Transducer Drive Circuit And Signal Generator

Abstract

A transducer such as a piezoelectric element forms part of the diaphragm of a speaker in a sonic signal generator. A timing circuit and the piezoelectric transducer element are coupled to a level detector and voltage comparing circuit. This circuit monitors the phase and voltage of the piezoelectric transducer and compares the same with the signal from the timing circuit. When a selected condition is reached a controlled switch connected between the transducer and the power supply is operated to cause a small signal to be applied to the transducer at the appropriate time to maintain the transducer in an oscillatory state corresponding to its resonant frequency with the expenditure of a very small amount of energy from the power supply.

Description

BACKGROUND OF THE INVENTION

Audio signal generators are widely used at the present time, as for example in the telephone art. In such applications it is of importance to be able to operate a substantial number of the telephone ringing circuits without imposing an undue drain on the power supply. Various attempts have been made to provide these and other electrically operated audio generators using as little energy as possible while obtaining substantial audio energy output. Thus sonic transducers such as disclosed in U.S. Pat. No. 3,331,970 have been devised wherein a transducer such as a piezoelectric crystal is connected to a thin metallic diaphragm forming part of an audio speaker. In such an arrangement audio signals corresponding to the resonant frequency of the piezoelectric element are generated. Various other uses of transducer elements such as piezoelectric devices are well known, particularly those wherein the transducer is driven at its resonant frequency.

An oscillator carefully designed to have exactly the same frequency as the piezoelectric transducer has been typically used as the driving circuit in applications such as mentioned above. U. S. Pat. No. 3,277,465 to Potter suggests such an approach. While a carefully tuned oscillator can be used to drive a transducer, I have found that the energy required to drive the transducer and the oscillator itself is excessive, and in fact will severely limit the number of ringing devices which can be driven by a single source.

It is therefore an object of the present invention to provide an efficient transducer drive circuit. Another object of the invention is to provide a driving circuit for a piezoelectric transducer which circuit requires an extremely small amount of electrical operating energy.

Another object of the present invention is to provide an audio signal generator utilizing a piezoelectric element in combination with a controlled power switch which provides a small boost to the piezoelectric element at a selected point in the physical movement of the piezoelectric element.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention a piezoelectric transducer is connected to a power supply so that when power is applied to the element the element will be distorted in the manner well known in the art and attempt to vibrate at its resonant frequency. At the time of applying energy to the piezoelectric element a timing circuit is also provided with power. The timing circuit and the piezoelectric element are connected to a level detecting and comparing circuit which compares the voltage level of the piezoelectric element with the level of the signal from the timing circuit. The timing circuit is adjusted such that its voltage level achieves a selected amplitude at a point in time corresponding to the signal of the piezoelectric element approaching a maximum. At this point in time a controlled switch circuit connected to the piezoelectric element and to the power supply is actuated so that a very small signal is applied to the piezoelectric element. The applied signal is of the proper polarity and accurately timed so that only a small boost signal is applied to the piezoelectric element, causing it to swing through its maximum signal condition. The controlled switch then turns off and the transducer continues on through another cycle, the timing circuit being reset for another cycle of operation.

By thus monitoring the voltage of the piezoelectric element and accurately timing the application of a small "boosting" signal, the piezoelectric element is maintained in a vibrational mode corresponding to its inherent resonant frequency. The system is such that the amount of energy applied to the piezoelectric element is maintained at a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and additional advantages and objects of the invention will be more clearly understood from the following description when read with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating the principles of a preferred embodiment of the present invention.

FIGS. 2, 3, 4, 5 and 6 are schematic circuit diagrams illustrating preferred embodiments of the present invention.

FIG. 2A is a waveform diagram illustrating voltages appearing at selected points in the circuit of FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings there is illustrated in FIG. 1 a piezoelectric transducer 10 which is provided with energy from the power supply 11 whenever the main switch 12 is closed. The power supply 11 also applies power to the timing circuit 13 and to the controlled switch 14 when the main switch 12 is closed. A level detector and comparison circuit 15 is connected to the timing circuit 13 and to the transducer 10 and serves to compare the voltage level appearing on the transducer 10 with the voltage of a signal from the timing circuit 13. When the voltage levels are substantially equal, the controlled switch 14 connected to the detector and comparator 15 is closed. The controlled switch 14 is connected by circuit 16 to the piezoelectric transducer 10 with the arrangement being such that closure of the switch 14 causes a signal to be applied to the transducer 10 at the appropriate time in the cycle of movement thereof to assure continued vibration of the transducer 10. The transducer 10 is preferably a part of a sonic signal generator such as disclosed in U. S. Pat. No. 3,331,970 and thus the system of FIG. 1 provides an intense audio signal when switch 12 is closed.

