How is this Capacitive Touch Pot Wired?

T

Thundercat

Well-known member
Hi all, I'm keenly interested in creating a capacitive touch pot. I've settled on the Alpha endless potentiometers, RV112FF-40B1-15F-0B20, https://www.mouser.co.uk/ProductDetail/317-1240FF-15F-0B20K, and I know these have been used for this purpose.

They do not have a dedicated touch wiper and the metal shaft has a bit of connectivity with the body - when I test on my meter it bounces around from 75R to 200R and anything in between (not sure why that's the case). But there does seem to be some conductivity between the pot shaft and the body.

I'd love to be able to have capacitive touch on these pots, but I'm not clear how to proceed. Since the pots don't have high conductivity, I'm not sure they would work with just attaching a wire to the body of the pot through a 1M resistor to a Teensy pin.

Maschine made by Native Instruments (NI) has touch-sensitive pots. You can see a video of it in action here:

What you're seeing is the pot board, and the Alpha pot being used:
Screenshot 2025-03-19 at 2.27.03 AM.jpg


And a couple closeups:
Screenshot 2025-03-19 at 2.27.13 AM.jpg


Screenshot 2025-03-19 at 2.27.21 AM.jpg

I see some kind of interesting PCB pattern on the underside of them, which I assume is related to the capacitive touch, but I'm not sure.

Also there's a setting in the options for low or high touch sensitivity too.

Does anyone have any insights into what this curious PCB pattern on the underside of the pots may be doing, and how they might be achieving capacitive touch with it? And also even without this mysterious PCB do you think cap touch can be implemented with a slightly conductive pot knob to body?

Thanks for your thoughts.

Mike
 
From my limited understanding I would guess:
That pattern on the underside acts as the capacitive touch sensor for each potentiometer / encoder. It seems that the housing of the encoders are not connected to ground or anything, right?

In this handbook you can find a little bit about "Mutual Capacitance Sensors" which is one way to build such a sensor on a PCB.

Unfortunately there is no information about encoders or so.

I tried some patterns a few years back and remember that size of the planes, length of wires and grounding can become highly critical to the point where the whole system does not work anymore at all. It can be a challenge to get it reliable and robust. So I would assume that something like the solution on the PCB is the preferred way. But if you are curious and have some patience you could simply try out if you find a way to directly connect the housings and use it as sensors. Maybe there is a way to make that work.
 
Excellent thoughts Tom. Thanks for the advice, wisdom, and the link. I really appreciate it.

From the PCB picture it does not appear the housing is grounded (two large blobs on top and bottom of each pattern). In fact you can see they carefully routed traces around it (although we can't really tell from the top pic as it's at least a double sided board, so it may be connected).

I appreciate it, and I will check out your link.

If you or anyone else has any further insights based on the photos and video above, I would be grateful.

Thanks brain team :)
 
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