Capacitive touch panel
Changes of the stray capacitances of a large number of sensing electrodes can be detected simultaneously. Further, even if a stray capacitance changes slightly, an input operation to a sensing electrode can be detected based on elapsed time until the electric potential of the sensing electrode becomes the same as a threshold potential. The stray capacitance of a sensing electrode and a resistor of the sensing electrode form a CR time constant circuit. Charge or discharge of the stray capacitance of the sensing electrode is controlled while a rest time in a predetermined time ratio is set. A stray capacitance that slightly increases in response to an input operation made near a sensing electrode can be detected by detecting the stray capacitance in extended elapsed time until the electric potential of the sensing electrode becomes the same as the threshold potential.
1. A capacitive touch panel for detecting an input operation made at a position of a sensing electrode based on elapsed time that increases as a result of increase of a stray capacitance, the capacitive touch panel comprising:
a sensing electrode arranged on an insulating panel, the sensing electrode increasing in stray capacitance in response to approach of an input unit;
a resistive element forming a CR time constant circuit between the resistive element and a value of the stray capacitance of the sensing electrode;
a charge and discharge switch for raising an electric potential of the sensing electrode from a ground potential to a predetermined charge potential or reducing the electric potential of the sensing electrode from the charge potential to the ground potential by connecting a common terminal on one side of the resistive element to a switching terminal that is either at the charge potential or at the ground potential from a base point in time to charge or discharge the stray capacitance with the time constant of the CR time constant circuit; and
time measuring means for measuring elapsed time for the electric potential of the sensing electrode at the charge potential or ground potential to become the same as a predetermined threshold potential between the charge potential and the ground potential by charging or discharging the stray capacitance from the base point in time,
wherein the switching of the charge and discharge switch is controlled by a PWM modulated signal obtained by pulse-width modulating a rectangular wave pulse signal at a fixed frequency by a predetermined modulation value, and the common terminal on one side of the resistive element is connected with or separated from the switching terminal in response to a binary signal value of the PWM modulated signal.
2. The capacitive touch panel according to claim 1 , wherein the PWM modulated signal controls switching of connection of the common terminal of the charge and discharge switch among a first switching terminal at the charge voltage, a second switching terminal at the ground potential, and an open third switching terminal, and
wherein the connection of the common terminal is switched in response to the binary signal value of the PWM modulated signal between the first and third switching terminals to charge the stray capacitance of the sensing electrode, and/or between the second and third switching terminals to discharge the stray capacitance of the sensing electrode.
3. The capacitive touch panel according to claim 1 , wherein the resistive element and the charge and discharge switch are prepared for each one of a plurality of sensing electrodes arranged on an insulating panel while being insulated from each other, and
wherein the elapsed times of the sensing electrodes measured by the time measuring means are compared, so that an input position is detected based on the position of a sensing electrode that increases in elapsed time.