BACKGROUND Analogue display units and digital display units are known. Analogue display units of the type where a needle moves across a display have an advantage over digital display units in particular applications. These applications are ones where it is useful to have an indication of the direction or rate of change of the parameter being displayed. An example would be a speedometer in a car. The human eye and brain appear to assimilate this kind of information significantly faster and more effectively from an analogue rather than digital display. However in certain conditions analogue display units may not be suitable, for instance when the display unit is subjected to shock loads or is excessively vibrated causing the needle to flicker unintelligibly.

GB 2,301,894A describes a display which uses light emitting diodes and an array of individual reflective lighting elements each associated with one of the diodes. Each diode is positioned under its associated element and on activation of the diode the emitted light reflects off internal reflection surfaces of the individual element to light the whole element as a display.

SUMMARY OF THE INVENTION It is a general object of the present invention to provide improved forms of display units. Another object of the present invention is to provide a display unit with an easily-readable distinctive display. Another object is to provide a light weight and compact display unit. It is a further object of the invention to provide an analogue- type display unit which can function in harsh environments and in particular, it is a further object of the invention to provide an analogue-type display unit which is reliable in environments where excessive vibrations or shock loads may be experienced.

According to the invention there is provided a display unit incorporating, or for use in conjunction with, a scale, the display unit comprising processing means, light emitting means and light directing means such as a refractive lens, wherein when the unit is powered and in use and connected to an input signal source supplying an input signal, the processing means analyses the input signal and activates the light emitting means which produces light which interacts with the light directing means to produce at least one discrete beam, the positioning of which in relation to the scale corresponds to the input signal.

Preferably the light emitting means is a multiplicity of discrete emitters, such as light- emitting diodes (LEDs), which may be positioned around the edge of at least part of a circle. The light directing means may be positioned between the light emitting means and the scale and may be an annular lens. The processing means, preferably in the form of a microprocessor on a printed circuit board, can analyse the frequency or amplitude of the input signal and energise one or more of the emitters.

In a preferred embodiment the whole unit is encapsulated in a clear or tinted resin and the resin forms the lens.

Another embodiment of the invention provides light emitting means with variable intensity.

A scale is directly incorporated into the unit in yet another embodiment. The annotation or legend for a scale, whether directly or indirectly incorporated into the unit, may be provided by a set of programmable display elements. These elements may be controlled by the processing means or by another processing means.

The invention also provides a method of displaying a variable signal representing a parameter said method comprising analysing the signal, energising at least one of a plurality of light emitters according to the signal, and directing light from the one light emitter with the aid of a refractive lens to produce a linear beam which registers with a scale to display the signal.

The invention may be understood more readily, and various other aspects and features of the invention may become apparent, from consideration of the following description.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying dru. rings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing the interactions between parts of a display unit constructed in accordance with the invention; Figure 2 shows a plan view of a display unit constructed in accordance with the invention; Figure 3 is a cross-sectional side view of the display unit of Figure 2; Figure 4 is a circuit diagram of an implementation of the invention; Figure 5 shows a plan view of a placement of the components of Figure 4 on the unit of Figure 2; Figure 6 shows the track artwork corresponding to Figure 5; Figure 7 is a block diagram showing the interactions between parts of a display unit having a programmable display legend; Figure 8 shows a plan view of a display unit constructed in accordance with the invention and having a programmable display legend; Figure 9 is a cross-sectional side view of the display unit of Figure 8; Figure 10 is a circuit diagram of an implementation of a programmable display legend; and Figure 11 is a diagrammatic cross-sectional representation of the refractive lens of the unit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is a block diagram showing the interactions between component groups of a display unit constructed in accordance with the invention. The essential features of the display unit are processing means 2, light emitting means 3 and light directing means 4.

The processing means is connected to a power source 6 and an input signal source 5 which delivers an input signal. The processing means 2 receives the input signal and selects which part of the light emitting means 3 to activate and activates this part. The light from the activated part of the light emitting means 3 interacts with the light directing means 4 to produce a discrete beam. This beam is directed towards a scale 7 and in co-operative alignment therewith to enable a reading to be taken. The reading corresponds with the input signal. If the input signal is fluctuating then the beam will move accordingly as the processing means 2 selects another part of the light emitting means 3 for activation.

Figures 2 and 3 show an exemplary display unit 10 in plan view and cross-sectional side view respectively. It can be seen from these Figures that the display unit 10 is generally disc-shaped.

