Expression Pedal Interfacing

Here is an expression pedal:

Expression pedals are passive devices that take in a supply voltage and ground, and output an adjustable level on a third wire.  The electrical part of the expression pedal is simply a potentiometer (volume control or “pot”), usually 50kohms.  Power at 5 to 9V and ground go to each end of the pot, and the middle wiper conductor is the output control voltage.  The difficult part of the expression pedal is the pedal/track/gear mechanism that causes foot movement on the pedal to turn the rotary pot or move the slider pot.

It takes skill to construct an actuator mechanism that is precise enough and sturdy enough to work over the long run and not slip or grind or wear and fall apart.  The mechanical aspects of the pot, if poorly constructed, can cause trouble over time as well.  Sometimes the travel of the sweeper is not sufficiently limited to keep its contact from running off the resistive track.  The resistive track can wear out, causing resistance changes or disconnects with the wiper contact.  Heavy jamming on the pedal makes this wear a common occurrence.  Many musicians excel at finding choice pots online and replacing their worn-out relics. 

The expression pedal is great in that it can sit somewhere away from the effects pedal, giving maximum flexibility for control.  One wants to avoid setups that run the audio out through the expression pedal, given that noise pickup on the cable can ruin the audio.  It is better to use the control voltage from the expression pedal to “affect the effect” in the box into which it is plugged.  Usually, the effect control is administered circuit-wise with a variable resistance.  This variable resistance may be implemented with a junction field effect transistor (JFET), a bipolar junction transistor (BJT), or a light-dependant resistor (LDR).  Using an LDR, the expression pedal control is fed to a white light LED (light-emitting diode) which is placed in front of the LDR.  It is best to encase the two devices in heat shrink tubing to eliminate the effect of ambient light on the LDR.  In darkness, the LDR resistance is about 100kohms, and as the light from the LED increases with the expression pedal output voltage, the resistance of the LDR drops linearly to about 100ohms.  A simple block diagram of the control chain is shown below:

The Input Circuit for the expression pedal performs three functions:

1. Filter out noise from a scratchy pot; a low-pass filter and the optical isolation of the LED/LDR combination are very effective in reducing noise from the pedal and attached cable.

2. Set the range of the pedal movement that you want for effect control

3. Set the (foot) position of the above range for that control

The figure below shows how the Input Circuit uses bias (Center Position) and gain (Range) settings to transform the full range of the expression pedal for optimum control of an audio effect. 

The range and position are adjusted to give optimum foot-feel for effect control.  The linearity of the four functions in the control path can be summarized as:

1. the foot position to control voltage of the expression pedal: linear

2. the current to luminous intensity function of the LED: probably not linear

3. the luminous intensity to resistance function of the LDR: may not be linear

4. the resistance to audible effect function: definitely not linear, and very subjective

By reducing the control voltage range, and positioning it for maximum effect, the Input Circuit takes the full pedal position range (dashed straight line) and applies it over the shaded region (solid straight line).  Two trimmer potentiometers, one for bias, the other for gain, respectively set the position and range of the effect modulation over the pedal’s full range of motion.

Bye for now!

Ken Buttle