[sdiy] * Piano roll reader project.. using Photo-transistors to read holes.. Sensitivity wise... ??

Ullrich Peter Peter.Ullrich at kapsch.net
Fri Dec 2 08:09:39 CET 2022

Hi Jean-Pierre!

What about adding Schmitt Trigger Gates at the phototranistor board, This makes the signals digital and low impedance – crosstalk should then be no problem.
74HC14 for example (inverting!)

But be careful – strong digital signal and edges normally lead to more EMC problems!


Von: Synth-diy <synth-diy-bounces at synth-diy.org> Im Auftrag von Jean-Pierre Desrochers
Gesendet: Donnerstag, 1. Dezember 2022 19:44
An: Synth-diy at synth-diy.org
Betreff: *EXT* [sdiy] Piano roll reader project.. using Photo-transistors to read holes.. Sensitivity wise... ??

So far my  Piano roll reader project goes well..

I’m now doing some experiments using T1 phototransistors (940nm IR wavelenght)
that are each inserted in the small one inch long copper pipes that prolong the rear of
each square holes of the main brass reading bar.
I noticed that rising the PT’s collector resistor will in fact also rise the PT gain
thus making it more sensitive. That’s good.. but there is a problem,
rising these resistors too much will create possible crosstalk between
the IR PT’s wirings (around 88 x 2 inches long wiring pairs) to the pcb that holds the readers IC’s (74HC165).

There will be 11 x 74HC165 parallel to SPI interfaces mounted on the main board

To read the 88 holes.. See one of the 11 SPI reader circuit here.

One thing that encourages me is that the rate of holes changing on the scrolling paper

Will never be higher than the humain hands that played the original song.. so not very fast for crosstalk..

I tried 10k’s but this needed a too strong IR beam facing the PT’s..

100k’s gave a much more sensitive result..

I could select a final value from 47k to 100k for all these resistors..

What do you think about  the possible crosstalk at that read speed, and the value to use ?

Thanks for your time !


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