[sdiy] Reading IR phototransistors in a 8X8 matrix

[Loscha]

I have seen this type of thing done with just LEDs, using the LEDs both as
photocells and light emitters.

You read the voltage generated on an LED one at a time, whilst the LEDs
around them are all on during that sense cycle.

I have not done this myself, however, so it may not be suitable for your
situation.

/Edward



On Tue, 16 Aug 2022 at 05:05, Jean-Pierre Desrochers <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:
>
>
>
> 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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