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Variable Threshold Method for Elimination of Key Bounce

IP.com Disclosure Number: IPCOM000074293D
Original Publication Date: 1971-Apr-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Fisher, DE: AUTHOR [+2]

Abstract

One technique for detecting when a particular key, in an array of keys, has been depressed is by AC coupling clock pulses to a detector arrangement when a key is depressed. One of the difficulties encountered in such an arrangement involves bounce of the key. The difficulty with key bounce resides in its tendency to effect a variation in the AC coupling. Thus, when the key shown is initially depressed so that key plate 1 is in close proximity to conductive lines 3 and 5 and the coupling capacitance, as shown, is at a maximum, a sufficient amount of the magnitude of the clock pulse, from clock pulse source 7, is coupled to the input of the detector to trigger it. However, the key thereafter bounces thereby decreasing the coupling capacitance and decreasing the amplitude of the clock pulse coupled to the detector input.

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Variable Threshold Method for Elimination of Key Bounce

One technique for detecting when a particular key, in an array of keys, has been depressed is by AC coupling clock pulses to a detector arrangement when a key is depressed. One of the difficulties encountered in such an arrangement involves bounce of the key. The difficulty with key bounce resides in its tendency to effect a variation in the AC coupling. Thus, when the key shown is initially depressed so that key plate 1 is in close proximity to conductive lines 3 and 5 and the coupling capacitance, as shown, is at a maximum, a sufficient amount of the magnitude of the clock pulse, from clock pulse source 7, is coupled to the input of the detector to trigger it. However, the key thereafter bounces thereby decreasing the coupling capacitance and decreasing the amplitude of the clock pulse coupled to the detector input. Although the threshold of the detector may be sufficiently reduced so that the reduced clock pulse amplitude can be detected, such reduction of threshold is accomplished at the cost of reducing the noise immunity.

The variable threshold arrangement, shown in the detector block, obviates the problem by employing both a triggering device, such as monostable multivibrator 9, and a feedback arrangement. In such an arrangement the initial threshold of the monostable multivibrator can be set sufficiently high so that the detector will not trigger unless the key is close to being fully depressed. When...