GB2201063A - Telephone switch hooks - Google Patents

Abstract

In a telephone in which, in use, the switchhook provides a shunt path across the telephone's calling device, the switchhook contacts (20, 21) comprise a conductive elastomer. The conductive elastomer can be carbon loaded, and provide a contact resistance of more than 10 ohms. Substantially all of the shunt path resistance may be provided by the switchhook contacts (20, 21). The switchhook's static contacts may be linked onto the telephone's circuit board (23). In a keypad telephone a single elastomeric bubble mat (22) may be used for the switchhook and the keypad. <IMAGE>

Description

TELEPHONE APPARATUS This invention relates to a telephone apparatus, for example a telephone subscriber's set, and more particularly to the arrangement of the switchhook within such telephone apparatus.

In a conventional telephone, when the handset is lifted, the switchhook operates to connect the handset (hereinafter telephone circuit") and signalling circuitry to the line, simultaneously completing a shunt path across the calling device (bell or tone-caller) of the telephone.

The switchhook is conventionally a double pole switch, one pole connecting the previously unconnected side of the telephone and signalling circuits to line, the other pole switching in the "anti-tinkle circuit" which comprises Zener diodes and a resistor in a shunt arrangement across the calling device. Because the switch contacts, which complete the shunt path, are exposed to arduous line conditions such as lightning strikes and short durations of ringing current, substantial metallic contacts are used. Similarly, other components used in the shunt path are selected with a view to their ability to handle surge currents: the resistor is traditionally a half watt solid carbon type, and the two Zener diodes are usually 1.3 watt devices.Because substantial surge currents can be expected, it has always been accepted that the switchhook should use low resistance metallic contacts, and such contacts have been used in the high reliability microswitches used as switchhooks. Unfortunately, the microswitches used to ensure high reliability are relatively expensive, and even when bought in very large quantities typically cost 30 to 40 pence each.

As with any other mass produced item, telephones are the subject of continuing efforts to reduce manufacturing costs by reducing assembly time, and materials and component costs. Major savings have been achieved by replacing the complex dialling mechanism with keypad operated electronic dialling, by replacing the electromechanical bell with a tone caller, and by switching to surface mounting of components. However, since telephones are produced in large quantities, even seemingly minor cost savings per telephone can result in appreciable savings over the course of a production run.

However, as yet, despite the money to be saved, no-one has come up with a satisfactory cheaper alternative to the conventional switchhook microswitch.

The present invention seeks to provide a cost effective and reliable alternative to a conventional microswitch for use as a switchhook.

The present invention provides a telephone having a calling device and a switchhook which, in use, completes a shunt path across the calling device wherein the switchhook contacts which complete the shunt path are made of a conductive elastomer.

Advantageously, the switchhook contacts provide a resistance in the shunt path of at least 10 ohms.

Preferably, the switchhook contacts provide substantially all of the shunt path resistance.

In a preferred embodiment, the telephone further comprises a keypad having elastomeric contacts, the moving contacts of the keypad and the switchhook being provided by a single bubble mat.

Conveniently, the switchhook comprises static contacts provided on a principal circuit board of the telephone.

We have made the surprising discovery that, despite the previously accepted belief that low contact resistances are needed for switchhooks, it is possible to use elastomeric contacts in place of the conventional metallic contacts in the shunt path and still obtain the necessary good reliability.

A telephone constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically the principal elements of a conventional telephone circuit which has been modified by replacing the conventional hookswitch with an elastomeric hookswitch; Figure 2a is a side elevation of the hookswitch of the telephone circuit of Fig. 1 in the handset "up" position; Figure 2b is a side elevation similar to that of Fig.2a, but showing the hookswitch in the handset "down" position; and Figure 2c is a plan view of the hookswitch of Figs 2a and 2b.

