Resistor Ladder Aux Switch

I see in the 3.12 beta manual that there’s now an option for a resistor ladder aux switch. But I googled around and couldn’t find any info on what a resistor ladder aux switch is (just a resistor ladder circuit more generally). Does anyone actually make one you can purchase? Has anyone seen a schematic for how to build one? How do they work and why do they allow so many more switches into a single omniport?

To be honest I don’t really know, but my guess would be it works like a quantized expression pedal. Just a couple of switchable resistors in series.
So if you put a couple of expression pedals in different positions and route them through a passive loop switcher you’d have a resistor ladder footswitch.
But I’m just fooling around :grin:

I found a feature request by @goosecheese which might answer your questions

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We have currently no plans to make one, but I think there are some builders who are already designing one.

Here’s the schematic:
Main.pdf (24.8 KB)

Thanks for sharing the schematic. It looks simple enough to build. And the idea is that if you only wanted, for example, 6 switches (but didn’t want to give up 2 omniports), you could just follow the schematic for the first 6 switches and leave it at that?

Yes.

I think Pedalnetics and SparkleFish might be building these.

Yes, the idea is that the resistance increases as you add more switches (i.e, if you press on switch 7, it is the sum of 7 resistors), so the controller can differentiate.

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There are a few ready made options on the market that use this method. TC helicon make a six switch version already, that is used in their voice effect units.

But it is also a very simple DIY project if you’re that way inclined, and would allow you to make something truly custom to your needs without having to spend very much.

@james and the team have been really responsive on this one, another example of how fantastic their support for the product has been. Unfortunately I’ve had to deal with adult things like renovating my house, so haven’t been able to really dive into this and test the limits yet, but it’s certainly on my backlog for when I have the time available! There’s a lot that opens up with this functionality that goes beyond just simple switching.

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Google searching suggests the TC Helicon Switch-6 uses 10K resistors, which I believe would make it not compatible with the circuit diagram james posted (using 1.5K resistors). Just in case anyone (like me) was thinking about existing off-the-shelf options.

Not so sure about that. 10K resistors might/could work…

The editor has a calibration process where it learns the resistor values for each switch. It might work with 10K resistors.

There are probably two limits that matter for designs:

  • The minimal amount of resistance that the device can detect differences without noise. A 10 or 100 ohm difference might be too low.
    • This is likely a percentage of the resistance (based upon the step from 1.5K resistors to 2.2K resistors in diagram above). That is, 1.5K to 3K might be ok to detect, but 10K to 11.5k might be too small a jump)
  • The max resistance (total) that the devices can drive and detect. I assume a gigaohm might be too much.

Certainly possible. You’re probably right that the calibration routine may be able to handle it. Without knowing what the actual logic inside the MC6 Pro looks like, or what the software routine looks like, I don’t want to speculate.

I put an enclosure and switches in an Amazon cart and figured I’d mess around. In the manner and location I was thinking about using it I really only wanted six switches, any more that that (in the space) would be closer together than I wanted. Then I looked up that TC Helicon switch and thought, “hey, I like that form factor better” and was going to just buy and try. I saw the 10K resistors and posted the info here, but now that I think about it I can just replace the resistors in it anyway. I’ll probably do that.

It should work. I just tested with 10k resistors for up to 8 switches. As long as you can get unique values for each switch in the calibration menu, it should be ok

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How about 50k trim pots instead of resistors? That way you could adjust each switch to an exact percentage.

possible but I’m not sure how precise and stable are trimpots over time. If it goes off by a few hundred ohms then a recalibration may be needed.

67WR100KLF TT Electronics/BI | Potentiometers, Variable Resistors | DigiKey
This is what I had in mind. 20 turns.

Can I just check before I build this. Am I looking for spst momentary switches.
some nice softies will be great

“The footswitches need to be Momentary and Normally Open” from the manual if that helps?

Just to make sure I’m reading the schematic right–for the TRS input, sleeve goes to ground, tip connects to the switches, and ring is left unconnected?

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Also, what is the procedure for calibrating the resistor ladder switch? I built a switch and am trying to calibrate it. I get into the calibration menu and it asks me to press a switch. I do, and a value shows up, but then disappears when I let off the switch and nothing happens. I can press each of the switches and it does the same thing (but showing a different value). It’s not clear to me what I’m supposed to be doing with this.

The values I’m getting when I try to calibrate are:

Switch 1 - 11
Switch 2 - 18
Switch 3 - 24
Switch 4 - 38
Switch 5 - 34
Switch 6 - 29

Is it a problem that switch 5 is a smaller value than 4 (and 6 lower than 5)?