Siren from the sound annunciator Aurora. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The source of a powerful sound signal, necessary for watchdog and signaling devices, can be the Aurora annunciator, the main part of which is a piezoelectric sound emitter. But it needs a voltage converter, the arrangement of which is described in the article.

The Aurora annunciator is rather small-sized, economical, and the sound pressure generated by it exceeds 100 dB. which is very unpleasant and even painful for a person's hearing. Since the siren based on the sound emitter of the annunciator is supposed to be installed on objects with autonomous power supply (battery, etc.), it must operate at a supply voltage of 10 ... 12 V. The design of just such a siren is offered to the attention of readers.

First, some information about the emitter. Experiments show that it is capable of quite effectively emitting sound frequencies from hundreds of hertz to tens of kilohertz, but has a maximum radiated power at frequencies of 2...3 kHz. Therefore, if necessary, it can be used, for example, as a subscriber loudspeaker by connecting directly to the radio network without any matching devices. The volume of the sound signal will be sufficient to listen to the 1st program, but the sound will not be very pleasant due to the large unevenness of the amplitude-frequency response.

To obtain the maximum level of the sound signal, an alternating voltage of 150 ... 220 V must be supplied to the emitter, therefore, the main unit of the siren must be a voltage converter. Since the emitter has a relatively large capacitance - 22 pF, a DC-to-AC converter is required that can operate on a capacitive load.

A schematic diagram of such a converter is shown in Fig.1. It operates at the resonant frequency of the emitter (approximately 2...2,5 kHz). The converter consists of a multivibrator on an operational amplifier (op-amp) DA1, which controls an electronic key on a transistor VT1. A step-up transformer T1 is included in the key circuit - an emitter HA1 is connected to its secondary winding.

The device uses a unipolar power supply, therefore, to ensure the normal operation of the op-amp, it is powered using the so-called midpoint - it is formed by a voltage divider across resistors R1, R2.

The parameters of the transformer are chosen such that the inductance of its secondary winding, together with the capacitance of the radiator, constitutes an LC circuit tuned to the resonant frequency of the radiator.

All parts of the device are placed on a printed circuit board (Fig. 2) made of one-sided foil fiberglass, which is fixed on the upper wall of the housing (Fig. 3).

The emitter HA1 is mounted on the bottom wall (Fig. 4).

The device can use the K140UD6, K140UD7 or similar low-power op-amps. Capacitors C1, C2 - KLS, KM, K10 - 17, C3 and C4 - K52, K53, K50 - 6. Fixed resistors - MLT, C2 - 10, tuning resistors - SDR - 19a, and when changing the drawing of the printed circuit board, any others will do, for example SP5-2, SPZ-3. The diode can be any of the KD103, KD105 series, the transistor - any of the KT827, KT834 series.

The transformer is wound on a ring of size K32x16x8 made of ferrite 2000NM1: winding I contains 50 turns of PEV wire - 2 0,6 ... 0,8, winding II - 750 turns of PEV wire - 2 0,12 ... 0,15. The voltage on the secondary winding reaches 150 ... 200 V, so it should be done carefully and in the future, when setting up the device, observe electrical safety measures.

In the manufacture of the transformer, it is necessary to break the ring in half, round off the sharp edges with a needle file and wrap it with a layer of varnished cloth or electrical tape. On each part, you need to wind half of the secondary winding, and then on one of them - the primary, and then glue the ring with BF-2 glue, laying thin paper spacers between the parts. Winding should be carried out carefully, and so that the turns of only one hundred overlap.

Set up the device in the following sequence. First, you should tune the circuit formed by the winding II of the transformer and the capacitance of the emitter to the resonant frequency of the latter. To do this, pre-determine the frequency at which the sound output of the emitter is maximum. Having disconnected the emitter from the transformer and connected it to the 3-hour generator, a signal with an amplitude of 0,5 ... 1 V is supplied from the generator. By rebuilding the generator, the frequency at which the sound volume is maximum is determined.

After that, a signal with an amplitude of 0,05 ... 0,1 V is applied from the generator output to the I winding (it is disconnected from the transistor) of the transformer. An emitter and an AC voltmeter are connected to winding II. Rebuilding the generator, determine the frequency of electrical resonance at which the alternating voltage is maximum. If this frequency turned out to be lower than the frequency of the maximum sound output, then the number of turns of the winding II must be reduced by several tens and after each change, re-control the resonant frequency. If the frequency is higher, the number of turns should be added.

Next, connect the transformer to the transistor, apply power and carry out the final adjustment. Resistor R4 sets the duty cycle of the current pulses through the key, resistor R5 - the generation frequency. First, the engine R4 is placed in the middle position, and the frequency at which the sound volume is maximum is set with the resistor R5. By shifting the slider of the resistor R4 to the left according to the scheme, it is possible to reduce the duration of the current pulses through the transformer, thereby reducing the volume of the audio signal, shifting to the right - to increase the volume of the signal.

All manipulations with the resistor R4 lead to a change in the generation frequency, therefore, after each change in its position, it is necessary to set the maximum signal volume again with the resistor R5.

During the adjustment, the volume of the beep is so high that it is sometimes impossible to endure. Therefore, the emitter must first be wrapped with some kind of sound-absorbing material, such as a towel.

The device can be powered from any, including an unstabilized source with a voltage of 9 ... 30 V. The current consumption at a voltage of 12 V, depending on the volume of the signal, can reach 100 ... V00 mA. If the voltage differs from 12 V, the number of turns of winding I should be changed accordingly. For example, if the supply voltage is doubled, the number of turns should be increased by the same amount.

To protect the transistor from voltage surges, it is desirable to include a zener diode between its collector and emitter (cathode to the collector) with a stabilization voltage of 50 ... 70 V.

Author: I.Aleksandrov, Kursk

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