Capacitive keyswitch sensor and method
First Claim
1. The method of repetitively determining whether each of a plurality of capacitive keyswitches is actuated or deactuated during each of a plurality of keyswitch-scanning cycles, the keyswitches being of the type which change from a first capacitance value to a second capacitance value when the keyswitch is actuated, which comprises:
- prior to the start of each keyswitch-scanning cycle, sampling the capacitance of a reference capacitor having a capacitance value approximately midway between the first and second values of the keyswitches, with a capacitance testing circuit, and generating a first data signal having a parameter which is a function of the sampled capacitance of the reference capacitor;
next, sequentially sampling the capacitance of each successive keyswitch with the same testing circuit and under the same testing conditions, and generating a second data signal having a corresponding parameter which indicates the capacitance of each individual keyswitch in the sequence relative to the reference capacitor; and
thencomparing the second data signal with the first data signal, and generating an output control signal whenever the comparison of the second data signal with the first data signal indicates that one of the keyswitches exhibits the second capacitance value thus indicating that the corresponding keyswitch has been actuated.
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Abstract
Each of the inputs of an analog multiplexer are connected to a plurality of capacitive keyswitches and a reference capacitor. At the first multiplexer scan position, the reference capacitor is charged through a charging resistor and when a predetermined voltage is reached a threshold amplifier is caused to respond. The time period required to charge the reference capacitor is stored and compared with the time period required to charge each of the keyswitches. A control signal is generated in response to this comparison providing a signal representative of the capacitive value of the keyswitch in relation to the reference capacitor and thus the operative condition of the keyswitch.
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Citations
18 Claims
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1. The method of repetitively determining whether each of a plurality of capacitive keyswitches is actuated or deactuated during each of a plurality of keyswitch-scanning cycles, the keyswitches being of the type which change from a first capacitance value to a second capacitance value when the keyswitch is actuated, which comprises:
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prior to the start of each keyswitch-scanning cycle, sampling the capacitance of a reference capacitor having a capacitance value approximately midway between the first and second values of the keyswitches, with a capacitance testing circuit, and generating a first data signal having a parameter which is a function of the sampled capacitance of the reference capacitor; next, sequentially sampling the capacitance of each successive keyswitch with the same testing circuit and under the same testing conditions, and generating a second data signal having a corresponding parameter which indicates the capacitance of each individual keyswitch in the sequence relative to the reference capacitor; and
thencomparing the second data signal with the first data signal, and generating an output control signal whenever the comparison of the second data signal with the first data signal indicates that one of the keyswitches exhibits the second capacitance value thus indicating that the corresponding keyswitch has been actuated. - View Dependent Claims (2, 3, 4)
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5. A method for determining the operative condition of a capacitive keyswitch forming a component part of a keyboard said keyswitch being operable between a first condition and a second condition, comprising the steps of:
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charging a reference capacitor to a predetermined level; measuring the time interval required to charge the reference capacitor to the predetermined level, charging a keyswitch to said predetermined level; measuring the time interval required to charge the keyswitch to said predetermined level; and comparing the reference capacitor charge time interval with the keyswitch charge time interval so as to determine the relative capacitive value of the keyswitch with respect to the capacitive value of the reference capacitor. - View Dependent Claims (6, 7)
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8. An apparatus for sensing the operative condition of a plurality of capacitive keyswitches forming a keyboard, each keyswitch being operable between a first condition and a second condition comprising:
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a reference capacitor; means for measuring the capacitive value of said reference capacitor and providing a first informational signal related to the value of said reference capacitor and for measuring the capacitive value of a selected keyswitch and providing a second information signal related to the capacitive value of said selected keyswitch; means for storing said first informational signal related to the value of said reference capacitor; means for comparing said stored first informational signal with said second informational signal related to the capacitive value of said selected keyswitch and providing a control signal in response thereto, said control signal serving as an indication of the operative condition of said selected keyswitch; and said measuring means including means for charging said reference capacitor and said selected keyswitch, and voltage sensitive means selectively responsive to the charge upon said reference capacitor and said keyswitch, said voltage sensitive means serving to determine said first and second informational signals in response thereto. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. Apparatus for repetitively determining whether each of a plurality of capacitive keyswitches is actuated or deactuated during each of a plurality of keyswitch-scanning cycles, the keyswitches being of the type which change from a first capacitance value to a second capacitance value when the keyswitch is actuated, which comprises:
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a reference capacitor having a capacitance value approximately midway between the first and second values of the keyswitches; means for sampling the capacitance of the reference capacitor prior to the start of each keyswitch-scanning cycle during a first testing interval, and for sequentially sampling the capacitance of each successive keyswitch under the same testing conditions during subsequent testing intervals; means for generating a first data signal having a parameter which is a function of the sampled capacitance of the reference capacitor; means for generating a second data signal having a corresponding parameter which indicates the capacitance of each individual keyswitch in the sequence relative to the reference capacitor; and means for comparing the second data signal with the first data signal, and for generating an output control signal whenever the comparison of the second data signal with the first data signal indicates that one of the keyswitches exhibits the second capacitance value, thus indicating that the corresponding keyswitch has been actuated.
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Specification