Are there pots, or variable resistors in other formats, that change their resistance depending upon the pressure/force put upon them? I'd like to make an expression pedal that reacts to foot pressure. I don't want to have a regular expression pedal on my board - its too big, and something like an expression wheel is a little difficult to control. Something small that I could press with my foot would be ideal.
I'm not aware of anything like that.
The only thing I can think of that would change resistance with outside manipulation that's not an expression pedal would be a joystick.
http://www.mammothelectronics.com/joysticks-s/56.htm
There are pressure sensitive resistor strips available. Sparkfun's has some. I think they would be hard to control by foot and may need a microcontroller/digital pot to do what you want however.
The term you need is "force sensitive resistor." I found them on google shopping for about $7.
Most, if not all, of them would decrease resistance when you push down, and are the highest resistance at idle, since they work by bringing two materials closer together. And you can't do a voltage divider. So keep those limitations in mind for what you want to use them for.
I don't know how they do this, but maybe of interest:
http://www.jacquespedals.com/index.php?page=products&product=trinity&PHPSESSID=a79e12c07fc95dd21365073d2727a618 (http://www.jacquespedals.com/index.php?page=products&product=trinity&PHPSESSID=a79e12c07fc95dd21365073d2727a618)
This circuit looks great, maybe worth a try: http://www.diystompboxes.com/smfforum/index.php?topic=88413.0
I was talking about piezo pressure sensors such as this one:
https://www.sparkfun.com/products/8712
(https://cdn.sparkfun.com//assets/parts/1/8/9/2/08685-03-L.jpg)
They are available in differing LB ratings. The one I linked to has a max resistance of 300k when stepped on and 25lb rating. The circle part at the end is the pressure sensor. The mechanics will be the most difficult to control..much more than say a wah pedal. With this being said, it may be well suited to a "fun" switch which does something like the feedback loop on a delay or similar.
You can search for "Flexiforce Pressure Sensor" or "Piezo Pressure Sensor"