Ultrasonic security device. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Ultrasonic security device. Encyclopedia of radio electronics and electrical engineering

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

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In security systems for premises, various sensors are used. Feature of ultrasonic - ease of installation. When used indoors, there is no need to lay a security loop.

The device consists of a motion sensor, a sound signal and an autonomous power supply united in one housing. It can protect a room up to 20 square meters. Place it on the wall inside the protected object. An audible signal is triggered when an object is moved, with a short warning beep first. If the owner entered the room, this signal will warn him that the device has worked and needs to be turned off. If this is not done, then in a minute the device will give a loud beep sounding for several minutes, and then switch to armed mode again.

The displacement sensor includes an acoustic emitter and a receiver. The emitter generates a signal in the ultrasonic range of stable amplitude and frequency. It is desirable to choose a frequency within 25 ... 35 kHz. Sound waves propagate in all directions from the emitter and enter the receiving sensor in different ways. The direct signal goes directly from the transmitter to the receiver. In addition, signals reflected from surrounding objects are received at the input of the receiving sensor. The amplitude and phase shift of the reflected signal relative to the direct signal have a random but constant value and depend on the size of the room, the location of the sensor and the objects in the room.

In the receiving sensor, the direct and reflected signals are mixed, forming a total received signal of a certain amplitude. When moving at least one object, which hits the sound wave, the phase and amplitude of the reflected signal change. Moving the reflective surface by approximately 1 cm will change the phase of the reflected signal by 180°, so a long movement of the reflective surface will cause the total received signal to ripple at a frequency of 1 to 100 Hz, depending on the speed and direction of movement. When this kind of ripple appears in the received signal, an alarm device is triggered and an audible signal is given.

The diagram of the device is shown in the figure. The emitter generator is built according to the capacitive three-point scheme. The emitter BQ1 is included in the feedback circuit of the transistor VT1.

(click to enlarge)

The oscillation frequency of the generator depends on the resonant frequency of the emitter BQ1 and the parameters of the circuit L1 C1. The radiation power is regulated by the selection of the resistor R3, and the frequency is adjusted by the selection of the capacitor C1.

The receiver consists of an ultrasonic microphone BM1, an amplifier of the received signal on the op-amp DA1.1, a detector on the elements R11, VD2, C8, R13, an amplifier of the detected signal on the op-amp DA1.2 and a transistor switch VT2VT3. The parameters of the detector are selected in such a way that the suppression of the carrier frequency in the range of 25...35 kHz is maximum, and the attenuation of low-frequency ripples 1...100 Hz is minimal. Circuit C7R12C9R14 sets the gain and bandwidth of the op amp DA1.2. When an alternating voltage appears at its output, a positive half-wave through the capacitor C10 opens the transistor switch VT2VT3, and a negative half-wave through the diode VD3 recharges the capacitor C10.

The signaling device includes a Schmitt trigger on the elements DD1.1, DD1.2, a control unit on the elements DD1.3, DD1.4, a current amplifier on transistors VT5, VT6, a thyristor VS1 and a sound signal emitter BF1. When the power is turned on, the capacitor C12 is charged. After about 1 ... 1.5 minutes, a high level occurs at pin 2 of the DD1.1 element. Now, if the displacement detector is triggered, transistors VT2, VT3 and VT4 open, a high level at pin 1 of the DD1.1 element will switch the trigger. A low level will occur at the output of DD1.1, and a high level at the output of the trigger (pin 4 of DD1.2). Circuit C13R23 sets the duration of a short beep - 0,1 s, and circuit R21C14 - a delay in the supply of a long beep - 60 s. The R20C12 circuit determines the duration of the beep and the delay in the operation of the device after power is turned on.

The consumed current in standby mode does not exceed 70 mA, and in the sound signal mode - 1 ... 2A.

Bimorph piezoelectric elements tuned to the same resonant frequency, for example, 1 kHz, are used as the BQ1 emitter and BM34 receiver. The distance between the piezoelectric elements should be 3 ... 5 cm. Between them, it is necessary to lay a soundproof gasket made of foam rubber. In principle, if there are no bimorph piezoelectric elements, you can use an ordinary high-frequency dynamic head and microphone, while reducing the radiation frequency down to 10 kHz. But this will worsen the noise immunity of the device, as the frequency selectivity of the receiver will worsen. The emitted sound will also be heard, but for the protection of small enclosed spaces, objects, such as a car, sensitivity will be quite enough, and sound radiation will shield the car body well. In this case, the design of the generator must be changed.

The sound siren BF1 is a car signal with a current consumption of 1 ... 2 A. The coil L1 is wound on a ferrite ring of the M2000 brand with dimensions of 20x12x6 and contains 100 turns of PEV-0,3 wire with a tap from the middle. The body of the device must be made with a margin of safety and securely fixed to the wall inside the protected premises.

Establishment begins with setting up the generator. To do this, turn off the receiving piezoelectric element BM1 and connect it to the oscilloscope. By placing the piezoelectric elements against each other and applying power to the generator, by selecting the capacitor C1 and the resistor R3, the maximum amplitude of the received signal is achieved. You can measure the frequency of the generator - it must correspond to the resonant frequency of the emitter. Then you need to restore the connections, place the piezo elements in the case and apply power to the entire device. The voltage at the outputs of the op-amp DA1.1 and DA1.2 (pins 10 and 12) must be equal to half the supply voltage.

In conclusion, the amplitude of the amplified alternating voltage at the output of the op-amp DA1.1 is checked, it should be approximately equal to 0,1 V. A strong difference in amplitude from this value will lead to some deterioration in sensitivity. If you hold your hand in front of the piezoelectric elements, the amplitude of the AC voltage at the output of the op-amp DA1.1 will begin to pulsate. The frequency of pulsation will be the higher, the higher the speed of movement.

The rest of the device does not need to be configured and, if installed correctly, should work immediately.

Author: A. Koinov, Nakhodka, Primorsky Territory; Publication: N. Bolshakov, rf.atnn.ru

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