[sdiy] Opto-switch Kybd (was Reed and Switch)

[Tim Parkhurst]

> From: Roman [mailto:modular at go2.pl]
> Sent: Friday, December 12, 2003 2:33 AM
> To: Tim Parkhurst
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Opto-switch Kybd (was Reed and Switch)
> 
> >keyboard with photo interrupter type switches? They're very reliable,
> 
> I had that idea years ago, have bought lots of photo interrupters
> and still no time to finish it.
> My idea was to use no solid plate to interrupt the light, but gradually
changing
> darkness printed on foil. I made PCB design with dense traces changing
thickness
> from 1 to 10 mils. After making photomask it looked not too good, but now
there may
> be other ways to make such gradual thing.
> Anyway, when such thing crosses photo-interrupter slot, we can obtain
velocity info
> by reading current from phototransistor like we would do in 2-contact
keyboard. Set
> 2 thresholds and voila.
> And what's more interesting, we get aftertouch from each key too.
> I'm only affraid it might be difficult to select so many optos with
similar
> light/current response.
> 
 Roman

Wow, that sounds very interesting. Years ago, I wanted to try something
similar with polarizing material (one piece stationary, one piece moved by
the key and the amount of transmitted light would change), but I could never
get it to behave. I would wonder about the response of the phototransistors,
and whether you could get them to gradually conduct like that. I thought
they were designed to have a Schmitt-like "snap" response, since most
interrupter switches are used in applications where they are either full ON
or full OFF. Even then, you're right about having to carefully select them
to get relatively even response across the keyboard. 

The more I think about it, the more I think the best way is to use two
switches per key to get velocity (after touch is still an issue). For those
not familiar with this, a common way to sense velocity is to use two
switches that close at slightly different points when a key is pressed. You
measure the time difference between the two switch closures (in analog by
charging a cap or digital by starting a counter) and viola! you have a
measurement of how fast the key was pressed down. Perhaps the polarizing
material thing might work for a pressure sensor (remember where you heard it
first). 


Tim Servo
"No pessimist ever discovered the secret of the stars, or sailed to an
uncharted land, or opened a new doorway for the human spirit." -Helen Keller



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