Touchplates

Don Buchla was a master at designing control interfaces. Every control based module was designed in a way that would challenge the musician to think outside the box. While he was not against standard keyboards, as most people theorize, he felt that the keyboard commanded the performer to think within a certain set of preconceived motions. The touchplate that we all know and love was originally based on an invention of Don’s from his days as a freelance engineer working for NASA; they were installed as fuel sensors in rocket fuel tanks.

Detail of the 113 Touch Controlled Voltage Source touchplate, David Tudor Collection 09/23/2019

Detail of the 113 Touch Controlled Voltage Source touchplate, David Tudor Collection 09/23/2019

Capacitance Touchplates

The standard for Buchla interfaces have always been known as capacitance based touchplates. What if we told you that this has not always been the case?

At first glance, in the world of capacitance based touchplates, the larger plane of touchplate (or common) is considered ground. The positive pad connects the common to it via your body, completing the circuit and causing the pad to trigger whatever circuit is behind the panel.

Resistance Touchplates

Resistance based touchplates are used in the earliest Buchla designs, at least all of the San Francisco Tape Music Center modules. Did you ever wonder about the alternate keyboards on the older modules? The switch to blue keyboards was not mainly for aesthetics, the circuits on these touch controllers are completely different as well. This tidbit of info came to us when we visited Wesleyan University to study the 113 Touch Controlled Voltage Source.

Resistance based touchplates rely on the skin resistance of the performer. The larger “common” plane is actually electrified with positive voltage, the skin completes the circuit when you bridge the gap with a finger. This introduces some issues that may have been the impetus to adjust the design to use the more reliable capacitance method. For starters, skin resistance is very unreliable, as it can drastically change when it comes to the skin thicknesses of different performers. A calloused hand will have a very high resistance, and may not trigger the keying circuit properly. Also, keep those stray bananas and audio jacks from brushing against this touchplate.

I’m sure Underwriters Labs would love to have people touching 15V all day.

113 Touchplate, positive voltage plane. I sing the body electric.

113 Touchplate, positive voltage plane. I sing the body electric.

Detail of the San Francisco Tape Music Center 112 Touch Controlled Voltage Source, red wire is the 15V input, which loops around the board, through a 10K resistor and then to the orange wire, that goes to the touchplate common plane.

Detail of the San Francisco Tape Music Center 112 Touch Controlled Voltage Source, red wire is the 15V input, which loops around the board, through a 10K resistor and then to the orange wire, that goes to the touchplate common plane.