Guitar effects are comprised of various tonal characteristics. In most cases, you adjust these characteristics by changing the parameters of the effect. For example, you increase the intensity of a chorus effect by adjusting the "depth" dial. An expression pedal lets you increase and decrease the intensity of an effect with your foot. The treadle of an expression pedal is attached to a potentiometer. The position of the potentiometer dictates the intensity of the effect. A common application of the expression pedal is for wah and volume effects. Building your own expression pedal gives you greater scope for customisation.
Source your parts. There are two ways of doing this. You can source the pedal enclosure, circuit board, potentiometer, gear strip and board-mount components separately or as part of a single kit. The advantage of the latter approach is that the parts come made to measure and are mutually compatible.
Colour-code the schematic. The schematic comes with your parts. It illustrates the layout of the circuit and the relationship between each board-mounted component. Use a set of coloured marker pens to code the parts as they appear on the schematic with the corresponding components and the turrets on the blank circuit board.
Populate the circuit board. Push each component into its blank turret on the circuit board. Start with the capacitors, then diodes, resistors, transistors, transformer and potentiometer. Use the colour-coded schematic as a reference. Push each component into the turret with sufficient force that the connector pins poke through to the other side.
Flip the circuit board over and gently push each connector pin flat against the metal conductor strip that runs along the base of the board. Put a small amount of solder onto the tip of a soldering iron and press the tip against the connector pin and the metal strip of each component in turn. The solder joint fuses the connector pin to the conductor strip.
Screw the fully populated and connected circuit into pre-drilled mounting holes in the base of the bottom half of the pedal enclosure using a Phillips screwdriver and four 40-gauge screws.
Connect the serrated end of the gear strip to the potentiometer. Attach the teeth of the gear strip to the serrated side of the potentiometer. The gear strip adjusts the position of the potentiometer, which governs the intensity of the effect.
Load the chassis-mounted parts. Mount a 1/4-inch jack in each of the two pre-cut holes on the top half of the pedal enclosure. Glue the battery snap to the empty space on the chassis base above the circuit board. Solder a power wire between the output terminal of the input jack and the eyelet on the board in front of the nearest resistor. Solder a piece of power wire between the output jack and the eyelet directly behind the nearest resistor.
Put a thin layer of glue on the non-serrated end of the gear strip.
Mount a 9-volt battery in the battery snap. Fit the top half of the chassis enclosure. The bottom of the treadle will fit over the non-serrated end of the gear strip. Leave the glue to bond over night before using.
Clean the tip of the soldering iron with a sponge before applying solder.
Only solder in a well-ventilated area.