Prior art devices used to detect the proximity of a person or object, have as their principle of operation, the utilization of a change in capacitance of the sensing component altering the frequency of an oscillator, and the detection of this change in frequency as indicative of the presence of a person or object. Such frequency dependent devices have had limited application particularly in 'situations where compact size is required and where environmental influences and objects, not being the desired subjects for detection, interfere with the frequency detection and generate undesired and spurious results.

This is found to be the case for capacitive sensing devices such as switches or keys that have been used in keyboards and the like where there is a plastic or glass overlay. The presence of the overlay, tinting, moisture, dust, and finger grease have been known to affect the frequency of oscillation resulting in inaccurate detection.

In an effort to try and overcome the aforementioned disadvantages, the present invention provides a specially adapted capacitive sensor and the detection of a change in a differentiated signal caused by the proximity of a person,

(or other capacitive object) , and particularly, the accurate detection of a persons finger through glass or plastic on a capacitive sensor according to the invention.

In the case of a keyboard, where a number of sensors are required, the present invention further provides a method of scanning the sensors for the differentiated signal and any change thereto, which minimises interference from the scanning circuitry.

It has been found that the detection of a differentiated signal and any changes thereto rather than detection of a frequency change, provided advantages in that

such previous environmental disturbances from cover plates, dust moisture etc., have a minimal or no effect upon the differentiated signal.

The specially adapted capacitive sensor comprises, a planar capaci ^' tor having an outside area and a central plate. The planar capacitor is preferably etched directly on a printed circuit board, and this contributes to the space and cost savings of the device.

It has been found that the size of the central plate affects the required dielectric distance between the outside area and central plate for the proper functioning of -the capacitor as part of the proximity device. For a one finger size central plate a ratio of 1/3 between the central plate size and the dielectric distance is preferable. For a two finger size central plate, a ratio of 2/3 is preferable. The layout of the planar capacitor can also be determined alternatively, and preferably, by the ratio between the outer perimeter of the dielectric between the outer area and central plate, and the perimeter of the central plate. A preferable ratio of about 2.1 has been determined for proper functioning.

Preferably connected to the back of the central plate of the planar capacitor, are a transistor, preferably bipolar, and a bias resistor for the transistor in a configuration such that the circuit behaves as a electronic differentiator of the drive signal. The connection to the back of the central plate allows a users finger to approach the sensor without obstruction, and also the configuration is preferred as it minimises electrical interference whilst being as close as possible to the plate for a usable amount of signal. The gain is set for maximum output without introducing finger noise. (It has been found that finger noise manifests itself as a 50 hertz oscillation) .

The drive signal, generated externally, is applied preferably to the outside area of a planar capacitor, and picked by up the central plate. The drive signal needs to

be a pulse with a sharp leading edge and preferably is a square wave pulse. It has been found that a CMOS driver circuit provides a suitable quality pulse.

As the circuit is behaving as an electronic differentiator, the square wave input results in a corresponding output of a sharp spike, at the collector of the transistor.

The detection of a change in the amplitude of the spike is the means by which the presence of a user is determined.

The presence of a users finger through glass or plastic over a planar capacitor interferes with the pickup of the drive pulse by the central plate. It is considered that the finger acts as a third plate for the capacitor, absorbing some of the drive pulse and thereby reducing the amplitude of the spikes coming from the transistor output. A finger on glass or plastic should preferably have a capacitance comparable to the capacitance of the planar capacitor and it has been found that where the value of capacitance is about the same a change of approximately 25% in the amplitude of the spike is observed.

The detection of the change in the differentiated signal (spike signal) is done by comparison of the differentiated signal, preferably after amplification, with a reference signal in ^' a comparator circuit.

The comparator circuit is part of a spike processing circuit which comprises, in general an operational amplifier/filter circuit connected to a microprocessor. The microprocessor generates the square wave drive pulse and controls the delivery of such pulses to the planar capacitor sensor.

