5. Pedal Action
Both the Vox and Crybaby pedals use a mechanical system called rack and pinion. The linear motion of the pedal is translated to rotary motion, as shown in the diagram. This in turn rotates an ordinary potentiometer (variable resistor) which is connected to the filter electronics, thereby affording control of the wah effect.
Rack and Pinion
This is a very popular system and is used by many wah pedals, both vintage and contemporary. It does however have a well-known disadvantage - general purpose potentiometers were never designed to work in this repetitive manner, and with constant movement, the internal carbon tracks wear down and become uneven. This can introduce a scratching noise when the pedal is operated. In fact the pot can degrade over time even if the pedal is not used for a while, as the track is liable to oxidization and the oxide is non-conductive. With models based on this principle, the only solution is to replace the pot if and when the problem arises. It should be noted though that some players favour the grit of a slightly worn track as it can lend an edge to the sound. Unfortunately, just like a valve amp or a worn plectrum, it will only be at its subjective prime for a limited time! Some modern replacement pots use a conductive plastic track instead, which is far more resilient.
In an attempt to overcome the wear-and-tear problem, some manufacturers introduced new techniques. Morley developed their electro-optical system, which is currently employed in all varieties of their wah pedals. The light from an LED (light emitting diode) shines onto an LDR (light dependant resistor). These two components are separated by a small gap, and a shutter-strip connected to the pedal is placed between them. The strip is transparent at one end and opaque at the other, with a continuous change from clear to dark along its length. As the pedal is moved, a varying intensity of light falls on the LDR, which in turn controls the wah filter. This unique design with only one moving part ensures smooth control, without the inevitable unwanted noise that is typical of potentiometer designs. Some other manufacturers now also employ the optical approach in one form or another.
An alternative technique, which isnt seen so much these days, is based on the so-called Hall-effect. A small magnet is secured to the underside at the toe of the pedal. As the pedal is moved forward, a Hall-effect device - whose electrical characteristics change smoothly with the increasing proximity of a magnetic field - detects the change in position and controls the wah filter proportionately.
Either approach renders the unit completely maintenance-free, which is certainly worth bearing in mind.