The plan view of the unit shows a circular peripheral edge 12 which defines a main axis 14. The unit is composed of three concentric regions. These regions are an inner region 16, an annular lens region 18 and an annular display region 20. In the centre of the unit is the inner region 16 which has a circular outer edge 22 centred on the main axis 14. The outer edge 22 of the inner region 16 adjoins an inner edge 24 of the annular lens region 18. The annular lens region 18 extends radially outwards to an outer edge 26. This outer edge 26 of the lens region adjoins an inner edge 28 of the annular display region 20 which extends radially outwards to the circular peripheral edge 12 of the unit 10.

The lens region 18 comprises an annular refractive lens 30, here a continuous lens, which provides the light directing means (4 Figure 1) and a series of 30 LEDs 32 constituting the light emitting means (3 Figure 1). The LEDs 32 are evenly spaced on a part-circumferential arc centred on the main axis 14. The display region 20 defines a circumferential scale 34 which is likewise centred on the main axis 14 and is graduated through the angular extent defined by the arc of the LEDs 32.

Figure 3 shows the cross-sectional side view of the unit. The three regions are formed by a main body 34 which has an upper surface 36 and a lower surface 38. In the inner and display regions 16,20 the upper surface 34 is generally parallel to the lower surface 36. However, the annular lens 30 causes the upper surface 36 to protrude in the lens region 18. A printed circuit board 40 is disposed in the main body 34 between the upper and lower surfaces 36,38 and generally parallel to the lower surface 38. The printed circuit board 40 extends across the regions 16,18,20 and holds components including the LEDs 32 which are disposed beneath the lens 30. In this embodiment the printed circuit board 40 also serves to define markings forming the scale 34. One end of an input cable 42 which supplies the power and input signal (6,5 Figure 1) is connected to the printed circuit board 40.

The whole unit 10 may be encapsulated in a clear or tinted material such as a resin, for example an epoxy resin. Acrylic or rubber, possibly a soft rubber, may also be used.

The material may form the lens.

When any one of the LEDs 32 is activated the light is directed by the lens 30 into a discrete beam which registers on the scale 34. This simulates the visual effect of a conventional analogue meter with the discrete beam being equivalent to the needle of the analogue meter. This may be appreciated with reference to Figure 11 which simply shows a cross-sectional side view of a refractive lens with an LED positioned beneath the lens. The arrows represent the path of the light emitted from the LED. Refraction of the light produces a collimated beam which when viewed from above appears as a line of light lying across the lens above the LED. In the embodiment shown in Figure 2 a radial line of light spanning across the extent of the lens region 18 would be observed above an activated LED. However, it is not essential for the lens to be annular. Any shape of lens is envisaged including linear lenses; triangular, square and polygonal l lenses; and lenses formed from any number of linear and/or curved portions. Alternative embodiments of the display unit incorporating these alternative lens shapes are also envisaged.

Figure 4 shows a circuit diagram of an implementation of the invention. The processing means (2 Figure 1) is a PIC16C54 microprocessor operating on a digital frequency input signal. The light emitting means are in the form of 30 LEDs which are driven directly by the microprocessor. By both sinking and sourcing the current, the microprocessor can multiplex many more LEDs than it has outputs. In use the processor selects and activates the LED which corresponds to the input signal. There is no LED representing zero in this implementation, instead the first LED is flashed on and off at appropriately 1 Hz when there is zero input.

The circuit shown in Figure 4 can be combined with the display unit shown in Figures 2 and 3 to give a functional display unit. Examples of the component placement and corresponding track artwork for such a combination are shown in Figures 5 and 6 respectively. If an input signal is used which rises uniformly from an appropriate minimum to an appropriate maximum then each of the LEDs will be activated in turn and a discrete beam will be observed moving across the display. The intensity of the LEDs 32 in the circuit shown in Figure 4 is varied according to the ambient light level through a light dependent resistor acting to change the drive voltage of the microprocessor and hence the drive vc ! tage of the LEDs. The light dependent resistor works by altering the bias circuit around a variable voltage regulator to change the voltage between the upper limit in full sunlight and the lower limit in darkness.

This means that the LEDs will be visible in high light situations yet will not be blindingly bright in the dark. However, if a fixed intensity is required the light dependent resistor can be disabled.

The software on the microprocessor can be altered to scale both digital and analogue inputs to the required output range, taking into account any offsets required.

