[sdiy] Reading IR phototransistors in a 8X8 matrix

[Jean-Pierre Desrochers]

Mike,

 

Here is what I meant when I said there is a diode in series with each switches (phototransistors)
in the matrix like shown here:



So I think none of these PT’s should ‘leak’ in the readings.. right ? 

 

De : Mike Bryant <mbryant at futurehorizons.com> 
Envoyé : 15 août 2022 15:30
À : Jean-Pierre Desrochers <jpdesroc at oricom.ca>; Synth-diy at synth-diy.org
Objet : RE: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

The transistors aren’t diodes.  Some (but I accept not all) phototransistors can conduct in either direction as they are being turned on by light, not a bias current on the base referenced to the emitter.

 

N            collector

P             base

N            emitter

 

Isn’t too different to :

 

N            emitter

P             base

N            collector

 

Gain is less in the second instance, but not zero.

 

From: Jean-Pierre Desrochers [mailto:jpdesroc at oricom.ca] 
Sent: 15 August 2022 20:26
To: Mike Bryant; Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org> 
Subject: RE: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

> Test the transistors and if this does occur you’ll need to add series diodes with each transistor.

But if you look at the shown matrix you will see series diodes on each switches (phototransistors in my case).
Or I am missing something on what you say..

 

De : Mike Bryant <mbryant at futurehorizons.com <mailto:mbryant at futurehorizons.com> > 
Envoyé : 15 août 2022 15:12
À : Jean-Pierre Desrochers <jpdesroc at oricom.ca <mailto:jpdesroc at oricom.ca> >; Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org> 
Objet : RE: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

One possible problem is that the phototransistors turn on just based on light, and so may conduct in the reverse direction.  Thus you may find lots of paths to conduct some of the current through the selected transistor finding its way to ground and reducing the voltage output.  Test the transistors and if this does occur you’ll need to add series diodes with each transistor.

 

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Jean-Pierre Desrochers
Sent: 15 August 2022 20:00
To: Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org> 
Subject: [sdiy] Reading IR phototransistors in a 8X8 matrix

 

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:

 



But instead of switches like in this circuit I would use 2 pins phototransistors like this one:


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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