Referring now to FIG. 2 a preferred embodiment of the invention following the concepts of FIG. 1 will be described. In the arrangement of FIG. 2 the power supply is connected via lead 21 to resistors 22 and 23. Resistor 23 is connected in series circuit with resistor 24 across the power supply so that the mid-point 25 thereof is at a voltage dependent on the relative values of resistors 23 and 24. A programmable unijunction transistor (PUT) 26 has its control electrode 27 connected to the point 25. The anode 28 is connected by lead 29 and resistor 22 to the positive terminal of power supply 11.

Capacitor 30 is connected between resistors 22 and 31 and forms part of the timing circuit referred to in FIG. 1. The piezoelectric transducer 10 which also forms part of the diaphragm of an audio signal generator is connected between points 25 and 32. It will be seen that when switch 12 is closed current is provided to the piezoelectric transducer via resistors 23 and 31. Thus it is stressed and moves in the manner well known in the art. When the switch 12 is closed the capacitor 30 also starts to charge at a rate controlled by resistor 22. Thus the voltage on the anode of the PUT 26 rises in a substantially linear fashion. When the voltage on the anode 28 exceeds that on the gate 27 the PUT conducts causing capacitor 30 to discharge and also discharging the voltage across the transducer 10. The transducer is thus excited and starts a positive excursion of point 25. Two complete cycles are shown in FIG. 2A, with the time T.sub.1 corresponding to point 25 starting to go positive. The voltage at gate electrode 27 is shown by waveform 27A and the voltage on anode 28 shown by waveform 28A. The voltages are shown relative to the negative voltage of supply 11. As is well known in the art the PUT will remain nonconductive when the gate is positive relative to the anode and will start to conduct when the gate is negative with respect to the anode.

Since the gate 27 is connected to the transducer 10 the gate voltage follows the generally sinusoidal waveform indicated with the PUT non-conductive till time T.sub.2. Between times T.sub.1 and T.sub.2 the excited transducer undergoes a positive excursion and then starts taking point 25 negative. At time T.sub.2 the piezoelectric transducer is nearing the completion of a cycle of oscillation, but due to friction and losses due to driving the diaphragm the excursion amplitude is diminishing. When the voltage at point 25 becomes less than the voltage on the anode 28 (at time T.sub.2), the PUT 26 conducts. Capacitor 30 therefore discharges to the negative voltage of battery 11, and point 25 is also lowered to that voltage via the PUT gate circuit. Thus resistor 24 is shunted and the negative excursion of the transducer receives an in-phase signal which accelerates the excursion. This small in-phase signal brings the excursion to full amplitude so that oscillation is sustained.

After the transducer achieves its maximum negative excursion and starts in the opposite direction, the voltage at point 25 again rises above the voltage of the anode 25 at time T.sub.3 and hence the PUT 26 is held non-conductive during the major portion of the next cycle of the transducer. The operation is then repeated with the voltage at point 25 again falling below the voltage of the anode 28 at time T.sub.4. It will be seen that the oscillation of the transducer is thereafter sustained by the repeated application of the boosting signals.

Since the switch element 26 conducts for only a small portion of each cycle of the transducer, and the remaining portion of the time the switch appears as a very high impedance, the circuit illustrated has been found to draw very little current and permits the use of a large number of such systems on a single telephone bell ringing circuit.

During conduction of the PUT anode 28 and gate 25 are both switched negative at the same time. The capacitor 30 thus drives point 32 negative. However, resistor 31 performs somewhat of a cushioning effect for the transducer from the fast wave-shape of the switch, and thus in practice is has been found that the voltage across the transducer is substantially simusoidal.

FIG. 3 is a schematic circuit diagram of an arrangement similar to that of FIG. 2 and also making use of a programmable unijunction transistor as the controlled switch. In the circuit of FIG. 3 resistors 43, 44 and 45 are connected in series circuit across the power supply 11 with the piezoelectric transducer 10 being connected in parallel with resistor 45. Resistor 42 and capacitor 50 are connected in series circuit from the positive terminal of power supply 11 to one side of the transducer 10. The other side of the transducer 10 is connected to the gate 47 of the PUT 46. The anode 48 is connected to the junction of resistor 42 with capacitor 50.

