[sdiy] Reading IR phototransistors in a 8X8 matrix

[Ben Stuyts]

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://github.com/nai-kon/PlaySK-Piano-Roll-Reader>
> https://www.youtube.com/watch?v=8lkMRsoG9cg <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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