Homebrew Vactrols

[Harry Bissell]

Ironically.... I built a footpedal to emulate the "Hat-Kat" hi-hat pedal.
It too...
used a shutter LED/LDR.  Originally I had the shutter interrupt the light
beam
in the closed (down) position. This worked... but the trigger came about
2mS
LATE.  I had to rework the shutter so the cell was DARK, and then became
LIGHT (suddenly) right at the trigger point (closed).  The response of
the cell was
strongly affected by the fact that
...whereas it is possible to BOMBARD the LIVING SH!T out of the cell with
PHOTONS and get it conducting REAL FAST...
...but it is Impossible to "UN-BOMBARD" the target... it must decay at
its own sweet
rate.  (at least before Bissell Industries patented "Black Hole"
technology....)

The ultimate resistance (dark) of the cell is often specified after
several hours or DAYS
of being dark (Dark Adaptation) .... its an exponential decay.  And if
the cell has been strongly illuminated it will lose the ability to go to
ultimate high resistance for some period of time (Light Memory). Higher
resistance cells are always slower than lower resistance cells... and
wide range cells are slower than those with a narrow light/dark
resistance ratio.  The color of the illumination is very important... it
should match the spectral curve of the cell...

One often overlooked favtor is voltage rating of the cell. These things
are often used by people (including yrs truly...) in vaccuum tube
applications. Switching high voltages requires a cell that can withstand
(block) these voltages, and also not exceed the power ratings.

The downside of the LDR/LED is that it isn't repeataable. If you can
calibrate each one... fine. If not the values will be all over the map
(from unit to unit...)

H^)   "did I actually say 'vaccuum tubes' ???"

Paul Perry wrote:

> Ironically, I was recently repairing an Ibanez wah pedal
> which uses a led/ldr arrangement with a shutter connected to
> the footpedal.
> The original ldr was missing... after trying every ldr I could find,
> the only one that worked was one I got by cutting a CLM600
> in half!!! (the critical requirement was small size, and a very low
> resistance at max illumination)
>
> paul perry melbourne australia
>
> BTW for diy use I think the main variable would be the response times
> of the ldr. Which is always faster for attack than decay.