GB2235082A - Audible alarm arrangement - Google Patents

Abstract

An audible alarm arrangement comprises a transducer, eg of rocking armature form, with actuating means employing first and second wire coil components (9, 10) connected to a driver circuit incorporating a four-arm H-bridge network of switching devices (15, 16, 17, 18). A d.c. voltage supply (Vs) is connected to input terminals of the bridge network. The first wire coil component (9) is connected in series with a first capacitor (11) between output terminals (13, 14) of the bridge network as is also the second wire coil component (10) in series with a second capacitor (12). The switching devices (15, 16, 17, 18) are switched repeatedly between first and second states by an astable or bistable multivibrator circuit (Figs 3A, 3B) such that the switching devices (15, 16) in two diagonally opposite arms of the network are in the first state when the devices (17, 18) in the other two opposite arms of the network are in the second state. Increased drive voyage is effectively achieved in the transducer coils and hence increased sound output power compared with the prior art. <IMAGE>

Description

AUDIBLE ALARM ARRANGEMENT This invention relates to electrically operated audible alarm arrangements, or sounders, of the type comprising a transducer adapted and arranged to provide an audible alarm output and incorporating an actuating means involving wire coil means which is electrically connected to a driver circuit. A well-known example of such a transducer is a rocking armature transducer which incorporates two wire coils mounted on suitable pole pieces on a metal yoke and connected in series to the driver circuit. The coils are arranged such that the magnetic field produced by one is in the opposite direction to that produced by the other. As is known, a magnetically susceptible beam, or armature, is pivotally mounted like a see-saw, with opposite ends adjacent the two coils.In operation, energising of the coils causes the beam or armature to rock about its pivot in sympathy with the alternating current flux produced. This movement of the beam is transmitted to a diaphragm, by way of a semi-flexible rod, to produce a sound output.

Existing methods of driving transducers in alarm sounder arrangements involve applying to the transducer a switched or modulated voltage supply. It is known to increase the available drive voltage to the transducer by employing a four-arm bridge of switching devices, known as a full bridge or an H-bridge. Such an arrangement is illustrated schematically in Figure 1 of the accompanying drawings. In Figure 1 four switching devices 1, 2, 3, 4, suitably comprising transistors, are respectively connected in the four arms of the full or H bridge. A voltage supply Vs is connected to input connections on the bridge and the coil means 5 of the transducer is connected to output terminals 6,7 on the bridge.It is arranged for the switching devices 1, 2, 3, 4 to be switched on and off in pairs at a predetermined frequency such that switching devices 1,2 in one pair of diagonally opposite arms of the bridge are open when the switching devices 3,4 in the other pair of diagonally opposite arms of the bridge are closed and vice-versa.

voltage levels at points 6 and 7 on the bridge are therefore out'of phase with each other. A capacitor 8 is connected in series with the coil means S between the output terminals 6 and 7 and prevents any direct current components from affecting the transducer coil means 5.

In operation, current flows alternately through switching device 1, coil means 5 and switching device 2 and then through switching device 3, coil means 5 (in the opposite direction) and switching device 4. An alternating magnetic flux is produced in the coil means 5. The capacitor 8 is effectively charged to almost twice the voltage of the supply Vs, thereby increasing the drive voltage to the transducer and hence the sound output power. In the case of rocking armature transducers used in prior art arrangements, the two coils of the coil means 5 are connected in series with one another and with the capacitor 8, between the output terminals 6, 7 of the bridge. The available voltage is sensed by the two coils in series such that the voltage across each coil is effectively one half of this available voltage.

It is an object of the present invention to provide an audible alarm arrangement in which increased drive voltage to the coil means of the transducer is effectively achieved without requiring an increase in the voltage supply, thereby enabling increased sound output power to be obtained.

The present invention provides an electrically operated audible alarm arrangement comprising a transducer adapted and arranged to provide an audible alarm output and incorporating an actuating means employing wire coil means electrically connected to a driver circuit, said driver circuit incorporating a four-arm full or H-bridge network of switching devices having input points arranged for connection to a direct current voltage supply and means to effect switching of the switching devices repeatedly between first and second states such that the switching devices in two opposite arms of the bridge network are in the first state when the switching devices in the other two opposite arms of the bridge network are in the second state, the said wire coil means comprising first and second components, a first combination being provided comprising the first component of the coil means connected in series with a first electrical capacitor, and a second combination being provided comprising the second component of the coil means connected in series with a second electrical capacitor, the said first and second combinations being connected in parallel with one another between a pair of output points on said bridge network.

