Author Topic: pressure sensitive expression mounted on guitar  (Read 317 times)

macsearcher

pressure sensitive expression mounted on guitar
« on: March 28, 2021, 11:41:55 PM »
Hello folks.
I'm wondering if anyone has any ideas on the best way to accomplish this.
I have a Morningstar midi box that takes several expression inputs and converts them to MIDI.
I'd like to mount a few pressure sensitive buttons on my guitar to create momentary expression controllers. I've rigged up a cruel one for now which is simply a potentiometer from an old expression pedal taped to the guitar and aside from it's obtrusiveness, it works quite well.
I've seen force sensitive resistors (see pic below) but I don't know anything about what I'd need to do to wire them up so that they work as a pedal type expression control message.


« Last Edit: March 28, 2021, 11:49:03 PM by macsearcher »

Mark Hammer

Re: pressure sensitive expression mounted on guitar
« Reply #1 on: March 29, 2021, 08:21:39 AM »
I salute your adventure.  :)
There are a few things to consider.  One is certainly having a variety of sensor-types to complement different approaches to picking, phrasing, etc.  Another is, as you rightly note, having sufficient space for more than one sensor.  A third is having the "right conditions" for a given sensor (more about that later).  Finally, there is the matter of electronic requirements.

I'm unfamiliar with the Morningstar unit, and can't see anything on their site that provides greater clarity.  What is the unit expecting as an expression input?  Is it assuming a TRS setup, where a designated voltage is divided down?  Is it comfortable with a simple variable resistance?  If it demands a control voltage, what voltage range?

As for "right conditions", once I realized that Line 6 products used a simple 0-10k variable resistance to ground, I simply taped a photocell to the top of my guitar, where I could cover and uncover it with my pinky, while playing.  MUCH smaller than the FSR you show, making positioning more flexible.  Of course, that assumes one would have a source of ambient light yielding sufficient illumination of the photocell to block with one's finger.  I suppose one could always devise a little bracket to mount a superbright LED pointed at the photocell, with sufficient space to move your hand in.

A device I first learned about at NAMM a few summers back is the TruV Trem unit - https://www.truvtrem.com/ .  It is an intriguing and easy to use retrofit for a Strat-style tremolo bridge/unit that permits standard pitch-wiggle with up/down movement, but allows the user to implement volume swells with side-to-side (i.e., towards/away from the control knobs) movement.  I spoke at length with the developer, and gather it employs a magnet near the base of the tremolo arm, and hall-effect electronics, that can translate lateral arm movement into a control voltage or similar.  I found it very easy to use, with the steeper learning curve being how you want to use it, rather than how to make it work.  But the bottom line is that it represents a form of real-time control that can be integrated within phrases with minimal disruption.  Things like pressure sensors, or the Source Audio Hot Hand that I have, tend to be forms of control one can only use AFTER strings have been picked/strummed, rather than while picking.  The photocell approach falls somewhere between.  Basically, there is a division between controllers that can be embedded within picking, and those that demand picking be interrupted in some way.

Mark Hammer

Re: pressure sensitive expression mounted on guitar
« Reply #2 on: March 29, 2021, 10:24:15 AM »
Just to add, the FSR  simply mimics a single resistor.  If your MIDI box is expecting to encode a voltage, provided via a TRS jack.  Then you'd need to divide a voltage.  And for that you'd require a second resistance to form a virtual pot/voltage-divider.  The FSR could either be the "input" leg of that virtual pot, OR the ground leg.  The junction between the FSR and that second resistance serves as the "wiper" of that virtual pot.

But again, I'd need to know a little more about what the Morningstar unit expects at its expression-control inputs.

macsearcher

Re: pressure sensitive expression mounted on guitar
« Reply #3 on: March 29, 2021, 11:51:12 AM »
Hi Mark.
Thanks very much for the response.
I must admit that I am a total novice when it comes to wiring.
The Morningstar has 4 TRS inputs which can be configured to accept expression pedal input. This is then converted to MIDI data and sent out via the Morningstar.
The information that I have about what the Morningstar controller expects is as follows.

The Morningstar works best with expression pedals fitted with 10k - 25k Linear potentiometers. The potentiometer wiper should be connected to the Tip of the stereo cable, while the Ring and Sleeve should be connected to the outer lugs on the potentiometer.

ideally, I would have three or four of these small sensors mounted on the body of the guitar with the wires running down to my Morningstar device. I'd like to try to keep things as compact as possible.
If a lot of additional hardware is needed, I would possibly build a box to go between the guitar and the Morningstar but ideally, everything would be attached to the body of the guitar (and eventually fully built-in).
The idea would be that I would play the guitar with my thumb and use my other fingers to apply pressure to the "buttons" as needed.


aron

Re: pressure sensitive expression mounted on guitar
« Reply #4 on: March 29, 2021, 12:09:03 PM »
What Mark is saying is that you have 1 of 2 resistors needed to make a voltage divider using that force sensing resistor. That's why you have 2 lugs instead of 3. So you need to basically make this circuit.


