Acoustic motion sensor. Scheme, description

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Acoustic motion sensor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Safety and security

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The operation of many burglar alarm systems is based on a very simple principle: there should be no movement in the protected area at odd hours. To detect it, the room is "filled" with radiation - most often radio or acoustic. Having repeatedly reflected from the walls and objects in the room, the rays reach the receiver. Any change in the environment will cause the modulation of the received signal, which will fix the sensor.

Acoustic (ultrasonic) sensors of this type have a rather significant advantage over those using radio waves - they do not emit anything into the "air", they do not require permits for installation and operation. Readers are offered a description of one of these sensors, relatively simple and sensitive enough to protect a room up to 20 m2.

Unlike acoustic sensors, descriptions of which were previously published in the journal "Radio" [1 - 3], the proposed one operates according to a slightly different principle, protected by a patent [4].

Main Specifications

Sound frequency, kHz ...... 10
Radiated acoustic power, mW, no more......5
Supply voltage (constant), V......10...16
Power consumption in standby mode, mW......120
Dimensions, mm......150x50x30

The output circuit is the "dry" relay contacts, in addition, the operation is signaled by the lighting of the LED.

The scheme of the device is shown in fig. one.

Acoustic motion sensor
(click to enlarge)

A piezoelectric microphone BM1.1 is connected to the input of the amplifier on the op-amp DA1.2 and DA1, and a piezoelectric sound emitter BF1 is connected to the output. As a result, the amplifier is covered by acoustic feedback through a controlled gas volume, due to which self-oscillations occur in the system. Their frequency depends on the frequency response and phase response of the elements (primarily the microphone and emitter) and on the acoustic properties of the protected premises. The oscillation amplitude is maintained by a constant AGC system from a detector on diodes VD2, VD3 and an amplifier on one of the elements of the DA2 K176LP1 microcircuit. The AGC control elements are separate field-effect transistors available in the same microcircuit, the drain-source sections of which are included in the local feedback circuits of the cascades on the op-amp DA1.1 and DA1.2.

If any object (intruder) moves in the sensitive zone of the sensor, the attenuation and delay of the acoustic waves reflected from it change, which leads to a change in the amplitude of the oscillations generated by the sensor. The R7C10 and R6C1C6 circuits set the frequency characteristics of the AGC loop, which are necessary for the stable operation of the sensor in various conditions while effectively tracking changes in the signal amplitude.

The variable voltage component at the output of the AGC amplifier, caused by movement, is fed to the input of the DA1.3 comparator. The response threshold is set by a trimming resistor R8. The HL1 LED is connected to the output of the comparator through a buffer amplifier of two elements of the DD1 microcircuit connected in parallel, flashing indicating movement in the protected room.

In addition, the signal from the outputs of the elements DD1.1 and DD1.2 starts a single vibrator on the elements DD1.3 and DD1.4, the pulses of which open the key on the transistor VT2, causing relay K1 to operate. The single vibrator generates pulses only if the input 13 of the DD1.4 element is a high logic level. Thanks to the R14C16 circuit, this level will not be reached until some time after power-up, giving the sensor the opportunity to enter a steady state without giving alarms.

If alarm pulses are repeated too often, capacitor C16 is discharged through resistor R16 and diode VD5, which blocks the start of the single vibrator and prevents unnecessary trips of relay K1. Thus, a significant saving of the relay resource and power consumption is achieved.

The supply voltage stabilizer is built according to a somewhat unusual scheme with a regulating transistor VT1 in the negative circuit, which made it possible to reduce the number of parts in the device. Diode VD1 protects against reverse polarity connection to the power source.

The appearance of the sensor is shown in fig. 2. It is assembled on a printed circuit board placed in an insulating material such as polystyrene. The BM1 microphone and the BF1 transducer are installed on the top cover of the case, acoustically isolated from the case and from each other with the help of foam rubber washers 3 mm thick. The greater the distance between the emitter and the microphone, the higher the sensitivity of the sensor. In the author's design, it was 100 mm. The same cover has a hole for the HL1 LED.

Acoustic motion sensor

As BF1 and BM1, the same VUTA-1 piezoelectric transducers manufactured by the Alfa-Optim enterprise (Volgograd) were used. Replacing them with higher-frequency and more sensitive ones is desirable, but this will require some modifications to the sensor that change the frequency characteristics of the auto-generation circuit.

The sensor is equipped with oxide capacitors K50-35, ceramic capacitors K10-17, resistors MLT-0,125, relay RES55A (passport RS4.569.600-01). Transistors KT361B can be replaced with KT361G, KT361E and other low-power pn-p silicon structures.

When adjusting the sensitivity of the sensor (with a tuning resistor R8), sometimes it is necessary to swap the conclusions 12 and 13 of the DA1.3 element to achieve the desired result.

Literature

Will W. Ultrasonic autoguard. - Radio, 1996, No. 1, p. 52-54.
Volkov A. Ultrasound sensor for security alarms. - Radio, 1996, No. 5, p. 54-56.
Koinov A. Ultrasonic security device. - Radio, 1998, No. 7, p. 42.
Guskov V., Guskova M. A method for determining the change in the state of a volume filled with an elastic medium, and devices (options) for its implementation. - Patent of the Russian Federation No. 2104494 MKI 6G 01D1 / 18, declared on January 26, 1995, published on February 10, 1998

Authors: V.Guskov, V.Sviridov, Samara

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