JPS6248840B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a drive circuit for a piezoelectric buzzer.

[Conventional technology]

In recent years, electronic wristwatches that use a buzzer to notify the hour or provide an alarm have rapidly become popular. Buzzers used in electronic wristwatches include electrodynamic buzzers, electromagnetic buzzers, and piezoelectric buzzers. Among these, piezoelectric buzzers have recently come into widespread use due to their low price and simple structure. Hereinafter, a piezoelectric buzzer (a general term that includes a sounding body that uses the piezoelectricity of an object to electrically drive it to cause mechanical displacement, as well as a vibration system around the piezoelectric buzzer) will be referred to as a piezoelectric buzzer. In an electronic wristwatch, the piezoelectric buzzer is driven by a rectangular wave electrical signal. At this time, a method is generally used in which a booster coil is used to increase the voltage of the electrical signal, thereby driving a piezoelectric buzzer.

FIG. 1 shows a specific example of a drive circuit for such a piezoelectric buzzer. In Figure 1, 1 is an inverter, 2 is a buzzer drive transistor, 3 is a boost coil, and 4 is a piezoelectric buzzer, L ₁ is the inductance of the boost coil 3, C ₁ is the capacitance of the piezoelectric buzzer, and this L ₁ and C ₁ constitute a resonant circuit. FIG. 2 is an explanatory diagram of electrical signals in the conventional drive system, where a is the buzzer drive signal applied from the inverter 1 to the base of the buzzer drive transistor 2, b is the buzzer drive signal induced at both ends of the boost coil 3,
The drive voltage c applied to the piezoelectric buzzer 4 is the collector current that flows when the buzzer drive transistor 2 is turned on.

In the case of a piezoelectric buzzer used in an electronic wristwatch, the period T of the buzzer drive signal a is T = 244.14μsec
(4096Hz = 2 ¹² Hz) is the most commonly used. This is because the frequency of the crystal oscillator, which is the time standard for electronic watches, is 32768 Hz (2 ¹⁵ Hz).
Since the frequency is divided by cascading 1/2 frequency dividers, 2 ⁿ (n
This is because there is a restriction that frequencies other than those (integer) cannot be used. Furthermore, there is a restriction that a signal with a period of 1/2 or twice the period T described above cannot be used due to the resonant frequency and tone color of the piezoelectric buzzer.

When such a buzzer drive signal with a period T is used, the drive voltage b applied to the piezoelectric buzzer 4 is equal to the second
The waveform will be as shown in b in the figure. During the period Ta, the buzzer driving transistor 2 is turned on, and a collector current as shown in FIG. 2c flows through the boosting coil 3. When the buzzer driving transistor 2 is turned off during the period Tb, a driving voltage b is induced across the boost coil 3, and an oscillating current with a frequency determined by the inductance _L1 and the capacitance _C1 flows, and the piezoelectric buzzer 4 is turned off. Drive. By repeating this operation, a buzzer sound is generated from the piezoelectric buzzer.

[Problem that the invention seeks to solve]

In such a conventional drive method, a driving force acts on the piezoelectric buzzer 4 during the period Tb, causing it to vibrate forcibly, but during the period Ta, the piezoelectric buzzer 4 vibrates without any force acting on it.
Moreover, during this period Ta, the drive voltage b is almost zero due to only the potential difference between the emitter and collector of the buzzer drive transistor 2, so the electric charge generated by the vibration of the piezoelectric buzzer 4 is neutralized. , acts as a brake to dampen vibrations of the piezoelectric buzzer 4. That is, in the conventional driving method, the operation of applying braking immediately after driving is repeated at a cycle T, which has the disadvantage that sufficient sound pressure (volume) cannot be obtained.

Furthermore, due to space constraints, many electronic wristwatches have recently adopted a method in which a thin piezoelectric element is attached to the back cover of the electronic wristwatch to create a piezoelectric buzzer. When worn on the arm, the sounding section comes into close contact with the arm, which has the disadvantage of further reducing the sound pressure (volume). Therefore, there is a strong desire to increase the sound pressure (volume).

However, since electronic wristwatches are subject to significant space constraints, it is almost impossible to improve the design by enlarging the piezoelectric element or changing the position of the sounding section. In addition, increasing the effective value of the drive voltage is an easy way to increase sound pressure (volume), but in the case of electronic watches, battery capacity is limited, so this increases power consumption. Such a method has the problem of shortening the battery life and significantly reducing the commercial value.

In view of the above points, it is an object of the present invention to provide a drive method that can increase sound pressure (volume) without increasing power consumption.

