It’s been a long time I wished to document the building of the control voltage footpedals I use together with the gnusb and the videobass.

The basic idea was to build an expression pedal that outputs a voltage according to it’s angular position. I wanted it  to stay in position when not used, so there has to be a mechanism to adjust the friction. And I was reluctant to use potentiometers as previous experiences with guitar volume pedals showed that potentiometers quickly wear out and get cranky, especially when they are mounted on the axis of a foot pedal.

And it should be quick and easy to build.

The idea I came up with was to rotate a reflection sensor around a hollow axis that has a hole in it. At one extreme position the sensor will only see the metal of the axis, and then gradually see less reflexions as it looks more and more into the hole, until at the other extreme there’s almost no light coming back. It turned out to work quite well – the response is not totally linear, but this doesn’t really bother me as long it is always similar. The sensor needs a supply of 5V and it’s output varies between 1.5 and 3.5 V in this configuration, so additional scaling and calibration is done on the microprocessor.

It’s certainly not perfect, but the easiest way I found to build a reliable foot pedal with a good feel – connected via the gnusb to the USB port of my computer.

Please read through the whole how-to before starting and leave me a comment with your experiences and suggestions. So here we go:

Material needed

• 2 pieces of plywood, 110mm x 200mm x 6mm
• 2 pieces of plywood, 40mm x 40mm x 6mm
• 1 piece of plywood, 40mm x 40mm x 3mm
• aluminum tube 60mm dia: 12mm
• 2x M5 threads, 28mm
• 8 nuts M5
• 4 washers

• 4 screws M3 x 20mm
• 4 M3 nuts you can screw into wood (don’t know how you call them…)

• SFH 9201 Reflective Sensor
• 1 resistor somewhere around 300-500 Ohms
• 1 resistor somewhere in the 10 kilos
• small piece of PCB: pedalsensor-eagle

Step 1

Step 2

While the glue is setting, let’s prepare the axis.

We’ll want two 5mm holes exactly 49mm apart.

And a 8mm hole in the center, but 45 degrees off.

Spray paint the inside of the tube in black (matte finish), but leave the outside metallic.

Step 3

While the paint is drying, put together the 9mm and the 6mm 40×40 pieces and drill a 12mm hole all the way through.

We want the final assembly to be a bit tighter, so I put a piece of paper folded 4 times in between as a spacer.

Step 4

Drill a 12mm hole through the center of the face of the 9mm piece of plywood (the thicker one).

Step 5

Glue the thinner piece to one of the large ones.

The axis should be at 70mm from one of the small sides.

Step 6

Insert threaded rods into axis and tighten the nuts firmly.

For the next step, the glue needs probably some more time to set (~30 min with standard white glue) – so it’s time for a coffee….

See you after the break.

Step 7

Next, put the axis assembly together to keep everything perfectly aligned.

Then drill four holes through the whole thing.

Step 8

Widen the holes in the loose part to 5mm and insert these screw-in-nut-thingies

Step 9

Drill 5mm holes through the bottom plate, again at a distance of 70mm from one side.

(I’m not able to work precisely enough, so I assembled the top part of the pedal to see where my threaded rods land exactly)

You might want to widen the holes a bit on the underside to make place for the washers and nuts.

Step 10

Mount the axis to the bottom plate.

Big center hole goes to the far side of the pedal.

Mount the top part to see if everything fits well

Friction can be controlled by adjusting the four screws

Now you’ll probably want to take it all apart again to do some sanding and paint finish – before soldering and glueing on the reflexion sensor assembly:

Step 11

I use hot glue to attach the sensor board to the axis plate.

The sensor should look right trough the middle of the big hole in the center.

It’s a good idea to spray paint the whole assembly in black so that not light can get through (we found that sometimes on stage a powerful light source can influence the output of the foot pedal)