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4 wire swell potentiometer?

PostPosted: Mon May 29, 2023 5:08 am
by ajt
I've recently acquired an old Miller "Norwich" console. It's very well made, but very old.

The swell pedal is confusing me - it appears to have 4 wires going to it in 2 pairs.

Pair 1 - this went to what appears to be a combined amplifier and power source so I'm assuming is power:
White insulated wire x 2

Pair 2 (in grey insulation):
Blue insulated wire
"Bare wire"

I'm assuming one pair is positive and ground and the other is "signal" and ground.

If I wire up Pair 1 as +/- and Pair 2 as Signal/Ground, I get 0 signal. If I remove the ground from either pair, then I get signal but nothing that relates to the movement of the pedal.

Same if I flip Pair 1 to be Signal/Ground and Pair 2 to be +/-.

So I'm clearly wrong. Any ideas?


Re: 4 wire swell potentiometer?

PostPosted: Mon May 29, 2023 6:28 am
by rayjcar
I did a bit of searching, and it appears that the potentiometer is actually an LDR (light dependent resistor).

If you look at the top left corner of the attachment, two wires supply voltage to the lamps, and the other two provide the resistance value. There is a mechanical shutter which varies the amount of light.
So without the lamps powered, you won't read any resistance. Even if you do, you may not have the value and taper required for a typical midi encoder (i.e. 5k to 10k resistance, linear taper. Your best bet would be to replace the potentiomenter with a new one with correct shaft size and rotational arc that matches the existing rotation.


Re: 4 wire swell potentiometer?

PostPosted: Mon May 29, 2023 7:06 am
by ajt
Wow, your internet searching skills are better than mine - really useful, thank you!

Re: 4 wire swell potentiometer?

PostPosted: Mon May 29, 2023 2:16 pm
by larason2
I agree with Ray, trying to get a light dependent resistor to work with an arduino is a headache. You could also try a linear pot with an inexpensive plastic mount.

Re: 4 wire swell potentiometer?

PostPosted: Tue May 30, 2023 8:55 pm
by Coenraads
When MIDifying a Galanti some years ago, I retained the LDR based swell pedal by adding a pullup resistor as shown in the bottom picture:

But the resulting small voltage range (Voltage readings 2.06 volts (closed) to 3.79 (open) ) required code different from that used with a potentiometer. In my parallel scanner program: ... el-scanner

I therefore provide two different methods of scanning the expression pedal. (Method 2, for LDRs is commented out but is there if needed). This method worked just fine with the Galanti.