Piezo siren in burglar alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Safety and security

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The high price of branded piezo sirens (especially with a built-in power supply) hinders their mass use in amateur radio practice. But you can make such a siren yourself. If you supplement it with a switch of a special design, you get a simple security system that is quite suitable for protecting personal belongings.

The basis of the siren is the piezoelectric element SP-1 (SP - piezoceramic siren). The sound pressure developed by it at an excitation voltage of 40 V can reach 110 dB. On fig. 1 shows a diagram of an electronic converter that provides the required operating mode for SP-1. A low-frequency generator is assembled on the transistor VT1 and transformer T1, which is excited at a frequency that depends on the reactance of the piezoelectric radiator and the inductance of the step-up winding. The amplitude of the alternating voltage on the piezo emitter can be calculated by the formula Uampl = Upit N1/N2, where Upit is the supply voltage of the device; N1 and N2 - the number of turns of the windings l and ll.

Acoustic radiation acquires an alarming character only after a special modulation of a monotonous signal. This is done by an electronic manipulator on the transistor VT2, included in the generator power circuit. The generator controls the manipulator, assembled on the elements DD1.1 and DD1.2. The frequency of its operation depends on the time constant of the R4C1 circuit, and at the values ​​\u5b\u6bspecified in the diagram, it is about XNUMX ... XNUMX Hz.

Transformer T1 - output from a transistor receiver (for example, B 4: 731.083) or similar with a transformation ratio of 5 and an output from the middle of the primary winding. The DD1 chip can be replaced with a K561LE5 or a similar K176 series. Transistors VT1 and VT2 must have a current gain of at least 100 and a saturation voltage of less than 0,3 V. All resistors are MLT-0,125. Capacitors C1 - KM6; C2 - any electrolytic suitable size, its capacity may be more than indicated. Diode VD1 - any silicon.

If the siren assembled without errors did not sound, the reason should be sought in the incorrect phasing of the transformer windings. The siren must be immediately turned off (the current consumed in this mode can be dangerous for transistors) and, having swapped the ends of one of the windings, turn it on again. You can experiment with resistors R1 and R2, especially if a different type of transformer is used. In this case, it is better to adjust with an oscilloscope.

The power of the acoustic signal emitted by the siren largely depends on its design, i.e., on the size and configuration of the case, the presence of acoustic "sinuses" in it, on the place and method of attaching the piezoelectric emitter, etc. But all this is usually found out experimentally. Acoustic calculations are hardly useful here.

Structurally, the siren can be made as shown in Fig. 2.

Its dimensions are 58x58x38 mm, weight - 95 g (with "Korund" battery). The case is glued from impact-resistant polystyrene 2...2,5 mm thick. It is recommended to seal the holes on the periphery of the SP-1 holder. This will significantly increase the volume of the siren. On fig. 3 - view of the siren with the cover removed.

The table shows the dependence of the current Iconsumed by the siren and the voltage on the piezoelectric emitter Ueff (effective value) on the voltage of the power supply Upit.

The siren can work in a wide variety of devices and installations, even as a doorbell. Small size and weight allow it to be used to protect personal belongings and passenger luggage. To do this, you only need to insert the switch shown in Fig. 4. Here: 1 - normally closed contact pair from the relay; 2 - check - a thin plate of electrical insulating material (getinaks, fiberglass, etc.), opening contacts; 3 - switch body; 4 - thrust (string, thin steel cable, etc.), extracting the pin from the contact pair.

A piezo siren with a switch is placed inside a protected object - in a suitcase, in a coat pocket, etc. The rod is passed through a suitable or specially made hole and connected to a luggage rack, a hanger, etc. When the stolen item is moved, the check leaves the contact pair, the siren's power is turned on and the attacker with a "screaming" object in his hands is in the center of attention of others.

Author: Yu.Vinogradov, Moscow

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