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

[harrybissell]

The phototransistors need not have a snap response, it depends on the
electronics
you use them with. In general if you use them with a collector load, they will
be
pretty unlinear because of their beta. If uoi use them as emitter-follower they
will have
pretty low output but much more linear...

instead of a gradiated film. why not just use a shadow mask. If the light source
is interrupted
with a simple shutter, there will be a large linear response when the light
source is half-open
etc... PROBABLY way more than you would like for good switching response.

Phototransistors turn on FAST but turn off slow, as a rule. Photodiodes can be
much much
faster, but require an amplifier per diode. Who cares ? we are DIY and component
count doesn't matter to us...

...well maybe except JH   :^P

H^) harry (who uses LOTS of components :^)

Tim Parkhurst wrote:

> > 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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