The operation of the circuit shown in FIG. 3 is substantially the same as the operation of the circuit of FIG. 2 with resistor 45 acting as part of the discharge circuit for the crystal 10. As in the case of the circuit of FIG. 2, when the piezoelectric element 10 starts its positive excursion, the voltage on the piezoelectric element holds the gate 47 positive with respect to the voltage on the anode 48 and hence the PUT is held against conduction until the crystal is on the negative portion of its excursion. Then in the manner previously described, the PUT becomes conductive so that the "boosting" signal is applied to the crystal in the manner described above.

FIG. 4 shows a circuit substantially the same as the circuit of FIG. 3 with the exception that the controlled switch in the circuit of FIG. 4 is a silicon controlled switch (SCS). With the circuit arrangements of FIGS. 3 and 4 the impedance of the power source is typically lower in the case of FIG. 2, which is adapted for use with a high impedance power supply.

In the circuit of FIG. 5 resistors 64 and 65 are connected across the power supply and provide the bias voltage for the base of transistor 63. The emitter of the NPN transistor 63 is connected to the gate of the PUT 66 and to one side of the piezoelectric transducer 10. The capacitor 70 is connected in series circuit with resistors 61 and 62 across the power supply and is also connected to the other side of the piezoelectric transducer 10. The anode of the PUT 66 is connected to the capacitor 70 and to resistor 62. A second capacitor 67 connects the emitter of the transistor 63 with its base. It will be seen that the circuit of FIG. 5 corresponds in principle to the circuit of FIG. 2 with the transistor 63 essentially taking the place of resistor 23 in FIG. 2. The transistor 63 looks like a few ohms on the positive cycles of the transducer 10, since during that time the transistor conducts. However, during the negative cycles the transistor looks like several megohms. As in the case of FIG. 2 will be seen that the PUT compares the signal from the timing circuit (which includes capacitor 70) with the signal from the transducer and controls the application of the small boosting signal in the small manner as previously described.

In the circuit arrangement of FIG. 6 a PNP transistor 75 serves as the level detector and the NPN transistor 76 serves as the controlled switch. As an aid to understanding the invention the various circuit components are contained within the functional blocks formed by the dashed lines. It will be seen that the base of transistor 75 and the collector of transistor 76 are connected to the junction of resistors 73 and 74. The emitter of transistor 75 is connected to the positive power supply terminal by the resistor 72. Resistor 71 connects one side of the transducer to the negative power supply terminal and resistor 74 connects the other side of the transducer as well as the base of transistor 75 to the negative power supply. Capacitor 80 connects one side of the transducer to the emitter of the level detecting transistor 75. The operation of the circuit of FIG. 6 corresponds in general to the operation of the circuit of FIG. 2. That is, when power is applied to the circuit, current momentarily flows through resistors 71 and 73, exciting the transducer 10. At the same time the capacitor 80 starts to charge via resistor 72. When the voltage on the emitter of transistor 75 exceeds the voltage on the base, transistor 75 starts to conduct current through resistor 72 and also from capacitor 80. Current is thus supplied to the base of transistor 76 so that transistor 76 conducts drawing current from the base circuit of transistor 75 and hence insuring complete conduction of both transistors. Resistor 74 is thus shunted by transistor 76, and the piezoelectric transducer 10 is therefore discharged, causing its complete excitation. It will be seen that capacitor 80 will also be completely discharged through the emitter-collector junction of transistor 75 and the base-emitter junction of transistor 76. Resistor 72 does not provide sufficient current to hold transistors 75 and 76 conductive, and therefore the circuit reverts to its original nonconducting mode. When this occurs, the now excited transducer begins a positive excursion on the side connected to the base of transistor 75, assuring non-conduction of transistor 75. At the same time, the capacitor 80 starts charging toward the positive voltage of the power supply through resistor 72. The rate at which capacitor 80 is permitted to charge is adjusted so that the voltage on the base of transistor 75 runs ahead of the voltage on the emitter of transistor 75 (in the manner indicated in the wave diagrams of FIG. 2A) and hence the transistor 75 remains non-conductive until the negative excursion of the transducer occurs. During the negative excursion of the transducer, the base of transistor 75 goes slightly negative with respect to the rising voltage on the emitter, and hence the transistor 75 acts as a level detecting circuit and voltage comparator, causing transistor 76 to become conductive at the appropriate point in the negative excursion of the transducer. Thus the small "boosting" signal is applied to the transducer in the manner previously described.

