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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Vibration sensor for car security device

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Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms

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The basis of the sensor is a piezoelectric element from the sound emitter ЗП-2, ЗП-4 or ЗП-5. The general view of the sensor (from the side) is shown in Fig. 1a. The piezoelectric element 2 of one of the plates is soldered to the foil pad of the printed circuit board 1. The rack 2 is soldered to the top plate of the piezoelectric element 4 in the figure, bent in the form of the letter L from an elastic steel wire with a diameter of 0,5 mm. View of rack 4 along arrow A is shown in fig. 1,6. The legs and saddle of the rack must be irradiated in advance.

The console 3 is bent out of the same wire and a load 5 weighing 10...15 g of lead or solder is securely fixed at one of its ends. After that, the console is soldered with one end to the board, and approximately in the middle - to the saddle of rack 4.

In order to avoid separation of the upper plate from the piezoelectric element, before soldering the console, it is slightly bent so that, after being put in place, it creates an excessive pressing elastic force on the piezoelectric element. The dimensions of the sensor parts are not fundamental, therefore, they are not given in Fig. 1. It is necessary to solder with low-melting solder.

Vibration sensor for car security device. Sensor

The sensor outputs are a foil pad, to which a piezoelectric element is soldered, and a console base soldered into the board. The board is fixed on the surface, the vibration of which must be controlled. When this surface mechanically vibrates, several weak pulses with a duration of 15 ... XNUMX ms appear at the sensor terminals.

Vibration sensor for car security device. Scheme
(click to enlarge)

In order to amplify these pulses and give them the shape necessary for further processing, the signal from the sensor is fed to the input of the shaping amplifier (see diagram in Fig. 2). Operational amplifier DA1 operates in maximum gain mode, and transistor VT1 - in switching mode. Diode VD1 increases the dead zone of the transistor with its cutoff voltage.

The op-amp, together with a diode and a transistor, form a voltage comparator, characterized by low power consumption. The threshold for the comparator is set by a trimmer resistor R2. If the amplitude of the negative half-wave of the sensor signal is less than the voltage across the resistor R2, the transistor VT1 remains closed, and the output voltage is zero.

Mechanical excitation of the sensor leads to the appearance at the output of the shaper of several rectangular pulses with a duration of 3 ... 15 ms, in amplitude suitable for their direct introduction into a digital analyzer made on CMOS microcircuits. The simplest such device capable of isolating a useful signal against the background of false positives is a counter (001 in Fig. 2), periodically reset at the input R by pulses of an electronic clock or a special generator. An alarm signal - a high level voltage - will appear at the output only when the number of pulses at the counter input in the interval between two adjacent zeroing pulses reaches a certain number set by switch SA1 (in Fig. 2 it is set to eight).

If you do not set a solution to the problem of eliminating false signals, then the signal from the collector of the transistor VT1 can be applied directly to the input of the alarm generating unit.

As experience shows, the sensor practically does not respond to acoustic signals propagating in the air. Sensitive primarily to the normal component of vibrations, it also perceives quite well perturbations lying in the plane of the piezoelectric element, apparently due to the occurrence of a reaction at the points of attachment of the rack. Thus, the sensor responds to vibrations of arbitrary orientation. The current consumed by the conditioning amplifier in standby mode at a supply voltage of 9 V does not exceed -18 μA, at 5 V - 10 μA.

Publication: cxem.net

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