JPH0157919B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a volume adjustment circuit for a piezoelectric buzzer.

Generally, a drive circuit using a self-oscillating piezoelectric buzzer is as shown in FIG. 1, and has a feedback electrode on the piezoelectric buzzer body, forming a modified circuit of a grounded-emitter Hartley oscillator. In this case, the voltage V applied to the feedback electrode deviates from V _p by 180°, and V/V _p ≧ R ₂ + h _ie1 /h _fe1・R ₃ (however, the input resistance of the transistor is h _ie1 and the current amplification h _fe1 )
There are circuit conditions such as satisfying the following conditions, making sure that the DC bias point of transistor T _r1 is 1/2 of the power supply voltage, and adjusting resistor R ₂ to prevent spurious oscillation.

Next, a conventional example of a volume adjustment circuit for a piezoelectric buzzer will be described with reference to FIG.

In Fig. 2, 1' is a microcomputer;
OUT indicates its output, and 2' is a piezoelectric buzzer drive circuit including a self-oscillating piezoelectric buzzer and a drive circuit, and the +V terminal of the circuit shown in FIG. 1 is connected to the semi-fixed resistor VR'.

As explained in FIG. 1, in FIG. 2, the voltage applied to the piezoelectric buzzer drive circuit 2' is adjusted by the semi-fixed resistor VR'. In this case, the power supply impedance seen from the piezoelectric buzzer drive circuit 2' is at least larger than the impedance of the semi-fixed resistor VR'. Since the current waveform flowing through the piezoelectric buzzer drive circuit 2' roughly approximates the oscillation waveform, the piezoelectric buzzer drive circuit 2'
There will be large fluctuations in the voltage applied to the As a result, the terminal voltage +V shown in Figure 1 changes due to changes in the current flowing through the piezoelectric buzzer under the circuit conditions explained in Figure 1, and as a result, the rate of change in the current flowing through the fixed resistor _R3 is fixed. It will be greater than the rate of change of the current flowing through resistor _R1 . Therefore, the voltage V ₀ applied to the positive electrode becomes a larger fluctuation voltage than the voltage V _f applied to the feedback electrode, and since the piezoelectric buzzer is capacitive, the voltage applied to the positive electrode V _p and The phase of the voltage V _f applied to the feedback electrode fluctuates. In addition, since h _ie1 and h _fe1 of the transistor vary depending on the operating current, the V _f /V _p value varies,
Furthermore, the operation becomes unstable, leading to spurious oscillation. As a result, the distortion of the oscillation sound increases, and in severe cases, the oscillation may stop.

In order to eliminate such problems, the present invention focuses on the power supply impedance seen from the piezoelectric buzzer, and aims to provide a circuit that provides a minimum power supply impedance at any setting point for adjusting the volume.

An embodiment of the present invention will be described below with reference to the piezoelectric buzzer volume adjustment circuit shown in FIG. 1 is a P-channel microcomputer, which serves as a controller, and is connected from the open drain output terminal OUT, which outputs an intermittent waveform, to ground via a semi-fixed resistor VR, a fixed resistor _R4 , and an NPN type resistor connected to the midpoint of both. The base of transistor TR ₂ is connected to it. This constitutes a kind of emitter follower circuit. That is, by outputting a pulse waveform to the output terminal OUT of the microcomputer 1, the drive circuit of the piezoelectric buzzer is controlled and an intermittent sound is generated. Since the power consumption of the _{piezoelectric} buzzer is extremely small, if the values of the semi-fixed resistor VR and the fixed resistor _R4 are made small to some extent, the base voltage _VB of the transistor _TR2 becomes _VB≈R4 /VR+ _R4V . In other words, the base potential V _B of transistor TR ₂ is the semi-fixed resistor VR
It becomes variable by changing. Further, the value obtained by subtracting the base-emitter potential V _BE of the transistor TR ₂ appears as the emitter voltage _VE . The emitter voltage V _E becomes a kind of stabilized power supply. The voltage applied to the piezoelectric buzzer drive circuit 2 in this way
V _E will be adjusted. On the other hand, the power supply impedance R seen from the piezoelectric buzzer drive circuit 2 is equivalently R if the voltage +V is a low impedance.
≒V _R R ₄ /h _fe2 . Here, h _fe2 is the current amplification rate of the transistor TR ₂ . Since this current amplification factor h _fe2 can usually be set to 100 or more, the power source impedance is dramatically lowered than in the conventional example.

As is clear from the description of the embodiments above, the piezoelectric buzzer volume adjustment circuit of the present invention includes a controller that outputs an intermittent waveform, an NPN transistor whose collector terminal is grounded to the power supply, and a variable resistor and a fixed resistor. Connect the devices continuously and connect the intermediate point to
Connect to the base terminal of the NPN transistor, connect both ends to the output terminal and ground terminal of the controller, and connect a piezoelectric buzzer oscillation circuit including a piezoelectric buzzer between the emitter terminal of the NPN transistor and the ground terminal. Therefore, it is possible to obtain a piezoelectric buzzer drive circuit that can vary the volume and has distortion-free sound quality with a simple circuit configuration. If a microcomputer is used as the controller, various controls can be easily configured by effectively combining other functions and the piezoelectric buzzer.

[Brief explanation of drawings]

Fig. 1 is a drive circuit diagram of a conventional self-oscillating piezoelectric buzzer, Fig. 2 is a volume adjustment circuit diagram of a conventional piezoelectric buzzer, and Fig. 3 is a volume adjustment circuit diagram of a piezoelectric buzzer according to an embodiment of the present invention. . 1...Microcomputer (controller),
2...Piezoelectric buzzer drive circuit, TR ₂ ...NPN type transistor, V _R ...semi-fixed resistor, _R4 ...fixed resistor.

Claims (1)

[Claims] 1. A controller that outputs an intermittent waveform, an NPN transistor whose collector terminal is connected to a power source, a variable resistor and a fixed resistor connected in cascade, and the midpoint between them connected to the base of the NPN transistor. volume adjustment of a piezoelectric buzzer, the piezoelectric buzzer is connected to a terminal, both ends are connected to an output terminal and a ground terminal of the controller, and a piezoelectric buzzer drive circuit including a piezoelectric buzzer is connected between an emitter terminal of the NPN transistor and the ground terminal. circuit. 2. The piezoelectric buzzer volume adjustment circuit according to claim 1, wherein the controller is a microcomputer.