The present invention relates to the device which might otherwise have a plurality of pushbuttons such as handheld calculators, telephones, PDAs, music players, remote controls, keyboards and the like. More particularly, although not exclusively, the invention relates to an improvement over the pushbuttons or keys of such devices.
The keys of such known devices are usually mechanical pushbuttons comprising an electrical contact beneath an actuator. A number of electrical contacts are often provided on a PCB. Such keys are prone to wear and resultant unreliability. They are also prone to the ingress of dust and liquid.
It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages.
There is disclosed herein a device comprising a plurality of input keys, wherein each key comprises an electrically conductive exposed surface to be touched by a user, the device comprising a discreet oscillator in circuit each key and providing an AC voltage, and wherein capacitance of said key changes when touched by a user to change said AC voltage, and switching means responsive to said AC voltage change.
Preferably, the capacitance of said key reduces when touched by a user.
Preferably, the oscillator generates a square wave.
Preferably, the device further comprises a transistor having a gate receiving the square wave and a drain whose voltage is controlled by the square wave.
Preferably, the device further comprises a peak detector circuit and a Schmitt trigger circuit.
A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings, wherein:
In
When a user's finger is not in contact with the keypad 11, the oscillator generates a 120 kHz (typically) square wave, which is passed to the gate of Q3. The square wave controls a voltage of the drain of Q3. The square wave will swing at the drain of Q3. The signal is then passed through the peak detector circuit. The voltage at this detector is sufficiently high to drive the Schmitt trigger IC to be low at its output. Thus the output formed by Q4 and Q5 is off.
When the user's finger is in contact with the pad, the capacitance of the pad is increased. This reduces the AC voltage generated by the oscillator to the gate of Q3, resulting in a decrease of AC voltage at the drain. The peak detector circuit outputs a voltage below the threshold of the Schmitt trigger IC U1-B which outputs at high. The output formed by Q4 and Q5 is then on.
The circuit of
It should be appreciated that modifications and alterations obvious to those skilled in the art are not be considered as beyond the scope of the present invention. For example, circuit details may be changed without departing from the scope of the invention.