While the values of the circuit components will vary in accordance with a particular circuit, the following values were used in the circuit of FIG. 2: resistors 22 and 24 were each 1 megohm, resistor 23 was 100,000 ohms, resistor 31 was 1,000 ohms, and capacitor 30 was 0.002 microfarads. The power supply was 26 volts, although it has been found that the circuits described herein will operate over a wide variation of voltage ranges. For example, the circuit of FIG. 5 has been found to work with voltages ranging from 1.5 volts to 160 volts.

It will be recognized, of course, that while the circuits disclosed in detail are shown for applying a small "booting" signal to the transducer on one side of each cycle, the concept can be utilized to apply a boosting signal to each side of the cycle. However, in practice I have found that with the circuit arrangements illustrated, a very small current drain on the power supply results, even through a substantial volume of audio energy is being generated.

While the invention has been described by reference to presently preferred embodiments, it is intended that the following claims will encompass those changes and modifications which become obvious to a person skilled in the art as a result of teachings hereof.

Claims

What is claimed is:

1. An audio signal generator for operation from a power supply, comprising in combination: a transducer operative to provide a damped output signal alternating between successively decreasing maximum values thereof when electrically energized; a switching means connected to said transducer for applying an in-phase, boosting signal to said transducer; means actuating said switching means when said output signal approaches a maximum value, said actuating means including means providing a threshold signal whose voltage is less than that of said maximum value of said transducer output signal, and wherein said switching means includes a comparison means operable to provide said in-phase boosting signal when the voltage on said transducer substantially equals the voltage of said threshold signal; and means connecting the power supply to said transducer, said switching means and said actuating means.

2. An audio signal generator as recited in claim 1, wherein said transducer comprises a piezoelectric element.

3. An audio signal generator as recited in claim 1, wherein said threshold signal means includes a timing circuit which produces an output voltage which increases with respect to time in synchronism with said output signal, and said comparison means in said switching means includes at least one semi-conductor element having first and second electrodes respectively connected to said timing circuit and to said transducer, said semi-conductor element remaining non-conductive until the voltages on said two electrodes become substantially equal.

4. An audio signal generator as recited in claim 3, wherein said semiconductor element comprises a programmable unijunction transistor having a gate electrode connected to said transducer element and an anode electrode connected to said timing means.

5. An audio signal generator as recited in claim 3, wherein said semiconductor element comprises a silicon controlled switch having a gate electrode connected to said transducer element and an anode electrode connected to said timing means.

6. A signal generator comprising in combination: an electrically sensitive transducer means including a piezoelectric element; timing circuit means; power supply means connected to said transducer means and timing circuit means; and signal comparison and switch means connected to said transducer means, said timing circuit means and said power supply means, said timing circuit means including means providing a signal having a voltage which rises to the voltage on said transducer means as the voltage on said transducer means is approaching a maximum value, and said comparison and switch means includes means operative when the voltage on said transducer means substantially equals the voltage of said timing signal to apply an in-phase, boosting signal to said transducer means.

7. A signal generator as recited in claim 6, wherein said signal comparison and switch means includes at least one semiconductor element having first and second electrodes respectively connected to the timing circuit means and to the transducer means, said element remaining non-conductive until the voltages on said two electrodes become substantially equal.

8. A signal generator as recited in claim 6, wherein said semiconductor element is maintained non-conductive during at least 50 percent of each cycle of said transducer means.

9. A signal generator as recited in claim 6, wherein said transducer means forms part of a sonic signal generator.

10. A sonic signal generator comprising in combination: power supply means having first and second terminals; a transducer element including a piezoelectric crystal having first and second electrodes; first and second impedance elements connecting said first and second electrodes to said first and second terminals of said power supply means; first circuit means including a capacitor having a first side connected to said first electrode, said first circuit means further including means connecting a second side of said capacitor to said first terminal of said power supply means; and second circuit means including a semiconductor element having a first electrode connected to said second terminal of said power supply means and second and third electrodes respectively connected to said second side of said capacitor and to said second electrode of said transducer element, said semiconductor element thereby being maintained non-conductive until the voltages on its said second and third electrodes are substantially equal.

11. A sonic signal generator as recited in claim 10, wherein said connecting means in said first circuit means further comprises a third impedance element connecting said capacitor to said power supply means so that the rate of charge of said capacitor is less than the rate of voltage increase on said transducer element.

12. A sonic signal generator as recited in claim 10, wherein said semiconductor element comprises a programmable unijunction transistor having an anode electrode connected to said semiconductor's second electrode and a gate electrode connected as said semiconductor's third electrode.

13. A sonic signal generator as recited in claim 10, wherein said semiconductor element comprises a silicon control switch having an anode electrode connected as said semiconductor's second electrode and a gate electrode connected as said semiconductor's third electrode.