Colorado Instruments Inc. was a US manufacturer. In 1970, they introduced an early capacitive keyboard based on metal domes.
Colorado Instruments was acquired by Mohawk Data Sciences Corporation on the 11th of February 1975.
Metal-dome capacitive keyboard
This is a capacitive keyboard design using metal domes as part of the variable capacitor assembly. This product is covered in US patent 3653038 “Capacitive electric signal device and keyboard using said device” filed in February 1970, and was advertised in the 9th November 1970 issue of Electronics magazine (pages 68–72, “Capacitive keys, simpler circuits add up to reliable keyboard”). The 7th December issue has a detailed article on the keyboards, including a photograph showing disassembly of a single keyswitch module. A truncated triangular metal dome is placed inside each module, pressed down by the plunger via a foam pad. As the dome snaps between its rest and pressed positions, the capacitance of the switch changes, and this generates a 0.5 microsecond pulse. The metal dome provides a click sound upon actuation. The use of snap-action domes means that this design is inherently digital, unlike the potentially analogue sensing in other capacitive keyboard types.
Each sensing element has three sections, which serve as three-axis co-ordinates within the encoding arrangement. Instead of a diode matrix, 13 (or 23, depending on the article) junction field-effect transistors (J-FETs) are used instead, together with the co-ordinate system. The 23 J-FETs are said to replace a matrix of 360 diodes.
US patent 3749859 “Keyboard switch assembly with improved hermetically sealed diaphragm contact structure” depicts the actual switch structure as depicted in Electronics magazine.
- Colorado Instruments, Inc. keyboard advertisement, Electronics, August 2 1963, page 75 (scanned by Bitsavers)
- Colorado Instruments Inc. capacitive keyboard announcement, Electronics, November 9 1970, page 120 (scanned by or for WorldRadioHistory.com)