Referring to the drawings Figure 1 shows what is essentially a typical telephone circuit. The two pairs of switchhook contacts, the shunt path contacts 10 and the line switch contacts 11, are each closed when the telephone's handset (not shown) is lifted. Traditionally, these two contact pairs would be provided by a 2-pole microswitch with metallic contacts, the line switch contact 11 normally being located at point LS just after the take off for the resistor for the dialler memory bleed supply. The shunt path contacts 10 are connected in series with a 100 ohm resistor 12 and a pair of back-to-back Zener diodes 13. When the handset is lifted, the shunt path contacts 10 connect the resistor 12 and the Zener diodes 13 in a shunt arrangement across the tone caller (or bell) 14 to prevent bell-tinkle during signalling.

Essentially, the same arrangement is used in a telephone according to the present invention, apart from the switchhook contacts, which are made of a conductive elastomer. The line switch contact 11 is unconventially located, and switches the line via a semiconductor switch 30 which may be a p-channel FET, a Hi-gain pnp transistor or a pnp Darlington pair. Since the elastomer of the shunt path contacts will have appreciable resistance, typically between 10 and 20 ohms, the resistance of the anti-tinkle resistor 12 will need to be reduced correspondingly to keep the sum resistance (contacts 10 plus resistor 12) to no more than about 100 ohms to avoid bell-tinkle.Provided that the tolerance on the resistivity of the elastomer from which the contacts 10 are made is sufficiently tight (better than +20% and preferably better than +10%), it is possible to do away completely with the resistor 12, the whole shunt path resistance (which will typically be about 100 ohms) being provided by the contacts 10.

Elastomeric contacts have, for many years, been widely used in keypads and similar low power CMOS type applications, and have proved to be very reliable. As far as we are aware, however, such contacts have not been used in applications where they may have to handle high transient or continuous currents - since it is to be expected that the high resistance of typical elastomeric contacts would lead to excessive power dissipation with consequent failure of the contacts.

Most surprisingly, we have found that elastomeric contacts are fully acceptable for switching the shunt path, despite the high transient currents which are encountered in such an application.

Initial endurance and reliability tests were carried out on elastomeric contacts having a resistance in the range 12-20 ohms. Surprisingly, the contacts appear to be unaffected both by the lightning test (as specified in standard S1626) and by the application of ringing current (at lOOV rms) for two hours. When a telephone is installed, ringing current is only usually applied to the shunt path for the few milliseconds after the handset is lifted, and until the exchange removes the ringing signal.

The elastomeric hookswitch contacts are preferably arranged to operate on the surface, for instance the printed circuit board (pcb), on which the telephone's components are mounted. One possible arrangement is shown in Figures 2a-2c. Here, the two poles 10 and 11 of the hookswitch are provided by two 'buttons' 20 and 21 on an elastomeric bubble mat 22 supported by the telephone's pcb 23. An arm 24 carrying an actuator 25 is arranged to depress the buttons 20 and 21 when the handset is lifted, the arm being pivotally-mounted on a pivot 24a within the casing 26 of the telephone. The actuator arm 24 is biassed, by means of a spring 27, towards the handset "up" position. The conductive elastomer pads within the buttons 20 and 21 may be loaded with carbon, and may co-operate with contracts, which may be tin-lead plated or gold plated, but are preferably carbon ink, on the pcb 23. Preferably, the actuator and spring assembly 24, 25, 26 is removable with the case, to ease assembly/ disassemby. Conveniently, where the case design permits, a single elastomeric bubble mat is used to provide the moving contacts for the telphone's keypad and for the hookswitch.

Claims (6)

1. A telephone having a calling device and a switchhook which, in use, completes a shunt path across the calling device, wherein the switchhook contacts which complete the shunt path are made of a conductive elastomer.

2. A telephone as claimed in claim 1, wherein the switchhook contacts provide a resistance in the shunt path of at least 10 ohms.

3. A telephone as claimed in claim 2, wherein the switchhook contacts provide substantially all of the shunt path resistance.

4. A telephone as claimed in any one of the claims 1 to 3, further comprising a keypad having elastomeric contacts, the moving contacts of the keypad and the switchhook being provided by a single bubble mat.

5. A telephone as claimed in any one of claims 1 to 4, wherein the switchhook comprises static contacts provided on a principal circuit board of the telephone.

6. A telephone substantially as hereinbefore described with reference to, and is illustrated by, the accompanying drawings.