[sdiy] organ pedals, was:Synth Keybards and Number of Keys

[Edward King]

>
> Are you using the slotted optical switches just for switching or also  for 
> displacement sensing?

In my case, Im using bus-bars for switching (and velocity sensing) and opto 
switches for displacement sensing.
However, over the last 48 hours, Ive built a couple of test pcb's with 
pressure sensors on them with a view to adding this setup to the keys as 
well. This setup is much simpler to implement, is cheaper than slotted 
switches, takes up much less room and has better sensitivity with very well 
documented and predictable behaviour.
You may want to consider this as an alternative.

> I was thinking about adding a displacement sensor to one of my 
> pedalboards, so the thing could double as an expression pedal. This 
> pedalboard (a very old one) already has a common switch for all keys, 
> probably used to trigger an envelope. That switch moves nicely when  any 
> key is pressed down, so I thought I could just attach a small  magnet 
> there and measure the field with an analog hall sensor. Of  course this 
> would only sense the displacement of one key at a time,  but I don't think 
> that this is a drawback. The individual key  switches are still there.
>
> One option I'd like to have when a synth parameter is controlled with  key 
> displacement: the setting should keep its value even after the  foot is 
> removed from the key. I was thinking about two modes: when  the key is 
> pressed first (mode 1) only values greater than the  current value are 
> output.

This may not be practical in base form. There is noise from these devices so 
you will have to conduct some experiments (or I'll be happy to share my data 
with you) to determine how bad this is in your implementation. 
Experimentation is tricky because ambient light will give you misleading 
data unless you do your experiments in the dark or with the project 
enclosed.
Essentially what this means is that you're going to have to go with 
averaging over a sample period

>Once the key is pressed all the way down it  switches to mode 2: then only 
>values smaller than the current value  are output. If it's completely 
>released it switches back to mode 1.  This way a value could be held as 
>long as the key isn't pressed all  the way down. Pressing it all the way 
>down then releasing it would  set the value back to 0. A LED for each key 
>indicating which mode it  is in is probably a good thing.
> Any better ideas?

I dont know about "better" but alternatives spring to mind:

1) A "hold" button on the keyboard itself which you press when the 
appropriate level is reached.
This could be a latcher. Press for hold, press for release type thing.

2) pressure sensitive pills have "memory". This functionality isnt normally 
used, but can be when a static charge is applied. This "memory" is reset 
when the pill is manipulated (pill is deformed or pressure applied). So you 
could do it like this:
Scan the pills, process data, if hold conditions are required, apply static 
charge, if not dont apply charge.

This way the memory function of the pills is only used as required (i.e on 
the way down, but before bottoming out) and disabled when not required (when 
the key is on its way back up having NOT bottomed out or when it is actually 
bottomed out).


EK

>
> Ingo 


		
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