[sdiy] Reading IR phototransistors in a 8X8 matrix

[Mike Bryant]

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
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:



[cid:image002.png at 01D8B0E3.4048B080]

But instead of switches like in this circuit I would use 2 pins phototransistors like this one:
[cid:image003.png at 01D8B0E3.4048B080]

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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