[sdiy] Mod/Pitch Wheel Thoughts

[Harry Bissell Jr]

The trouble with the cell is that the resistance is
never well defined. In the "on" position (low
resistance)... the absolute value will depend on
temperature and changes in illumination.  So if you
want to make that point a 'null' you'd need to adjust
out that particular resistance.

In pitch wheel uses, they often use back to back
diodes to create a deadband of 0.7V.  There is no
output until the input voltage goes +/- one diode
drop.

In the two terminal photocell, this is extra hard to
do.

In the MOD wheel case, the diodes are a major
liability because they cause distortion, you can't run
the mod signal through the photocell. Probably that is
the MOST attractive reason for choosing a photocell in
the first place. If you are going to use a VCA for
mod, then you could use phohotransistors for the
position sensing, screw the photocell.

Dark resistance is even worse, because it varies with
temperature, illumination, and the time that the cell
was exposed to light. Max resistance is spec'd at ten
minutes after removing the light source. OUCH

For s simple mod wheel, the photocell is very
attractive. For Pitch wheel, a photovoltaic cell would
probably be better.

or a POT ???

H^) harry


--- Ingo Debus <debus at cityweb.de> wrote:
> 
> Am Montag, 14.03.05 um 02:28 Uhr schrieb
> harrybissell:
> 
> > The flat area, with a single cell... is a loser.  
> The cells have a 
> > really
> > poor thermal stability, so changes in temperature
> render the optical
> > flat useless.
> >
> > You might do differential illumination on a
> center-tapped cell... at 
> > least the
> > two halves would track.
> >
> > I think trying to null the center is too hard. I'm
> thinking "mod wheel 
> > = good"
> > "Pitch-wheel = bad"
> >
> 
> How about a shutter where maximum light (or minimum,
> whichever is 
> easier) is exposed to the cell in the center
> position of the wheel, and 
> minimum (maximum respectively) at either end? It
> would need an 
> additional switch that detects whether the wheel is
> moved up or down 
> and reverses the polarity in one case. A flat area
> around the center 
> position would be easy to implement.
> 
> Ingo
> 
>