GB2179163A - Liquid crystal displays as detectors of static charges & electric fields - Google Patents

Abstract

The liquid crystal display of the type commonly used in digital watches, has been adapted to form a visual sensor of externally collected electric charges. It has been discovered that by applying a charge from a collecting plate to two separated segments within the lower face of the display, the whole register becomes activated. This also increases the sensitivity of the display as a detector and enables static charges and electric fields to be virtually seen when the device is held by an insulated handle 6 with contact made to ones person via a button 7 connected to one of the display terminals. In addition the device may be used to grade the seasoning of timber as well as the dielectric property of various materials. <IMAGE>

Description

SPECIFICATION Liquid crystal displays as detectors of static charges & electric fields This invention relates to visual detectors for finding the presence of static electrical charges and also for exploring the field of a sustained electrical charge.

Liquid crystal displays of the type commonly used in digital watches have an internal construction which differs from that employed within those used for other applications. The details of this particular construction will be described in this description because it has enabled me to modify the operation of these displays from that normally taking place to one more suitable for the detection of static electrical charges.

Using this new mode of operation the total register of the display becomes a sensitive detector than enables the presence of quite small electrical charges to be virtually seen. It is because the activation of the liquid crystal display requires only an extremely small passage of current and therefore a negligable amount of energy from the source that this form of display may be directly used to detect static charges. Thus with the addition of a small charge-collecting plate it is possible to construct a simple instrument for detecting static charges which embodies in its circuit the hand, body and feet of the person holding it. Since this circuit will work equally well in reverse it can become also an indicator of a person carrying an electric charge; (a useful adjunct for workers in the field of semi-conductors).In addition the electric field which surrounds a live mains cable can similarly be detected and identified from static charges by the sustained nature of its presence.

A description of the construction of the 'watch' type liquid crystal display and modified mode of operation will follow with reference to the accompanying drawings.

Figure 1 illustrates the metalic interconnections which lie beneath the top surface of a common form of digital watch liquid crystal display.

Figure 2 illustrates the metalic interconnections which lie above the lower surface of the liquid crystal display.

Referring to Fig. 1 of the drawing the pattern or format of the upper plane of metalic interconnections is illustrated. These are grouped in pairs of segments or parts which make up the display so as to lie across the lower plane of metalic interconnections. They lie under the glass face in contact with the liquid crystal chemical and they continue through the glass edge forming eleven separate contact-pads along the top outside edge.

Referring to Fig. 2 of the drawing, the pattern of metalic interconnections forming the lower plane is illustrated. These form two distinctly separate groups, one mainly in the upper part of the drawing and the other in the lower part. They are in contact with the lower surface of the liquid crystal chemical and each group is connected through the glass edge to a separate contact-pad at either end of the outside top edge.

In normal operation as a digital watch display these two outer contact-pads are fed a waveform of regular pulses from a common source but the phase of one is displaced 90 degrees apart from the other.

The eleven inner contact-pads are individually fed a square wave of the same frequency but phased so as to be either inphase or out of phase with either both or each or neither of the two outer contact pads. Thus a form of multiplexing takes place to effect a positioning of the electric charge between individual segments of the upper and lower planes. The liquid crystal chemical only becomes opaque when there is an electric charge between upper and lower planes, and thus where there are segments which lie directly infront of each other. Thus any or all individual segments may be actuated in accord with the phase relationship previously described where segments lie counterpoised.

Liquid Crystal Displays with internal metallic formats as described above will be refered to as 'Dual segmented' Liquid Crystal Displays.

I have discovered that by simply applying an electric charge between the two outer contact-pads, it is possible to cause the whole digital display to actuate. This indicates that the inner eleven unconnected contact-pads and the whole of the upper plane of metallic interconnections have acted as an uncharged link exactly similar to the connection point of two capacitors joined in series.

This mode of operation, which I shall call 'Series Mode' doubles the input impedance to the display and therefore greatly increases the sensitivity of the display to detect static charges. Also it would be reasonable to assume that because the upper plane metalic segments are at neutral potential relative to the lower plane metalic segments there can be no polarisation at the liquid crystal interface with the upper plane under D.C. conditions.