Each of said electrical capacitors has its capacitance value appropriately selected to provide optimum impedance match and hence highest power transfer in the arrangement.

The said first and second states of the switching devices may comprise 'conducting' and 'non-conducting' states, or 'off' and 'on' states, or 'open' and 'closed' states or relative 'high' and 'low' states of an appropriate electrical parameter.

The switching devices conveniently comprise solid state switching devices, eg transistors.

The means to effect switching of the switching devices conveniently comprises a multivibrator circuit, embodied in astable or bistable form.

The said multivibrator circuit suitably incorporates two switching transistors, one of which is connected to drive the switching devices in two opposite arms of the bridge network, and the other of which is connected to drive the switching devices in the other two opposite arms of the bridge network. When the switching devices comprise transistors, the multivibrator circuit is suitably connected to base terminals of these transistors.

For astable operation, the multivibrator circuit is arranged to be connected either to a fixed voltage supply, whereby it will function as a single-frequency oscillator, or to a variable voltage supply to achieve a modulated frequency oscillator effect.

For bistable operation, the multivibrator circuit is arranged to be connected to an external oscillator circuit whereby it operates with modulated oscillation.

The first and second components of the said wire coil means are appropriately wound relative to one another to provide essential magnetic fields in required directions relative to one another according to the type of transducer being provided.

The first and second components of the wire coil means may comprise either two separate wire coils or a single wire coil having two sections appropriately wound and with a point of electrical connection provided therebetween, according to the type of transducer being provided.

In a particular embodiment, the transducer comprises a rocking armature arrangement. In this embodiment the coil means suitably comprises two wire coils wound in such a way as to provide a magnetic field from one coil which is in the opposite direction to that from the other coil.

By means of the invention, sound output power from the transducer is considerably increased compared with the prior art, without increase in the supply voltage.

The invention is now described by way of example with reference to Figures 2, 3A and 3B of the accompanying drawings in which: Figure 2 represents a schematic circuit diagram of an audible alarm arrangement according to the invention; Figure 3A represents a schematic circuit diagram showing an astable multivibrator circuit applied to the arrangement of Figure 2; Figure 3B represents a schematic circuit diagram showing a bistable multivibrator circuit applied to the arrangement of Figure 2.

Referring to the drawings, an electrically operated alarm sounder arrangement is constructed having a rocking armature transducer as sounder. The construction of such rocking armature transducers is well known in the art and it is well known that they incorporate two wire coils which are electrically energised to cause a pivoted metal armature to rock in sympathy with the alternating current flux produced. This movement is transmitted to a diaphragm by way of a semi-flexible rod to produce a sound output. The difference in the transducer which is involved in the present invention concerns the electrical connection arrangements of the two wire coils. In Figure 2 the two wire coils of such a rocking armature transducer in the present invention are denoted by reference numerals 9 and 10.A first combination is provided comprising one of the wire coils 9 connected in series with an electrical capacitor 11 and a second combination is provided comprising the other wire coil 10 connected in series with an electrical capacitor 12. The two combinations are connected in parallel between a pair of output points 13, 14 on a four-arm full or H-bridge network. The bridge network has four switching devices comprising transistors 15, 16, 17 and 18 in its arms, and has input points 19A, 19B and 20A, 20B connected to a direct current voltage supply Vs.An astable multivibrator circuit, illustrated in Figure 3A is provided to effect repeated switching of the switching transistors between conducting and non-conducting states, such that the transistors 15, 16 in two diagonally opposite arms of the bridge network are both in a conducting state when the transistors 17, 18 in the other two diagonally opposite arms of the bridge network are both in a non-conducting state, and vice-versa. The heart of the multivibrator circuit is two switching transistors 21, 22, one of which (21) is connected to base terminals of transistors 15, 16 in the bridge network to drive these transistors 15, 16 and the other of which (22) is connected to base terminals of transistors 17, 18 in the bridge network to drive these transistors 17, 18. A voltage supply (not shown) is connected to terminals 23, 24.If this voltage supply is fixed, the multivibrator circuit functions as a single-frequency oscillator.

Alternatively, by providing a variable voltage supply connected to terminals 23, 24, a modulated frequency oscillator effect can be achieved. The frequency of oscillation of the circuit is determined by the values selected for the resistors and capacitors therein and the voltage of the voltage supply.

As an alternative to the astable circuit arrangement of Figure 3A, a bistable circuit arrangement may be employed as shown in Figure 3B. As in Figure 3A, two switching transistors 21, 22 are provided, connected respectively to base terminals of transistors 15, 16 and transistors 17, 18 in the bridge network. In the circuit arrangement of Figure 3B an external oscillator (not shown) is connected to terminals 25 and 26.

