[sdiy] source for thumbwheel switches?

[Tim Parkhurst]

On 4/30/07, J P <jonathan_science at hotmail.com> wrote:
> A man has a cool synthesizer called the Borgfotron...  some of you might
> have heard of this? ;-)
>
> He used some nifty thumbwheel pushbuttons on that synth.
>
> I would like to employ something similar....  but all the pushbuttons I can
> find are for digital systems...  I would prefer to use analogue routing as I
> haven't the understanding of a digital system that would function that way,
> nor do I want to try to figure that out...
>
> Anybody know where those are sold?
>
> Jonathan P.
>

Hi Jonathan,

Yes, I think most of us have heard of the Bergfotron. Nice synth, and
very interesting hardware.
http://hem.bredband.net/bersyn/

Off the top of my head, I think what Jorgen is doing is using
thumbwheel switches that put out a binary count (BCD), and then that
binary number is used to control an analog multiplexer. This
multiplexer is basically a chip that contains switches designed for
routing analog signals. A common analog multiplexer used in synths is
the 4051.
http://www.fairchildsemi.com/pf/CD/CD4051BC.html

This chip has 1 input and 8 outputs. A three-digit binary number
controls which output is connected to the input. NOTE that most analog
switches and multiplexers are bidirectional, so the signal can flow
either way. In other words, the 4051 can be used as a 1 in / 8 out, or
as a 1 out / 8 in.

So to sum it up, you use a BCD thumbwheel switch to control an analog
switch. You do have to be careful, as the 4051 and other "4000" series
analog switches will only handle +/-7.5V signals, but there are other
high-voltage analog switches that will handle the +/-10V signals that
modular synths sometimes see (look for "DG" series high voltage analog
switches).

Hope this helps.


Tim (all thumbs, but no thumbwheels) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein