On Sat, 27 May 2006 23:47:49 -0600, Hamad bin Turki Salami
put finger to keyboard and
composed:

On Sun, 28 May 2006 01:40:01 +0000, Matt J. McCullar wrote:


"Hamad bin Turki Salami" wrote in
message news
I'm just curious about a component that's on a board
in a synthesizer I have. It looks like a pot and it's
hooked to a knob that turns through about 315 degrees.
However, it has 4 leads instead of three. I've done
some measurements on it and they're not consistent
with it being a pot. What would it likely be?

When you turn the knob, does it feel smooth in its motion, as a volume pot
would? Or does it go through a series of "click-stops"?

You say it allows you to change a MIDI value. If the device has four leads,
and has click-stops in its motion, I would say it's a rotary switch. What
the switch is probably doing is connecting a nearby pullup resistor network
to Vcc and ground, or it could be doing it internally.

No, it's doesn't have clicks; it's smooth like a pot. In fact, this synth
has all three kinds of component: an ordinary pot for the volume control,
an encoder with click stops for selecting presets, and these 4 lead
components -- whatever they are -- for setting midi controller values.

I'm tempted to just bust one of these things open to see what's inside.

If these devices span 315 degrees, and if they are meant to "encode"
128 discrete values, then each binary increment must correspond to 2.5
degrees of rotation, assuming the devices are "linear".

I think your best approach is to measure the voltages on each pin as
you select known MIDI values. Otherwise look for changes as you switch
from 63 to 64 in midrange. This is the point at which all the bits
change state. To identify the "least significant pin", look for the
pin that changes state most often as you rotate the dial.

- Franc Zabkar
--
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