Expression pedal wiring

[skern]

Hey guys. I want to make an expression 'pedal' that's not actually a pedal but uses a force sensitive resistor instead, which is simply a piece of resistive material with two contact points on either 'side.' By pressing my finger I can vary the resistance from >1M ohms down to 70k ohms. This is a huge range but the pedal I want to use it with (Source Audio C4) has an expression calibration setting using software, so I'm optimistic.

Because the sensor has only 2 contact points I'm a bit confused how to wire it for TRS because normally you'd have 3 contact points if using a potentiometer. Further, I believe the Source Audio C4 uses a 'backwards' TRS wiring. From the manual:

Source Audio's expression pedals are configured as follows: the tip connection is power, the ring
is the expression signal, and the sleeve connection is ground.

Can anyone help me figure out how to wire this?

[ElectricDruid]

You need to turn it into half of a potential divider, by putting another resistor either above it or below it.

Say you put 470K above it, connected to +5V. The FSR goes from the bottom of the resistor to ground. The junction between the two becomes a potential divider between 0V and 5V. Because it's not a genuine pot, you won't get a full range 0-5V, but then you don't on a wah treadle either. That's where the software calibration comes in.

With the values you gave, you'd get:

(1M / (1M+470K)) * 5V = 3.3V
(70K / (70K+470K)) * 5V = 0.64V

You can play with the range by tweaking the top resistor. I chose 470K because it seemed like a midpoint between your two values, but that may no be the best option. With the software calibration, it probably doesn't matter too much.

[Mark Hammer]

Expression pedals/controls that use a control voltage can vary not only in what voltage they use, but also in whether the divided-down voltage comes via the tip or via the ring.

Source Audio uses a 0-3.3V control voltage, but I have no idea what the current requirements are.  The other thing to consider is that Source Audio uses TRRS plugs/jacks for expression control of two parameters.  So you would have to wire up your FSR to provide expression control of ONE parameter, and leave the other one isolated.

I have a SA Hot Hand 3 controller, and built a little breakout box for it, such that the two control voltages produced by hand-waving could be directed to separate devices.  I have yet to use it as such, so I can't say whether I've wired it up right. 

[skern]

You need to turn it into half of a potential divider, by putting another resistor either above it or below it

Alright, so something like this?

[ElectricDruid]

You need to turn it into half of a potential divider, by putting another resistor either above it or below it

Alright, so something like this?

Yeah, like that, assuming that this "expression" input is wired up as CV input with that pin out (which is fairy typical, but by no means universal). Mark was saying that Source Audio use a TRRS plug/socket, just to make things more awkward. Is that true on the C4?

[skern]

Yeah, like that, assuming that this "expression" input is wired up as CV input with that pin out (which is fairy typical, but by no means universal). Mark was saying that Source Audio use a TRRS plug/socket, just to make things more awkward. Is that true on the C4?

Great, thanks!

It seems the C4 uses a TRRS socket, but according to the manual, "Although the CONTROL INPUT port expects a 4-contact TRRS plug, it is still possible to use a 3rd party expression pedal with a 3-contact TRS plug (Note: you should make sure the expression pedal's TRS configuration is the same as the graphic below)"

And the graphic shown has tip = power, ring = expression, sleeve = ground. So I think I'm in the clear if I wire it that way.

[antonis]

Yeaapp.. It should be fine.. 

[Mark Hammer]

That looks about right, Antonis.
I guess the question is whether the sensor should best go between tip and ring, where pressing on it would decrease it's resistance and thus raise the control voltage fed via the ring, or between ring and shaft, where pressing on the sensor to reduce its resistance would reduce the control voltage.  You could use a DPDT switch to flip the contacts around for whichever pressure/CV relationship is best suited to what you want to achieve.

[antonis]

I'm not sure about OP's initial query, Mark..

As it's drawn, it should work as CW rotating pot, in the mean of the more the pressure the more the voltage..

[Mark Hammer]

Understood.  My point is that there are applications where you want to reduce a control voltage, and others where you want to increase it.  All of that depends on how CVs are used in the device.  So, for instance, if cutoff/resonant frequency of a filter gets lower with decreases in CV, then I can see where a person might want to be able to start with the filter "open" and apply pressure to the FSR to reduce the cutoff and achieve an "owww" sound.  Conversely, if LFO rate or modulation-depth increases with increasing CV, then one will usually want to start with a low CV and use FSR pressure to increase it.  The contact-flip I suggested allows the pressure-to-CV relationship to be tailored to the application.

[skern]

That looks about right, Antonis.
I guess the question is whether the sensor should best go between tip and ring, where pressing on it would decrease it's resistance and thus raise the control voltage fed via the ring, or between ring and shaft, where pressing on the sensor to reduce its resistance would reduce the control voltage.  You could use a DPDT switch to flip the contacts around for whichever pressure/CV relationship is best suited to what you want to achieve.

The software takes care of that well. I can choose the exact min/max of the parameter I'm controlling, so I can go from 0-100 or 100-0.

I've got it rigged up right now and it's working. I'm using it to control volume where the default (i.e. no pressure) is max volume and pressure causes it to quickly reduce to zero volume. It works fine although it's not always perfectly smooth when releasing. I think this is a reflection of the resistive material not immediately returning to baseline after releasing. I might experiment with different value resistors but I don't think it matters much because the calibration is so effective. When calibrating I can choose to push the sensor all the way or just a bit. Combined with the aforementioned ability to fine-tune the min/max, I'm not sure what effect a different resistor would actually have.

EDIT: I just switched from a 470k resistor to a 20k and it has the effect of requiring more pressure to achieve the same result. This is a bit different than not pushing all the way when calibrating because this is adding required pressure rather than reducing it.

[RickL]

Sorry to resurrect an old thread, but I just acquired a Source Audio Orbital Modulator and I'm wondering if I can DIY a dual expression pedal for it. The SA161 Dual Expression pedal appears to be passive (no mention of powering it in the user guide), with the option of using its TRRS output to control the Orbital modulator. So no actual control voltage output.

Is it as simple as connecting a dual pot with the end of each pot to the sleeve and tip (maybe through diodes) and the wipers to R1 and R2? Source Audio says the SA161 expression outs are 50k so that is what I would try first.

Anyone have an SA161 they can look inside to confirm? I don't want to duplicate the whole SA161 (although that would be cool too), just the TRRS part.

[ElectricDruid]

Yes, a passive pedal should be fine, and it *is* exactly as simple as you describe.

The TRRS socket provides the required voltage on the Tip connection, so the pedals can be wired up as voltage dividers between that and the sleeve/ground connection. The pedal wipers go to the two Ring connections, which are then your two CVs.

If you're using something that actually generates a CV in the right range itself, you can ignore the Tip connection - but that's not your situation here.