Employing this series mode of operation I have constructed a hand held device for the visual detection of static charges and electric fields which I shall now described with referrence to the accompanying drawings.

Figure 3 shows modifications mode to a digital watch.

Figure 4 shows in perspective the static charge detector in operation.

Referring to Fig. 3 of the drawing, here is shown the rear of the a digital watch with the back plate of the case removed also the circuit, battery, control buttons, strap and various contact pieces have been discarded, leaving just the plastic housing,"' the digital dis play'2' and the contact strip.(3 This contact strip or digital display connector is compounded from finely spaced individually insulated copper segments packed together to form a flexible continuous strip. When placed upon the contact-pads of the display it automatically forms separated contacts with any contact board placed on top of it.

Here strips of contact board (4) have been place across the display connector at the end contact-pad positions and screwed down into the plastic housing with the normal screws in their former positions. From the centre of the contact boards connecting wires are soldered.

One, a bare wire, is positioned to touch the inner surface of the back plate when replaced.

The other wire is insulated and is led out of the case via a hole which would normally house a button.

Referring to Fig. 4 of the drawings, here is shown the modified digital watch as described above mounted face up on a metal plate (5) so that its back plate makes good electrical contact with the metal plate. The insulated wire enters an insulating plastic handle (6) which is fixed to the surface of the metal plate, and connects with a metal stud (7) passing through the top of the handle forming a contact button.

When the plastic handle is held in the hand with finger on the button whilst the metal plate lies on a surface where a static charge is held then the whole of the liquid crystal display will actuate and remain visible until almost all the electrical charge has discharged.

An electrical field from a mains power cable can likewise be detected; in this case however the visible display will not fade away and this fact allows electric fields to be distinguished from static charges. If a partial insulator such as a piece of wood is placed between the metal plate and the mains cable then a thickness of wood can be found where conduction is just insufficient to sustain the display. This test can be used as a measure of the degree of seasoning of the timber. Similar tests can be devised to compare the dielectric properties of various materials.

Claims (7)

1. A liquid crystal display adapted so that it may be used as a static-charge detector by connecting a flat metal plate to one of its terminals and a wire making electrical contact with a person holding the device, via a metal button in an insulated handle, to one other terminal so that any static-charge collected upon the surface of the metal plate may be applied across the liquid crystal chemical and complete an electrical circuit through the body of the person and their connection with Earth thus causing a visual change in the physical property of the liquid crystal display.

2. A liquid crystal display adapted as claimed in Claim 1 but with the static-charge applied in the reverse wherein the charge collecting plate is Earthed and any charge presented by the person relative to Earth is applied to the display terminals to activate the liquid crystal chemical and thus detect the presence of a static-charge held by that person.

3. A liquid crystal display adapted as claimed in Claims 1 & 2 wherein both of the terminals to which electrical connections are made form part of a 'dual segmented' structure of the lower plane thus enabling operation of the display in 'series mode' and so modifying its operation to cause total registration from those terminals and increased sensitivity to static-charges.

4. A liquid crystal display as claimed in Claims 1, & 3 wherein the device may be used to detect the electric field surrounding a 'live' 'mains' cable and thus trace the position of said cable even when it is hidden within say a wall or channel.

5. A liquid crystal display as claimed in Claims 1, 3 & 4 wherein the device may be used to grade the seasoning of timber by interspacing a piece of timber between a 'mains' cable which is 'live' and the charge collecting plate of the device to an extent where the activation shown by the display becomes barely sustained: the distance between the collecting plate and the cable being an indirect measure of the amount of moisture in the timber.

6. A liquid crystal display as claimed in Claim 5 wherein the device may be used to grade the dielectric properties of various insulation materials wherein material other than timber is interspaced between a 'live mains' cable and the device to an extent where the activation shown by the display becomes barely sustained: the distance between the collecting plate and the cable being an indirect measure of the permittivity of the material.

7. A liquid crystal display adapted substantially as described herein with reference to Figs. 3 & 4 of the accompanying drawing.