[sdiy] DIY slide-bars

[Gene Stopp]

I've done two experiments with two different types of material.

First experiment: black antistatic bag material (opaque and rubbery), cut
into a strip. Second experiment: conductive rubber cord (maybe 1/8" dia.)
used for EMI gaskets. Both of these materials had a fairly high resistance
per distance (several K per inch). What this caused is non-linear behavior
and moving voltage points (i.e. yesterday 2 volts may be right *here*, and
today it's over *here*, a couple inches away). Something like the Moog
controller is more stable and repeatable, meaning you could mark the
fingerboard with the voltages and they'll always be at the same places. I
think it's because the resistive element was a pretty low resistance, driven
by higher current. This would imply some kind of heating element wire, like
in an electric heater. It may be only a couple of ohms so something like a
power regulator might be needed, perhaps configured in constant current
mode.

I've not run across such an element yet but I do have a note to self to be
on the lookout for the next trashed bathroom heater or hairdryer.

- Gene


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Rainer Buchty
Sent: Tuesday, July 27, 2004 6:57 AM
To: Magnus Danielson
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] DIY slide-bars


>1) How did these classic slide bars work? What materials etc. was there?

In the Elektor magazine they just used graphite, i.e. you literally drew
the slide bar, but then had a metal probe for sliding so your finger
stayed clean :)

>2) How could one make one today? I am looking for a fairly stable design,
so
>   antistatic foam is out, OK?

I wonder if recording tape could be used in a setup like this:

	========	cover foil
        --------	recording tape
	________	copper or some other conductor

The copper is attached to some sensible voltage. The recording tape
basically acts as a trimmer, so that touching -- thus closing the
contact -- is equivalent to the wiper. Now you take the difference (or
better: the ratio) between the voltages measured at the ends of the tape
and get the tap position. Could probably be further refined if the tape
is crescendo-shaped.

(I'm quite sure people on this list have done something like this before 
and can tell me why that's a bad idea.)

Rainer