[sdiy] Reading IR phototransistors in a 8X8 matrix

[Jean-Pierre Desrochers]

Hi Ben,

 

I like your suggestion about using cheap microcontrollers on a SPI bus.

Thanks !

 

De : Ben Stuyts <ben at stuyts.nl> 
Envoyé : 16 août 2022 07:47
À : Jean-Pierre Desrochers <jpdesroc at oricom.ca>
Cc : Synth-diy Mailing List <synth-diy at synth-diy.org>
Objet : Re: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

Ok, I thought I’d mention the PlaySK project in case you missed it.

 

About your plan with a matrix of phototransistors, how about this variation: You will basically have 90 phototransistors in a long row. So you will need a long pcb anyway. Why not get some 8-bit or 16-bit shift registers (parallel -> serial) and put them on the pcb? That way you have easier routing, and you can latch every phototransistor at exactly the same time. So no timing/jitter worries. Another variation would be using a bunch of (cheap) microcontrollers, bus them through SPI bus, and connect each phototransistor to and ADC input. That would also be great for calibrating the response curve.

 

Ben

 





On 16 Aug 2022, at 01:57, Jean-Pierre Desrochers <jpdesroc at oricom.ca <mailto:jpdesroc at oricom.ca> > wrote:

 

Yes I know that project..
But I want a self running unit (no PC needed).
Stuff that I’ll use:

Microchip uPC

MP3 player to play the paper roll notes (88 playable ‘Honkytonk piano’ wavefiles) on stereo audio outputs

MIDI output for any external device.. like a synth for instance !




I think so far the matrix way to read the notes could be the easiest way
with the less components used.

 

 

De : Ben Stuyts <ben at stuyts.nl <mailto:ben at stuyts.nl> > 
Envoyé : 15 août 2022 19:05
À : Jean-Pierre Desrochers <jpdesroc at oricom.ca <mailto:jpdesroc at oricom.ca> >
Cc : Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org> 
Objet : Re: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

Have you seen this project? It uses a webcam to read the piano roll:

 

https://github.com/nai-kon/PlaySK-Piano-Roll-Reader

https://www.youtube.com/watch?v=8lkMRsoG9cg

 

Ben

 






On 15 Aug 2022, at 21:00, Jean-Pierre Desrochers <jpdesroc at oricom.ca <mailto:jpdesroc at oricom.ca> > wrote:

 

I'm designing a piano Roll reader/player (yes antique paper piano rolls !)
that will read holes on a scrolling paper and play the actual musical notes
using MIDI standards..
This has already been done in the past but with very few documentations left
for DIYers like me.

So.. I'm thinking of using IR (940nm) LEDs, phototransistors and aligned short optical fibers
to read the actual scrolling holes. 90 phototransistors will be used as 'bit' readers.
I would use IR because paper is mostly opaque to 940nm wavelength
compared to visible light that could get through it.

Because of that high quantity of PT’s to be read I was thinking of using
rows/columns matrix of switches like keyboards in synths like this:

 

<image004.png>

But instead of switches like in this circuit I would use 2 pins phototransistors like this one:
<image001.png>





The resistors could be around 10K and this configuration would use only 2 x 8 bits ports.

But I was wondering about using PT’s in a matrix like that would bring glitches or noise

In the CPU readings.. 
Has anybody used PT’s in such a circuit ?

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