Control Voltage Pedal


The purpose of this project is to provide a control voltage pedal that outputs a continously variable DC voltage from zero to five volts, in order to control the hi-hat function in The Peasant's Monster Drum Synthesizer. This pedal design can also be used to drive any other voltage controlled equipment, with suitable adjustments to the buffer amplifier circuitry as necessary to match voltage swing requirements.

Virtually every component of this pedal came from the salvage of obsolete equipment, except for the foam cushions, solder, and a few resistors and capacitors on the pcb. In all, the project cost about $0.60 and a few hours of labour, not too bad a deal!

In order to achieve maximum durability, an optical system was chosen over using a potentiomenter for this pedal. When the pedal is pushed down, it moves a "gating lever" that in turn allows light to pass between an infrared emitter and phototransistor detector (an "opto pair"). This opto pair is positioned much closer to the lever rotational point than the pedal contact point, creating a much smaller optical movement compared to the pedal movement. A "V" shaped slot cut in the gating lever between the opto pair linearises the output voltage vs pedal position. The pedal uses bearings on the gating lever at both contact points to prolong pedal life.

Here you can see the area underneath the pedal including the bearing that slides along the bottom of the pedal as it pushes on the gating lever.

The "V" slot in the black gating lever can be seen between the opto pair in the center of this picture. The zero and span trimmers and test points are mounted to allow external access for ease of adjustment in case of electrical or mechanical drift.

Here you can see the gating lever and opto pair in the pedal down position.

The schematic is very basic, just a regulated power supply, ir emitter led resistor, and detector transistor buffer/invertor. The opto pair is from salvage, and has part numbers OP233 for the emitter and OPL101-1 for the detector. I have no specifications for them and do not know what other parts are correct substitutes, but most opto pairs should work with slight circuit adjustments, if any are required at all.

As well, smaller voltage regulators could be used, the TO-220 style were already "in stock", so in they went! The circuit draws less than 20 mA, mostly on the negative rail to partially balance the higher positive rail current draw of the Drum Synthesizer.

This pedal design appears to perform quite well so far, more use and testing with the Monster Drum Synth will show it's long term performance and durability.