Not much is known of Control Devices. Control Devices, Inc. was founded on the 16th of October 1969 and was a Massachusetts company, like various other keyboard manufacturers of the day. Three people are named on their sole keyboard patent: Donald C Gove, Barry W Mullins and Edmund G Rousseau. Donald Gove was responsible for the IKOR self-encoding capacitive system, filing patents for it in 1967. The Articles of Organization for Control Devices, Inc. names Donald Gove as one of the three directors as well as the treasurer; it would seem that he left IKOR to co-found Control Devices.
The business is listed as having a date of involuntary dissolution of the 7th of July 1980. The keyboard patent was assigned to Plessey Overseas Limited in April 1982; the circumstances surrounding this acquisition are not known.
CDK was Control Devices’ capacitive keyboard implementation. Described in US patent 3660838 “Multiple point switching apparatus” (filed in April 1970), the design as patented uses a capacitor-controlled transistor within each key, itself connected to a row in a diode matrix.
The advertised advantages of this design were as follows:
- Low cost: fully encoded keyboards for less than $100
- Low current draw: less than 200 mA at 5 V DC
- Low profile: as little as 0.75 inches from the bottom of the keycap to the bottom of the circuit board
- Low downtime: CDK keyboards can be made impervious to liquids spilled directly on them
The switch itself is somewhat involved. In addition to the self-contained plunger assembly, each key station requires a transistor, two resistors and a capacitor. The sense lines—the diode matrix columns—are held high by a fixed capacitor in each key position, which is charged from a separate shared feed. Pressing a key causes the base of the transistor to receive an AC signal delivered from the variable capacitor formed from the key’s capacitive assembly. With each AC cycle (suggested to be around 100 kHz) the transistor briefly activates and drains some of the charge from the capacitor, faster than it can recharge (as the charge line is fed through one of the two resistors). Once the capacitor is drained, there is a current path around it and through the transistor to ground, pulling that sense line low. The core portion of this circuit, specific to each individual key, is shown below:
Each column of the diode matrix is fed into a sense amplifier, and these in turn are synchronised by the strobe circuitry to ensure that no output is signalled until the diode matrix columns have stabilised. Additional circuitry is used to suppress output when two or more keys are held simultaneously. Non-encoded keys still have the same transistor–capacitor–resistor arrangement, but they are fed directly into a simplified sense amplifier that is not involved with the strobe or lockout circuitry. Although it is not specifically stated as such, some of the circuitry (such as in the sense amplifiers) may serve the purpose of converting the AC signal from the oscillator to a DC signal that forms a stable output: the per-switch transistor will be cycling at the same 100 kHz rate as the AC feed through the variable capacitor.
The patent does not detail how overtravel is managed. Most capactive keyboards use an overtravel pad made of foam, that allows the movable plate (a foil layer on the bottom of the pad) to directly reach the PCB. Control Devices instead use a thin metal sheet attached to what is seemingly a rigid plunger. Possibly the switch is able to actuate at a capacitance level lower than that of full travel. Claims that these keyboards are “foam and foil” suggests that possibly the design changed, or that the object in the advertisment photo above the conductive plate is formed of a compressible material.
The only known model is the CDK-3, advertised in Electronics magazine in January 1971. This is an ASR-33 format keyboard, described as “low weight” at 1.5 pounds (equivalent to 0.68 kg) and thus considered suitable for portable equipment.
Early DEC LK01 keyboards are reported to be Control Devices keyboards, with the description indicating CDK.
Foam pad capacitive
Control Devices are claimed to have introduced foam pad capacitive keyboards by 1970. No evidence is presented to back up these claims. It is possible that such keyboards were simply a revision of CDK using an overtravel arrangement similar to more conventional foam pad designs, but otherwise unchanged in their implementation.
All documentation was scanned by Bitsavers unless otherwise noted.