A horn for generating a sound

Abstract

 A horn for generating a sound comprising a diaphragm (280), vibration of which causes a sound to be generated, and switching circuitry (230, 260, 250, 270) connected to vibrate the diaphragm (280) in such a fashion that switching of the circuitry (230, 260, 250, 270) is synchronized with the oscillatory movement of the diaphragm (280). The switching circuitry (230, 260, 250, 270) comprises a main switching device (2510) through which a power electrical current passes at a power voltage to effect vibration of the diaphragm (280), and a position sensor (270) arranged to cause a switching signal to be delivered to the main switching device (2510) at a switching electrical voltage, in dependence upon the position of the diaphragm (280).

Description

[0001] The present invention relates to a horn for generating a sound comprising a diaphragm, vibration of which causes a sound to be generated, and switching circuitry connected to vibrate the diaphragm in such a fashion that switching of the circuitry is synchronized with the oscillatory movement of the diaphragm, in which the switching circuitry comprises a main switching device through which a power electrical current passes at a power voltage to effect vibration of the diaphragm.

[0002] A previously proposed construction of such a horn is shown in Figure 1, which shows an axial sectional view of such a horn.

[0003] The horn shown in Figure 1 comprises a diaphragm 10 held around its periphery by the lip 12 of a housing 14 so that central regions of the diaphragm are able to vibrate to and fro in an axial direction of the horn. A pole piece 16 is secured to a rear side of the centre of the diaphragm 10. It is formed with a switch protuberance 18 which projects in a radial direction from one side of the pole piece. A coil core 20 is secured to the housing 14 on the interior thereof rearwardly and in registration with the pole piece 16. The coil core is surrounded by a coil 22 which is seated within a cylindrical portion 24 of the housing 14. Two metallic power terminals 26 project rearwardly from the housing 14 on opposite sides of the cylindrical portion 24 thereof. A wire 28 extends from one of the power terminals to the coil, the other end of which is connected via a further wire 30 to one side of a make-and-break contact switch 32. The other side of this make-and-break contact switch 32 is connected to the other power terminal 26 via a further wire 34.

[0004] A nozzle-shaped horn tube 36 extends axially forwardly from the diaphragm 10.

[0005] When an electrical voltage is applied across the power terminals 26, the magnetic field created by the coil 24 magnetizes the coil core 20 which as a result attracts the pole piece 16 towards it. The switching protuberance 18 thereby comes into contact with one side of the make-and-break contact switch 32 so that the contact parts thereof are physically separated. This breaks the circuitry between the power terminals 26. The coil as a result no longer has an electrical current passing through it, a magnetic field is no longer generating along its axis, and the coil core 20 is no longer magnetized, so that the pole piece 16 is no longer drawn towards it and the natural resilience of the diaphragm 10 causes it to be restored towards its rest position and beyond. This results in the contacts of the make and break contact 32 to be brought together again to restore the circuitry across the power terminals 26. This now therefore causes the pole piece 16 to be drawn once again towards the coil core 20. This cycle repeats itself for as long as an electrical voltage is applied across the power terminals 26. The diaphragm 10 is thereby caused to vibrate, and because the switching on and off of the electrical current through the coil 22 is synchronized with the vibration of the diaphragm 10, it vibrates at its natural resonant frequency and a very powerful sound is thereby generated, the pressure waves of that sound being amplified by the tube 36 especially in a forward axial direction of the horn.

[0006] Disadvantages of such a construction are that any switch which is provided to turn the horn on has the full power current passing through it, or else it needs a relay to operate the horn indirectly. In addition, the make and break contacts tend to wear through arcing.

[0007] The present invention seeks to obviate one or both of these disadvantages.

[0008] Accordingly, the present invention is directed to a horn for generating sound having the construction set out in the opening paragraph of the present specification, in which the switching circuitry further comprises a position sensor arranged to cause a switching signal to be delivered to the main switching device at a switching electrical voltage, in dependence upon the position of the diaphragm.

[0009] Such a horn may have a greatly extended working life relative to a conventional horn.

[0010] The main switching device may comprise a field effect transistor.

[0011] The position sensor may comprise a make and break contact switch. This has the advantage that more of the parts which are already in use in a conventional horn can be used to create a horn in accordance with the present invention. As a result, switching circuitry can be retrofitted into an existing horn to create such an embodiment of the present invention, to convert the horn to a long-life electronic horn.

[0012] However, a make and break contact switch can be avoided altogether with consequent elimination of contact wear, pitting, arcing and corrosion, and with a cleaner switching of the coil current leading to reduced electromagnetic emissions, if a solid state detector is used as the position sensor. For example, the position sensor may comprise an optocoupler, a Hall effect switch, or a piezo electric device. The switch circuitry may include a pulse width modulator to vary the proportion of time within a given interval for which an electrical voltage is applied across the horn, in dependence upon the level of the voltage applied across the horn.