The comparator reference signal is initially obtained from the first square wave pulse differentiation when there is no finger present on the sensor. The reference is constantly being monitored and re-established by the microprocessor. Logic in the

microprocessor reads and checks the differentiated peak level to decide that it is within the expected range.

The spike processing circuitry serves the further purposes of amplifing the spikes and filtering out background noise.

In a keyboard configuration, comprising a number of capacitative proximity sensors, a multiplexer controlled by the microprocessor is provided to switch to each sensor in turn and thereby scan the whole keyboard matrix.

As the d.c. level of each transistor is slightly different (due to manufacturing characteristics and resistor biasing) , when each transistor level is switched through the multiplexer, a series of edges are produced which generate spikes that can swamp the spikes that need to be detected.

To overcome this, a window is provided by the spike processing circuitry under microprocessor control.

The window is a logical signal such as from a buffer or latch chip, driven by the micropressor, which prevents the receipt of signal from a sensor until the multiplexer has switched to that sensor. In this way, the undersired spikes associated with switching are not processed.

In a preferred mode of operation, the drive pulse is not activated until after switching has occured and the window is opened. The whole process has been found to take a period of approximately 50 micro seconds per key which is amply sufficient for detection.

The microprocessor performs the further function of key character or character string output and key debouncing which is the filtering out of unwanted threshold oscillations associated with the approach of a finger to a sensor key.

From the above, it can be seen that a keyboard (planar capacitor sensors, differentiators, and multiplexer) and a microprocessor and support circuitry (spike processing

circuitry comprising amplifiers/filters) are necessary for the operation of the proximity device as a keyboard in applications such as a consumer access keyboard.

Conveniently the keyboard can be a separate part of the remaining device, being joined by a cable to the microprocessor circuitry and thereby increase the possible applications of the device.

One preferred embodiment of the invention will be further described with reference to the figures as follows: Figure 1 layout sketch of a printed planar capacitor (shaded areas are copper on a printed circuit board) .

Figure 2 side view of a differentiator sensor comprising a planar capacitor, bipolar transistor and resistor.

Figure 3a single sensor circuit (the transistor and resistor of the differentator of figure 2 are shown externally for clarity) .

Figure 3b equivalent circuit for the circuit of figure 3a.

Figure 4 keyboard sensor circuit (4 sensors), showing connection to multiplexer circuitry. Figure 5 graphical representation of the square wave pulse in relation to the time sequence of the window and clock, (all produced by microprocessor)

Figure 6 spike processing circuitry. Referring to figures 1, 2, 3a and 3b, a planar capacitor 1 etched on a printed circuit board, has a bipolar transistor 2 and a bias resistor 3 connected from behind to the central plate la.

A square wave pulse received at the outside area lb is picked up by the central plate and differentiated by the circuit to produce a spike signal output at the collector of the transistor 4. Referring to figure 4, an analog multiplexer 5, is shown connected to a keyboard array 6 consisting of 4

- - - sensors. The multiplexer is controlled by a microprocessor (not shown) to switch to each key and thereby scan the keyboard continuously. A counter 7 driver by a clock pulse from a microprocessor (not shown) is used to regulate the scan. The microprocessor is also used for key character output and key debouncing as described earlier.

Referring next to figures 4, 5 and 6, a square wave drive signal is transmitted to a planar capacitor sensor from the microprocessor, only after the multiplexer has switched to that particular sensor. The spike processing circuit shown in figure 6 performs its function under software. control from the microprocessor.

The microprocessor software opens a window 8, which is a buffer or latch chip, which allows signal to pass into the processing circuitry.

-The graphical representation of this process is shown in figure 5, and the leading edge of the drive pulse (emphasized for clarity in figure 5) provides the detected spike signal. The spike signal is further amplified by the amplifier 9 in the top branch of the figure 6 circuit as shown, and is compared with a reference signal from the lower branch of the circuit at the junction 10 where an operational amplifier acts as the comparator. The lower branch of the circuit figure 6 develops the reference signal from the microprocessor which utilizes the signal on switching on of the device as previously described.