Guitar DIY #1: Build an Indestructible Expression Pedal

One area that few guitarists venture into is the world of digital controls.  With today's technology, many companies are now making effects pedals, amplifiers, and even guitars that implement USB or MIDI in some way or another.  One of the more often used controls for guitar gear is the expression pedal input.  The following is just a short list of examples of effects pedals that have expression pedal inputs:

  • Earthquaker Devices - Disaster Transport Sr. (repeats and/or bleed)
  • TC Electronic - Flashback X4 (delay, feedback, or level)
  • Boss PS-6 Harmonist (pitch bend)
  • Eventide "Factor" series

Using an expression pedal with any of these effects pedals opens up a whole new world of possibilities for "on the fly" parameter adjustments.  The problem is that most readily available expression pedals are not very durable and they often break during road use.  So how can we get an expression pedal that's road worthy without spending $100 or more on one of the well built, mass produced ones?  Hint: Crybaby wah pedals are one of the highest selling effects pedals of all time.

 

Well, it doesn't take a genius to figure out my proposed solution.  We're just going to recycle a Crybaby GCB-95 to make a versatile and rugged expression pedal.  You can sometimes find Crybaby pedals on eBay that are being sold as-is/for parts for $20-30.  That puts our DIY expression pedal at the same price point as the cheap commercial versions.  But how many times have you seen a Crybaby with a mechanical failure?  Never?  Me either.  Alternatively, you can find new wah enclosures and parts from several internet sources for around $45-50.

The first thing to do is remove the four screws from the underside of the pedal enclosure and remove the backplate.  Next, we can unscrew the nuts from the input and output jacks, remove the screw from the PCB, remove the wire harness from the pin header, and take the PCB out of the enclosure.  If you want to be able to return your pedal to stock condition it's probably best to de-solder the wires in Photo 2 from their lugs so you can reuse the wiring harness without worrying about the wires being too short.  Otherwise, you can just clip the wires as close to the connector as possible and use them for the expression pedal.

Photo 1 - The guts of a stock GCB-95

Photo 2 - The wires that need de-soldered

Now that everything we don't need is out of the way, it's time to reconfigure the pedal.  For the classic expression pedal circuit the setup is easy.  Refer to the schematic in Photo 3.  The potentiometer R1 is already in the pedal.  Many wah pedals use 100k pots instead of the 10k shown in the schematic, but the value doesn't matter in most cases.  It will still function the same way.  However, the taper of your pot will matter.  Most wah pedals use pots with a custom taper.  If your finished pedal seems to be too sensitive (or not sensitive enough) to your foot movements, I would recommend buying a new pot with a linear taper.  The pot in this particular pedal works quite well because it's pretty close to a linear taper except for the extreme ends of its sweep range.

Photo 3 - The most common expression pedal schematic

Photo 3 - The most common expression pedal schematic

The only additional parts we need are a 1/4" stereo (TRS) jack and a 1k-ohm resistor.  If you're feeling especially frugal (and patient) you can scavenge both of these parts from the original wah PCB.  A TRS jack is critical here because the expression circuit requires 3 connections.  A controlled, constant voltage (typically 5VDC) is applied to the ring of the TRS jack, which is then divided by the 10k pot.  Moving your foot changes this voltage divider and the resulting voltage is output to the tip.  This is how the connected device "reads" your foot position.  Ground is simply needed as a reference.  Also, the resistor R2 is only needed as a current limiter and its value is not critical.  Any similar value you have on hand will work.

First, install the TRS jack in one of the available holes.  Solder one end of the 1k resistor to the "tip" lug of the TRS jack.  Then solder a wire from the other side of the resistor to the wiper (middle) terminal of the potentiometer.  This connection is represented by the red wire in the pictures below.  All that's left is to solder one of the remaining pot lugs to the ring and the other to the sleeve.  In my pedal the black wire is connected to the sleeve and the white wire is connected to the ring.  Voila!  An expression pedal that will most likely never break.

If you find that your expression pedal is behaving in the reverse fashion from what you expected, simply swap the wires going to the ring and sleeve. You can prevent this problem ahead of time by using a multimeter if you have one.  Push the pedal's rocker into the "fully on" position.  Set the multimeter to measure resistance and place one probe on the wiper lug of the pot and the other probe on one of the remaining lugs.  If your meter reads somewhere close to 0 ohms, connect that lug to the ring.  If it reads close to the full value of the pot, connect it to the sleeve.

Bonus!  I'm going to use a second TRS jack as a latching on/off control by utilizing the SPDT footswitch that's still inside the pedal enclosure.  This circuit works in the same way as the expression circuit except that the output voltage on the tip will be read as either "fully on" or "fully off" and nowhere in between.  This will allow me to use the pedal exactly like an actual wah pedal by connecting each TRS jack to one of the two expression pedal inputs on my Voodoo Labs Ground Control Pro.  The GCP can then be programmed to control a virtual wah's position and bypass status via MIDI continuous control messages.  I've effectively turned my Crybaby wah into a controller for any wah that can be controlled by MIDI!  (such as emulations found in Eleven Rack, Axe FX, DAW plugins, etc.)  

Furthermore, by using the GCP, the pedal can be used to control any kind of parameter on any device that is controllable via MIDI.  On my Eleven Rack, I can now use my DIY expression pedal as a wah pedal, volume pedal, or as a controller for many other assignable parameters.

 

Here's a list of a few other ideas if you want to add more functionality:

  1. Use one jack for the common expression circuit and the other one for a Line-6 style expression circuit (not covered in this post).
  2. Build the basic expression pedal and buy a TRS Y-insert cable to optionally use it as a volume pedal.  Simply plug the ring cable into the output of the preceding pedal, the tip cable into the input of the next pedal in the chain, and the TRS plug into the expression circuit.  This trick also works for most other expression pedals on the market.  ;)
  3. Put an LED in the hole that was previously occupied by the DC jack for an on/off indicator.  You can use the reference voltage brought in by the ring as a power source for the LED.  However, use caution if you decide to try this!  The expression circuit from the connected device can only source a limited amount of current.  Use the proper components to ensure your LED circuit draws as little current as possible.

 

Watch the video below to see the DIY expression pedal in action.  Enjoy!

Photo 7 - Latching "expression" circuit

Photo 7 - Latching "expression" circuit

Photo 8 - The complete expression pedal with sweepable and latching outputs

Photo 8 - The complete expression pedal with sweepable and latching outputs

Posted on February 11, 2015 and filed under Guitar DIY.