[0013] An example of a horn made in accordance with the present invention will now be described with reference to Figures 2 to 4 of the accompanying drawings, in which:

Figure 2

is a block circuit diagram showing the circuitry of the example embodiment;

Figure 3

is a more detailed circuit diagram of the circuitry of the example embodiment shown in Figure 2; and

Figure 4

is an explanatory diagram.

[0014] The circuitry shown in Figures 2 and 3 comprises a voltage source 210 connected to a voltage regulator 220. The latter provides a regulated operating voltage of +5 volts DC to a pulse width modulator 230 and also to an oscillator 240. The pulse width modulator 230 is provided with an input connected to receive an output from the oscillator 240, and with a further input to have applied to it a voltage which is dependent upon the voltage level of the voltage source 210.

[0015] An output from the pulse width modulator 230 is connected (indirectly) to an input of a power switch 250. The latter is gated by gating switch 260, effectiveness of which is in turn controlled by a diaphragm position sensor 270 arranged in relation to a diaphragm 280 of the horn so that triggering signals issued from the diaphragm position sensor 270 are issued in dependence upon the position of the diaphragm 280.

[0016] An actuating coil 290 is arranged to intermittently attract a pole piece 300 of the diaphragm 280, the diaphragm 280, the actuating coil 290 and the pole piece 300 being arranged in similar fashion to the diaphragm 10, the actuating coil 24 and the pole piece 16 of the horn shown in Figure 1.

[0017] The actuating coil 290 is connected to be operated by the voltage source 210 in dependence upon the condition of the output power switch 250.

[0018] The component parts of the circuitry shown in Figure 2 are shown in greater detail in Figure 3, the components of the circuitry in Figure 3 being grouped by a number of boxes shown in broken lines, such that the components shown in Figure 3 which act together to function as one of the parts of the circuit shown in Figure 2 have their box labelled in Figure 3 with the same reference number as used in Figure 2.

[0019] The oscillator 240 comprises one half of a 556 timer 2410 constituting one half of a dual timer chip for the generation of a fixed frequency-square wave signal. The output 2420 of the 556 timer half 2410 is coupled via a capacitor 2430 to a triggering input 2310 of a second half of the 556 timer 2320. The 556 timer half 2320 is connected as a triggerable monostable. It has a control pin 2330 connected indirectly to the voltage source 210.

[0020] A horn on/off switch 310 is connected indirectly to the switching input of a field effect transistor 2610 of the gating switch 260 via a small gauge, low current signal wire 311. The latter is also provided with a switching transistor 2620 having its switching input connected to the output of the pulse switch modulator 230, and its collector side connected to the source connection of the field effect transistor 2610 (the drain of which is earthed) and also to the switching input of a further switching transistor 2630. The collector of the switching transistor 2620 is also connected via a resistor to the voltage source 210.

[0021] The collector of the switching transistor 2630 constitutes the output from the gating switch 260. The latter is connected to the switching input of a low internal resistance field effect transistor 2510 of the output power switch 250 via the diaphragm position sensor 270.

[0022] Operation of the circuitry shown in Figures 2 and 3 is as follows. When the horn on/off switch 310 is depressed, the field effect transistor 2610 is enabled for the passage of electrical current through its source and drain. Subject to the presence of an enabling output from the pulse width modulator 230 at the switching transistor 2620, the switching transistor 2630 is also enabled. If, in addition, the diaphragm position sensor 270 is also in an operative position by virtue of the diaphragm being in a position which causes that, a switching signal is applied at a switching electrical voltage to the triggering input of the field effect transistor 2510 by virtue of the completed circuitry from the voltage source 210 via the diaphragm position sensor 270 and the switching transistor 2630 to earth. This opens the circuit from the voltage source 210 via the field effect transistor 2510 and the actuating coil 290 to earth. As a result, a power electrical current passes through the field effect transistor 2510 and the actuating coil 290, with the field effect transistor 2510 being at the power electrical voltage of the power source 210. This in turn creates a magnetic field within the coil 290 and the consequent attraction of the pole piece 200 towards the coil, as well as consequent movement of the centre of the diaphragm 280 towards the coil.

[0023] At a certain position of the diaphragm 280, the diaphragm position sensor 270 causes the circuit through the switching transistor 2630 to be broken. This inhibits the field effect transistor 2510 so that the current through the coil 290 ceases and the pole piece 300 is enabled to return to its rest position by virtue of the resilience of the diaphragm 280 so that the position sensor 270 now causes the circuit through the switching transistor 2630 to be closed, the field effect transistor 2510 to be enabled, a current to be passed through the actuating coil 290, once again causing the pole piece 300 to be attracted towards the coil 290. This therefore causes vibration of the diaphragm 280 in a manner similar to that described with reference to the horn shown in Figure 1, such that the switching of the current through the coil 290 is synchronized with the oscillatory motion of the diaphragm 280.

