Controls Research C-Scan
C-Scan is Controls Research Corporation’s foam pad capacitive matrix scan keyboard. They are named and described in article Capacitive keys are priced low in Electronics magazine from December 1973.
US patent 3965399 “Pushbutton capacitive transducer” filed in March 1974, depicts a switch that sufficiently matches the description in the magazine. The design as patented employs a foam pad arrangement with a buckling rubber sleeve as the return spring. This is a rare example of a non-Topre capacitive switch with the return force device on the inside. Curiously, the patent was filed by Jr Frank A Walker and John B Shevlin, rather than Controls Research; the applicants are intead listed as “both c/o Controls Research Corporation”. The purpose of the foam pad is two-fold: it provides overtravel, and in conjunction with the flexible conductive film, its compressibility ensures that the entirety of the conductive film presses down evenly on the PCB to ensure consistent capacitance. The magazine article notes:
The flexible pad eliminates a common problem in systems using rigid plates—small dirt specks that prevent the plates from meeting, uneven wear, or differences that cause capacitance variations.
The switch housing has an angled base, for use on a sloping PCB, as with the CRC 02 switch design. The reported lifetime of the switches was 100 million cycles.
At the time of writing, the price for keyboards was $69 for 1,000-piece lots ($413 in October 2021), with the expectation that the price would be reduced to $30–$40 by 1976 ($180–$240 in 2021 prices). Adoption of MOS encoding was a factor in the predicted reduction in price.
No C-Scan keyboards are known to have been discovered. The only known model is SC-6000, described but not depicted in the Electronics magazine article.
Model SC-6000 is described as follows:
The standard SC-6000 keyboard, an ASR-33 type with “n” key rollover, uses standard ASCII coding. It includes 53 keys on ¾-inch centers. Key operating force is 3 ounces nominal, pretravel is 0.090 in., and total travel is 0.182 in.
In metric, this is 85 grams of operating force, 2.3 mm pretravel and 4.6 mm total travel.