiers can be treated as a building blocks - as long as you keep in mind offsets, biases and output loading, for me they were whole lot to easier to use than transistors on linear region. At the begining I was drawing capacitors between every stage to isolate DC-path (we are talking here audio signal) but that wasn't completely problem free either....sometimes signal riding on DC-offset is a good thing, just need to keep it in check and zero it before feeding it out of the jack. Pretty soon I lost interest on audio circuits and went different way.
With signal generator and scope you can figure out fast what works and where the problem is.
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