Piezo sensor in burglar alarm

Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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The main element of the sensor is a piezoelectric element, supplemented by an inertial transducer. The design of the sensor is shown in fig. 1. An acoustic piezoelectric emitter 1 with an open lining is attached to the printed circuit board 4, to which an M-shaped wire support 3 is soldered. An elastic spoke 2 is soldered to it, one end of which is flattened, bent into a semicircle and soldered to the board 1, and a load is fixed on the other 5.

Fig. 1

In the author's copy of the sensor, the spoke was made of steel wire with a diameter of 0,8 mm (paper clip), its flattened end had a thickness of 0,2 ... 0,25 mm, load 5 was a lead cube weighing 3 g. The experiment showed that the resonant frequency of such a converter Fpez = 23 Hz.

This design converts the vibrations of the load into a variable pressure on the piezoelectric element. The transducer responds to short shocks and shocks with exponentially damped oscillations (with frequency Fpez) of the voltage on the piezoelectric element (Fig. 2), the initial amplitude Ua of which depends on the acting force. If this signal is applied to one of the inputs of the comparator, and the exemplary (threshold) voltage Up is applied to the second, then a "pack" with a duration Tp of N = Frez-Tp pulses will be formed at its output.

Fig. 2

Obviously, with weak and rare shocks and shocks, a smaller number of pulses will be generated at the output of the comparator than with strong and frequent ones, and with high activity (crime?) It can exceed a certain limit. A schematic diagram of a device that generates an alarm signal (log. 1) in response to transcendental excitation of the sensor is shown in fig. 3.

Fig. 3

A comparator is assembled on a micropower op-amp DA1, its switching threshold Up is set by a tuning resistor R4. At rest, the voltage at the inverting input of the op-amp DA1 exceeds the voltage at the non-inverting one by 0,3 ... 3 mV, so a low level is set at its output - log. 0.

When an alternating voltage appears on the piezoelectric element B1 with an amplitude sufficient to switch the op-amp DA1, a “package” of pulses is formed at its output, which, after being inverted by the logic element DD1.3, will go to the input C of the counter DD2 and the input of the single vibrator (pin 6 DD1.1), assembled on elements DD1.1, DD1.2. This single vibrator generates a pulse with a duration Tact = 0.7*С1*R8 = 7 s, which sets the time interval for the active operation of the sensor - the duration of the pulse counting cycle. At the end of this interval, a short (t = 1.4 * R0.7 * C9 = 3 ms) pulse is formed at the output of the DD14 element, resetting the counter DD2.

Since the high logic level signal (alarm) at the output of the 2P counter DD2 occurs only with the receipt of the 2p-th pulse in the counter, the sensor threshold depends on which of the outputs of this counter is enabled. If this is done as shown in Fig. 3, then an alarm at "Output 1" will occur when the 2th pulse arrives at the input C of the counter DD64. Another sensor output ("Output 2") - an open drain of the field-effect transistor VT1 - will allow you to connect a load to it that has its own power source.

All elements are mounted on a printed circuit board made of double-sided foil fiberglass 1,5 mm thick, the drawing of which is shown in Fig. 4. Part of the foil on the side of the installation of the elements is used as a common wire. Connections to it "grounded" terminals of resistors, capacitors and other parts are shown with black dots. Protective circles with a diameter of about 2 mm should be etched in this foil in places where the leads of the elements are skipped (not shown in Fig. 4). The junctions of the metallized sections of the first and second sides of the board are shown as black squares with a light dot in the center.

Fig. 4

Fixed resistors are used: R8 - KIM, C3-14, the rest - MLT, C2-23, tuning resistor - SPZ-19a. The oxide capacitor is imported with a low leakage current, the rest are KM-6, K10-17. Piezoelectric element B1 - acoustic emitter ZP-19. Its base is soldered to specially designed contact pads on the board. To prevent the load from touching the board, a cutout can be made in it (shown in dotted line in Fig. 4). The assembled board is installed in a tin box from under the bouillon cubes and fastened with screws at three points. In this design, the sensor will have overall dimensions of 82x35x15 mm.

But from which of the outputs of the DD2 counter should the alarm be removed and what should be the time interval for the active operation of the Tact sensor? First of all, the condition must be met.

Fpez*Tact > 2 to the power of n, where 2 is the name of the counter output DD2. Otherwise, the sensor output alarm will not occur even if the piezoelectric element is continuously excited. For the ratings of the elements indicated on the diagram, this condition is met, since.

Frez * Cycle = 23 * 7 = 161, and the sensor output could be any of the four outputs of the counter DD2: 2 to the power of 4, 2 to the power of 5, 2 to the power of 6 or 2 to the power of 7 (2 to the power of 7 = 128 < 161). The highest sensitivity (and to interference too) will have a sensor in which the alarm signal is removed from output 2 to the power of 4, and the most noise-immune - from output 2 to the power of 7.

If the sensor must respond to short-term single impacts, the interval Tact should also be close in time. But if there is no such restriction, it is recommended to increase the Tact. This is due to the fact that as Tact increases, the probability of a false alarm occurring decreases. But the circuit solution used here will allow increasing the Tact only up to 35 ... 40 s, since, as experience has shown, the resistance of the resistor R8 should not be more than 30 MΩ, and the maximum capacitance of the capacitor C2 (ceramic or film) usually does not exceed 2,2 μF . The use of an oxide capacitor is undesirable, since its leakage current is much higher than that of a ceramic one. The threshold for the comparator is set by a trimming resistor R4.

With a "soft" shock, the amplitude of the signal on the piezoelectric element may be too small, so a significant increase in the sensitivity of the sensor to such shocks can result in a decrease in Fres. This can be achieved by increasing the weight of the load. The experiment showed that with a load mass of 5, 9, and 15 g, the resonant frequency was 18, 13, and 9 Hz, respectively.

"Exit 2" may be required not only to coordinate with a "foreign" security system. It is also suitable for direct control of a powerful load, such as an audible signaling device (siren) or an incandescent lamp. When setting up, a low-power sound signaling device with a built-in generator, such as HPM14AX, can be connected to this output. The extremely low power consumption in standby mode allows the use of a small-capacity galvanic lithium battery for power. It will outlive, most likely, the security system itself.

Publication: radioradar.net

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