[Means for solving problems]

The buzzer drive circuit of the present invention provides a drive voltage having a frequency substantially equal to the resonant frequency by turning on and off a buzzer drive transistor at a predetermined period in a resonant circuit consisting of a piezoelectric buzzer having a predetermined resonant frequency and a booster coil. In the buzzer drive method of generating and driving the piezoelectric buzzer, a buzzer drive signal having a frequency of 2f/3 (where f is the frequency of the drive voltage) and a duty ratio of 1/3 is applied to the buzzer drive transistor. It is characterized by

[Effect]

In this way, by applying a buzzer drive signal with a frequency of 2f/3 and a duty ratio of 1/3 to the buzzer drive transistor to turn it on and off, the peak value of the collector current flowing through the buzzer drive transistor becomes In the same case as before, even though the average current per unit time is 2/3 of that of the conventional case,
The effective value of the drive voltage applied to the piezoelectric buzzer is approximately
It becomes 1.5 times. Also, the peak value of the collector current is set to 1.5
Even if it is doubled, the average current per unit time remains the same as before, but the effective value of the drive voltage further increases to 9/4 times, and the sound pressure (volume) increases.

〔Example〕

The present invention will be described in detail below based on Examples.

Embodiment 1 FIG. 3 is an explanatory diagram of electrical signals in an embodiment of the drive circuit of the present invention, in which the buzzer drive signal d applied to the buzzer drive transistor 2 shown in FIG.
As shown in Figure 3, the repetition period is 3T/
2, and the pulse width (that is, the time for turning on the buzzer driving transistor 2) is T/2.
Therefore, the duty ratio (pulse width/period) of this buzzer drive signal is 1/3. As mentioned above, since T is normally 244.14 μsec (2 ¹² Hz), the period of this buzzer drive signal d is 366.21 μsec.
sec (2730.67Hz=2 ¹¹ /3Hz), which is 2/3 of the frequency of the conventional buzzer drive signal a.
Therefore, to create this signal, inverter 1
It is sufficient to further divide the frequency of the divided output one step before the 1/2 cycle to which the signal is applied to by 1/3. This 1/3 frequency divider can be easily constructed using C-MOS, and there is no problem in terms of power consumption, which only increases slightly.

Further, the values of inductance L ₁ and capacitance C ₁ in FIG. 1 are designed so that T≈2π√ _{1 1} . As a result, the frequency of the driving voltage becomes 1/T. At this time, the waveform of the driving voltage applied to the piezoelectric buzzer 4 is as shown in FIG. 3e. Further, the waveform of the collector current flowing through the buzzer driving transistor 2 is as shown in FIG. 3f. The peak value I _T1 of the collector current f is the same as the conventional collector current shown in FIG. 2, but if both have the same sawtooth waveform, the average current per unit time will be 2/3. On the other hand, the peak value V _c1 of the drive voltage e is also the same as b in FIG. 2, but the effective value is about 2/3 times as large. In other words, greater sound pressure (volume) can be obtained with less power consumption than before.

Embodiment 2 FIG. 4 is an explanatory diagram of electric signals in another embodiment of the drive circuit of the present invention, in which the buzzer drive signal d
is the same as in Example 1. In this embodiment, as the buzzer driving transistor 2, a transistor through which a larger collector current flows than the conventional one and the one in the first embodiment is used, and as shown in FIG. 4h, the peak value I
_T2 was set to approximately 1.5 times I _T1 in FIGS. 2 and 3.
In this case, the average current per unit time is
It will be almost the same as Figure c. On the other hand, the peak value V _c2 of the driving voltage g is also approximately 2/3 times as large as V _c1 , and therefore the effective value is approximately 9/4 times as large. If the sound pressure of a piezoelectric buzzer is proportional to the applied voltage, it is possible to increase the sound pressure by about 7 dB with the same power consumption as before.

〔Effect of the invention〕

As described above, in the buzzer drive circuit of the present invention, the buzzer drive transistor has a frequency that is 2/3 of the frequency of the drive voltage generated in the resonant circuit consisting of the piezoelectric buzzer and the booster coil, and a duty ratio of 1/3. By applying a buzzer drive signal to turn it on and off, a higher sound pressure (volume) than before can be obtained without increasing power consumption or changing the shape, dimensions, or mounting position of the piezoelectric buzzer. It has the effect of
Especially as a drive method for piezoelectric buzzers in electronic watches.
This has many practical effects.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a specific example of a drive circuit for a piezoelectric buzzer. FIG. 2 is an explanatory diagram of electrical signals in a conventional drive system. FIG. 3 is an explanatory diagram of electrical signals in the embodiment of the drive circuit of the present invention. FIG. 4 is an explanatory diagram of electrical signals in another embodiment of the drive circuit of the present invention. 1... Inverter, 2... Buzzer drive transistor, 3... Boost coil, 4... Piezoelectric buzzer,
a, d...buzzer drive signal, b, e, g...drive voltage, c, f, h...collector current.

Claims (1)

[Claims] 1 A resonant circuit consisting of a piezoelectric buzzer having a predetermined resonant frequency and a booster coil is turned on and off at a predetermined period to generate a drive voltage having a frequency approximately equal to the resonant frequency, and the piezoelectric buzzer A buzzer drive method for driving a buzzer drive system, characterized in that a buzzer drive signal having a frequency of 2f/3 (where f is the frequency of the drive voltage) and a duty ratio of 1/3 is applied to the buzzer drive transistor. circuit.