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

Roman Sowa modular at go2.pl
Fri Dec 2 09:09:47 CET 2022

Frankly I can't see any obvious reason to worry about crosstalk. This is 
only 2 inches of wire carying DC signal. Come on.
You may want to add 10n caps across the optosensors to get rid of HF 
noise of any origin, that might cause false triggers.

Do you have clean 0 to 5V levels on the optos? I dare to think it's not 
that beautiful. With input voltage varying barely above HC165 threshold, 
you may force poor HC165 inputs into linear region, making it demand 
more supply current. Maybe you could tweak power and GND ails of the 
sensors separate from logic supply to find a region where voltage 
changes almost rail to rail from HC165 point of view. And BTW, this chip 
works down to 2V so there's another way to tweak.


> *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 !
> JP
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