Modulated oscillation occurs in the multivibrator circuit, the frequency of oscillation being fixed by the external oscillating voltage supply.

In operation of the transducer arrangement, electric current flows alternately through transistor 15, coils 9, 10 and transistor 16 and then through transistor 17, coils 9, 10 (in the opposite direction this time) and transistor 18. Alternating magnetic flux is produced in the two coils 9 and 10, the magnetic field produced by one coil 9 being arranged, by appropriate selection of the direction of the windings therein, to be in the opposite direction to that produced by the other coil 10, whereby operation of the rocking armature in the transducer is obtained. The coils 9 and 10 with their associated series capacitors 11, 12 are in parallel with one another between the output terminals 13, 14 of the bridge network, as compared with the prior art in which the two coils of the transducer were connected in series with one another and with one associated capacitor, between the output terminals of the bridge network. As a consequence, the voltage at the coils 9, 10 is considerably higher (eg almost by a factor of two) than with the prior art arrangement and hence the sound output from the transducer is proportionately increased. In practice a sound output power of almost twice that available with the prior art arrangement is attainable without increase in the supply voltage.

Claims (16)

1 An electrically operated audible alarm arrangement comprising a transducer adapted and arranged to provide an audible alarm output and incorporating an actuating means employing wire coil means electrically connected to a driver circuit, said driver circuit incorporating a four-arm full or H-bridge network of switching devices having input points arranged for connection to a direct current voltage supply and means to effect switching of the switching devices repeatedly between first and second states such that the switching devices in two opposite arms of the bridge network are in the first state when the switching devices in the other two opposite arms of the bridge network are in the second state, the said wire coil means comprising first and second components, a first combination being provided comprising the first component of the coil means connected in series with a first electrical capacitor, and a second combination being provided comprising the second component of the coil means connected in series with a second electrical capacitor, the said first and second combinations being connected in parallel with one another between a pair of output points on said bridge network.

2 An alarm arrangement according to Claim 1 in which each of said electrical capacitors has its capacitance value appropriately selected to provide optimum impedance match and hence highest power transfer in the arrangement.

3 An alarm arrangement according to Claim 1 or 2 in which the said first and second states of the switching devices comprise 'conducting' and 'non-conducting' states, or 'off' and 'on' states, or 'open' and 'closed' states or relative 'high' and 'low' states of an appropriate electrical parameter.

4 An alarm arrangement according to Claim 1, 2 or 3 in which the switching devices comprise solid state switching devices.

5 An alarm arrangement according to Claim 4 in which the solid state switching devices comprise transistors.

6 An alarm arrangement according to any preceding Claim in which the means to effect switching of the switching devices comprises a multivibrator circuit.

7 An alarm arrangement according to Claim 6 in which the multivibrator circuit is embodied in astable or bistable form.

8 An alarm arrangement according to Claim 6 or 7 in which the multi vibrator circuit incorporates two switching transistors, one of which is connected to drive the switching devices in two opposite arms of the bridge network, and the other of which is connected to drive the switching devices in the other two opposite arms of the bridge network.

9 An alarm arrangement according to Claim 8 in which the switching devices comprise transistors and the multivibrator circuit is suitably connected to base terminals of these transistors.

10 An alarm arrangement according to Claim 7, 8 or 9 in which, for astable operation, the multivibrator circuit is arranged to be connected either to a fixed voltage supply, whereby it will function as a single-frequency oscillator, or to a variable voltage supply to achieve a modulated frequency oscillator effect.

11 An alarm arrangement according to Claim 7, 8 or 9 in which, for bistable operation, the multivibrator circuit is arranged to be connected to an external oscillator circuit whereby it operates with modulated oscillation.

12 An alarm arrangement according to any preceding Claim in which the first and second components of the said wire coil means are appropriately wound relative to one another to provide essential magnetic fields in required directions relative to one another according to the type of transducer being provided.

13 An alarm arrangement according to any preceding Claim in which the first and second components of the wire coil means comprise either two separate wire coils, or a single wire coil having two sections appropriately wound and with a point of electrical connection provided therebetween, according to the type of transducer being provided.

14 An alarm arrangement according to any preceding Claim in which the transducer comprises a rocking armature arrangement.

15 An alarm arrangement according to Claim 14 in which the coil means comprises two wire coils wound in such a way as to provide a magnetic field from one coil which is in the opposite direction to that from the other coil.

16 An electrically operated audible alarm arrangement constructed and arranged substantially as hereinbefore described with reference to Figures 2, 3A and 3B of the accompanying drawings.