An appropriate processor and any number of LEDs may be used to obtain the required resolution and an LED to represent zero may be included. 42 LEDs are used in an alternative embodiment. LEDs of any available colour can be used. The LEDs may be set out in different configurations in alternative embodiments which use different shaped lenses, for example the LEDs may be set out linearly on a display with a linear lens.

A further embodiment of a display unit constructed in accordance with the invention can have the LEDs positioned around a complete 360° circumference. This embodiment could be used to monitor wind direction for example. In such an application there is no difficulty in displaying a parameter value which may progress round and round in the same direction.

Another embodiment of the invention can be made to display more than one value at a time. This could be to monitor more than one value of the same parameter, such as on a barometer, or to monitor different parameters. Since the activation of the LEDs is controlled by a processor, any number of the LEDs can be activated at any one time and the unit can lend itself to a great number of applications.

In yet a further embodiment the markings for the scale are provided by another set of LEDs under another refractive lens operating in a similar manner to that described.

In the embodiment described with reference to Figures 2 and 3, the circuit board 40 serves to define markings forming the scale 34 and in such a construction all the markings including the legend are permanent. Figure 7 is a block diagram showing the interactions of parts of an alternative embodiment which has a programmable legend provided by a set of programmable display elements. The processing means 44, which may be either the same processing means (2, Figure 1) controlling the light emitting means or a separate processing means, is connected to a power source 46 and a legend data source 48. The processing means 44 receives the required legend data and accordingly activates the set of display elements. The required legend data may be input using a PC. Figures 8 and 9, which correspond to Figures 2 and 3, show the set of display elements 50. Each display element 50 is a double 7-segment LED display but other standard displays may be incorporated including LCD, starburst, alphanumeric or matrix-style displays. With the programmable legend any markings may be selected and programmed in, for example to grade the scale from 0 to 6 in steps of 1, from 0 to 30 in steps of 5 or from 0 to 60 in steps of 10. This means that the same unit can be programmed for a large number of applications. Figure 10 shows a circuit diagram of an implementation of a programmable display legend.

A digital display such as the 4 component 7-segment display 52 in Figure 8 can also be provided on the unit. This digital display can be any kind of standard display. Like the display legend the digital display can be programmable. The digital display can be programmed, for example, to identify the parameter being displayed (e. g. by displaying "MPH"for miles per hour). Alternatively the digital display could be programmed or fixed to display accurate readings of the parameter being displayed on the main display of the unit. More than one programmable or non-programmable digital display can be included on the display unit.

A unit with a programmable legend and a programmable digital display can be used for a wide range of applications. For example, a unit with a legend display composed of a set of 16 display elements and with an alphanumeric digital display can be programmed to display miles per hour from 0 to 150 or to display revolutions per minute in thousands from 0 to 7. In the first case the programmable legend would display the numerals 0,10,20,30,40,50,60,70,80,90,100,110,120,130,140,150 and the digital display would display"MPH". In the second case the programmable legend would display 0,0.5,1.0,1.5,2.0,2.5,3.0,3.5,4.0,4.5,5.0,5.5,6.0,6.5,7.0,7. 5 and the digital display"RPM (K)". All of the elements in the legend need not necessarily be used, for example alternative elements or upper-scale elements may be programmed to be unactivated. This also adds extra flexibility of use.

Generally a display unit constructed in accordance with the invention can be used instead of a conventional display wherever an indication of a parameter is required. In particular the display unit can function in harsh environments giving a precise display.

When encapsulated the unit is resistant or impermeable to dust, water, chemicals, powder or the like. The encapsulated unit may also provide intrinsic safety and is compact and light weight. Excellent visibility is attained in a wide range of lighting conditions by varying the intensity of the light emitting means. The display unit can give highly accurate readings with the incorporation of the digital display.

Display units constructed in accordance with the invention have a wide number of applications. Examples are in land vehicles including those for road maintenance, agriculture and horticulture, in boats used for business or leisure, in plants such as civil engineering construction plants and in instrumentation for industrial machinery such as machinery used in continuous process industries.

A specific application of the display unit would be in a sports boat. The display unit could be used for example to monitor RPM of the engine. Although exposed to sea spray and continuous shock loads from the pounding of the vessel, the display unit would be fully functional.

Those skilled in the art will appreciate that various other modifications may be made to the embodiments described herein without departing from the scope of the invention as defined by t : : e claims.