Your FSR can the the bottom or top resistor in this circuit. When you add up both values, they should basically equal anything from 10k to 25k.
Depending on the value of your FSR, it could sweep a portion of 0k-25k or the entire range. You just need a resistor and as usual trial and error.
You could start with a 10K FSR and 1K resistor. Connect the resistor leg to any FSR leg and now you have your 3 "lugs".
The connected leg (the one that is the resistor leg connected to the FSR) is your middle lug/wiper. Connect this middle lug/wiper to the tip of the stereo cord.
The other two leads connect to the ring/middle and sleeve of the cord.


(Advanced version). Instead of a 1K resistor, you could put a 10K pot and only connect two lugs. The outer lug could go to the FSR (and become your wiper/middle) and the middle lug of the pot can be connected to the other remaining lug and that can connect to either the middle/ring or sleeve. This would make a "range" control I think.


Try it and let us know.




« Last Edit: March 29, 2021, 12:13:36 PM by aron »

aron

Re: pressure sensitive expression mounted on guitar
« Reply #5 on: March 29, 2021, 12:12:41 PM »
In the posted picture, Vin would be one FSR leg and connected to ring, Vout would be the FSR leg connected to the 1K resistor and connected to tip, and the "bottom"/ground would be connected to sleeve.

Mark Hammer

Re: pressure sensitive expression mounted on guitar
« Reply #6 on: March 29, 2021, 02:07:36 PM »
Assuming that the TRS inputs on the MIDIbox can share a base V+, and ground, using  three sensor elements, as shown in the photo, would require a 5-conductor cable to connect the guitar to the midibox: V+ (whatever that is, whether 3.3V or 5V), a common ground, and the wiper outputs of your three virtual pots.

Again, absolutely NOTHING against them, but for sheer flexibility and expressiveness, it's nice to have some form of expression control that won't interfere with picking.  I like my Hot Hand, but it's something one can only apply after picking/strumming.  It's great for what it does, but one would like the ability to introduce control AS one picks.  OTOH, I guess that's what foot controllers are for.

aron

Re: pressure sensitive expression mounted on guitar
« Reply #7 on: March 29, 2021, 06:44:55 PM »
What if he had these on the stage marked and then pressed on them while playing. Could be cool I guess.

macsearcher

Re: pressure sensitive expression mounted on guitar
« Reply #8 on: March 29, 2021, 07:03:33 PM »
While it would be convenient to have the three sensors share a single cable, I'd be fine if it was three separate cables. They wouldn't be passing audio so I'm guessing that I could get away with using fairly light/thin cable and running them in a loom together until the connection point.
I may not be understanding the limitations of the design but I don't see why I wouldn't be able to use these while I'm playing. I would be using my thumb to pluck the strings and my fingers, when required, to apply pressure to the sensors.
I really do appreciate all of the input folks. Thanks!

Mark Hammer

Re: pressure sensitive expression mounted on guitar
« Reply #9 on: March 29, 2021, 07:16:18 PM »
What if he had these on the stage marked and then pressed on them while playing. Could be cool I guess.

I suppose.  But at that point...if I'm understanding you ...it's essentially 3 expression pedals.  A cheaper substitute for expression pedals,and less cable to run from the guitar to whatever, but expression pedals all the same.

While it would be convenient to have the three sensors share a single cable, I'd be fine if it was three separate cables. They wouldn't be passing audio so I'm guessing that I could get away with using fairly light/thin cable and running them in a loom together until the connection point.
I may not be understanding the limitations of the design but I don't see why I wouldn't be able to use these while I'm playing. I would be using my thumb to pluck the strings and my fingers, when required, to apply pressure to the sensors.
I really do appreciate all of the input folks. Thanks!

If you can swing it, more power to ya.  It's just that they're called "force-sensing resistors" for a reason.  Achieving the desired change in element-resistance may require enough finger-pressure that picking and pushing become mutually exclusive.  Again, not a reason to abandon your initiative.  Just be realistic about what you'll be able to do. I thought my photocell thing would provide freedom, but light isn't always where you need it.  Also, you'll need to factor in the time it takes for applied force to translate into resistance change.  These things are not instantaneous. Part of their interference with picking is the wait time.  It's not horrendous, but what works well for increasing synth modulation depth with your left hand, while you diddle the keys with your right hand, may be insufficiently responsive for modifying an effect while picking.

Finally, I think FSRs of some sort are also used for breath-controllers.  These are also capable of generating a control voltage using one's mouth (blowing into an enclosed space), in a way that doesn't/wouldn't interfere with other hand activities like picking.  FSRs are also used for the Igor controller in David Rainger's pedals.

I'll mention in passing, something I note once in a while.  "Racing car" games and driving simulators from the 90s generally come with two spring-loaded foot pedals, mounted in a chassis, that plug into the "games port" in the back of a PC.  One foot pedal is the accelerator and the other is the brake....for "realism".  The pots are generally 50k each.  Keep your eyes peeled for these in either thrift shops or yard sales.
« Last Edit: March 29, 2021, 07:19:29 PM by Mark Hammer »

aron

Re: pressure sensitive expression mounted on guitar
« Reply #10 on: March 29, 2021, 11:05:34 PM »
You are absolutely right, however, I can't recall the last time I saw someone stomp on the floor and create hopefully WILD effect sounds without an "obvious" pedal. Cover it with tape and you will have a bunch of people saying "what"?????