[0024] Whilst this switching cycle is occurring, a square wave signal is generated by the oscillator 240 to produce an output from the pulse width modulator 230 for a repeated predetermined period of time at the same frequency as the square wave signal it receives from the oscillator 240. The duration of the predetermined period is set by the voltage applied to the control pin 2330 of the 556 timer half 2320.

[0025] The variation of the set predetermined period as a function of the voltage source 210 is shown in Figure 4. Thus, if the latter is 12 volts, the predetermined period is the full duration of one cycle of the signal from the oscillator 240. If the voltage of the voltage source 210 is 24 volts, the predetermined period is one half of one cycle of the signal from the oscillator 240. If the voltage is 36 volts, the predetermined period is one-third of the full cycle period.

[0026] The operating frequency of the oscillator 240 is selected to be as low as possible, so as to minimize radio frequency interference emissions, whilst at the same time being high enough so as not to interfere with the resonance of the diaphragm 280 nor to be audible as an overtone of the horn sound.

[0027] It will be appreciated that the pulse width modulator 230 thereby enables the horn to be operated substantially independently of the voltage level of the voltage source 210. The coil will not overheat even with a high voltage source, because the modulated current through the coil has an average value which is the same as what it would be were the coil's nominal rated voltage applied across it.

[0028] It will also be appreciated that the synchronization of the switching of the field effect transistor 2510 with the oscillatory motion of the diaphragm 280 ensures resonance thereof substantially regardless of the environmental conditions in which the horn is used.

[0029] The position sensor 270 may comprise a make-and-break contact, such as that described with reference to Figure 1, although it more preferably comprises a Hall effect switch, an opto-coupler or a piezo electric crystal.

[0030] The circuitry of Figures 2 and 3 may be enclosed in the housing or casing of the horn, such as the housing 14 in Figure 1. Indeed, apart from the interposition of the circuitry shown in Figures 2 and 3 between the contact switch 32 and the coil 22 of the horn shown in Figure 1, the horn of Figures 2 and 3 may be otherwise the same.

[0031] The circuitry shown in Figures 2 and 3 enables the on/off switch 310 to be operated at low power, such as the power of digital TTL signals or CMOS signal level from, for example, a microcontroller or a control node of a multiplexed electrical system.

[0032] The voltage source 210 may comprise batteries of a discrete voltage from 6 to 80 volts, inclusive.

[0033] The use of a low internal resistance field effect transistor 2510 avoids the need for a heat sink.

[0034] The closed loop feedback obtained with the circuitry shown in Figures 2 and 3 avoids any problem that would occur with a fixed frequency oscillator being used to switch the field effect transistor 2510 to create the sound vibration. With such an oscillator, its frequency may become out of tune with the resonant frequency of the diaphragm 280, especially with extreme temperature changes.

[0035] Numerous variations and modifications to the illustrated circuitry may occur to the reader without taking the consequent construction outside the scope of the present invention. To give one example, whilst the pulse switch modulator 230 with the oscillator 240 is desirable, it could be omitted, with the switching transistor 2620 being replaced by an appropriate resistor. This would also avoid the need for the voltage control 220.

Claims

1. A horn for generating a sound comprising a diaphragm (280), vibration of which causes a sound to be generated, and switching circuitry (230, 260, 250, 270) connected to vibrate the diaphragm (280) in such a fashion that switching of the circuitry (230, 260, 250, 270) is synchronized with the oscillatory movement of the diaphragm (280), in which the switching circuitry (230, 260, 250, 270) comprises a main switching device (2510) through which a power electrical current passes at a power voltage to effect vibration of the diaphragm (280), characterised in that the switching circuitry (230, 260, 250, 270) further comprises a position sensor (270) arranged to cause a switching signal to be delivered to the main switching device (2510) at a switching electrical voltage, in dependence upon the position of the diaphragm (280).

2. A horn according to claim 1, characterised in that the main switching device (2510) comprises a field effect transistor.

3. A horn according to claim 1 or claim 2, characterised in that the position sensor (270) comprises a make-and-break contact switch.

4. A horn according to claim 1 or claim 2, characterised in that the position sensor (270) comprises a solid state detector.

5. A horn according to claim 4, characterised in that the position sensor (270) comprises an opto-coupler.

6. A horn according to claim 4, characterised in that the position sensor (270) comprises a Hall effect switch.

7. A horn according to claim 5, characterised in that the position sensor (270) comprises a piezo electric device.

8. A horn according to any preceding claim, characterised in that the switching circuitry (230, 260, 250, 270) includes a pulse width modulator (230) to vary the proportion of time within a given interval for which an electrical voltage is applied across the horn, in dependence upon the level of the voltage applied across the horn.

Drawing