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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Acoustic detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Safety and security Readers are invited to the original burglar alarm sensor, designed to protect windows in the room In recent years, many publications about security devices have appeared in the radio engineering literature. And this, of course, is not accidental, the topic of property protection is very relevant. There are many security devices based on various physical principles [1]. All of them are designed to protect cars, warehouses, offices, apartments and other facilities. An integral part of any such device is a "security detector" - a technical tool that signals an unauthorized intrusion into protected premises and generates alarms [2]. Let's consider one particular example. Let's say you need to block the possibility of burglars entering your apartment through a window opening. There are two ways to get through the window opening. The first is to open the moving parts of the frame (window) without breaking the glass cover, the second is to break the glass cover (break, cut, expose the glass) without opening the moving parts of the frame. Traditionally, for protection in the first case, magnetic contact detectors SMK-1, SMK-3, IO 102-4, IO 102-5, IO 102-6 are used. In the second case - electrical contact detectors (aluminum foil), surface shock-contact detectors of the "Window" type. These funds, of course, do not "decorate" the interior of living rooms and create additional problems for the owners, for example, when washing windows. In addition, the issue of secrecy of the security device is not solved. True, we can still recommend surface-sound detectors of the "Glass" type, but their cost is quite high. Acoustic detectors can satisfy many requirements [3]. The principle of their work is as follows. The BM1 microphone (Fig. 1) receives acoustic signals from the environment, which are converted into an alternating voltage of the appropriate frequency and amplitude, they are fed to a linear amplifier with a gain k, and from its output to the BA1 emitter, where the inverse conversion into sound takes place . The signal reproduced by the emitter propagates in the environment with a transmission coefficient β and, mixing with the sounds of the environment, is transmitted to the input of the BM1 microphone, where it is again converted, then amplified, etc. Thus, there is a feedback between the microphone and the emitter, which closes through external environment.
If the amplifier is made narrow-band, then from the entire spectrum of acoustic signals arriving at the microphone, the emitter will reproduce only those that fall within the frequency band of the amplifier. By selecting the area of operating frequencies ranging from 10 to 15 kHz. it is possible to tune out many audio interferences, which lie mainly in the range below 10 kHz. It is known from theory that undamped oscillations occur in an amplifying device with feedback (self-excitation mode), if the feedback is positive (phase balance), and the product of the forward channel transfer coefficients k and reverse β is greater than or equal to unity (amplitude balance), then kβ> 1. When the conditions for phase balance or amplitude balance are not met, then the device is in a stable state, i.e., in a linear amplifying mode. By changing the transmission coefficient β, it is possible to control the state of the considered device. This principle is used in the operation of the acoustic detector. To protect the window, the microphone is placed between the frames of the window opening (with a little effort it can be masked very well), and the amplifier and emitter are placed in the room. Thus, the microphone and the emitter are separated by a glass partition, and the acoustic feedback between them is weakened. At the output of the amplifier, the voltage amplitude is negligible. If an intruder tries to enter the apartment through a window (opens a window or window, breaks or exposes glass), an acoustic connection will occur between the microphone and the emitter and the device will be excited. The amplitude of the voltage at the output of the amplifier will increase many times over. By connecting a threshold device to the output of the amplifier, we get an acoustic detector (Fig. 2).
An active band-pass filter is assembled on the op-amp DA1. Its gain is 1000 at a resonant frequency of 11 kHz, the bandwidth is 800 Hz. The output transistors VT1 and VT2 operate in class B mode, due to which the power consumption in standby mode is minimal. The gain of the device can be adjusted by the resistor R4 in the range from 2 to 20 times. This is necessary to adjust the sensitivity of the detector after placing it on the object. From the output of the amplifier, the signal goes to the dynamic head BA1 and to the threshold device, which is assembled on transistors VT3, VT4, diode VD1 and zener diode VD2. In standby mode, transistors VT3 and VT4 are closed and there is a low level at the output of the threshold device. When the device, due to the circumstances mentioned above, is excited, a positive voltage appears on the base of VT3. If it exceeds the threshold voltage set by the zener diode VD2, transistors VT3 and VT4 open. An "Alarm" signal appears at the output of the threshold device - a positive voltage of about 15 V. This voltage can be used as a control voltage for various terminal devices. In addition to those indicated in the diagram, you can use the OU K140UD6, the MD-52 microphone, the dynamic head 10GDV-2 or 10GDV-4. The cable for connecting the microphone must be shielded. The detector is configured directly at the facility. With the window closed, resistor R4 sets the maximum gain (and hence the maximum sensitivity). If self-excitation occurs in this case, then the gain is reduced until it stops. After that, the window (window) is opened or the glass is removed - the device should be excited again, and the "Alarm" signal will appear at the output of the threshold device. It may happen that the device does not wake up. Then you need to choose the relative position of the emitter and microphone. It should be borne in mind that it is desirable to place them so that they are directed at each other. The current consumed by the detector in standby mode is 6 mA from the -15 V power supply and 8 mA from the +15 V supply. The current in alarm mode does not exceed 260 mA from each source. The performance of the device assembled according to the proposed scheme was tested for 30 days on windows measuring 70x115 cm (in the evening) and 120x170 cm (in the daytime). When opening the window (in this case, β changes by about 30 dB), the detector always gave an "Alarm" signal. No false positives were recorded during the checks. Thus, the operating experience of the described device allows us to speak about the prospects of its application. In addition, it may well be used to protect other objects, such as safes. Addition. Electret microphone in acoustic detector In the design described in my article "Acoustic" detector ", an electrodynamic microphone was used as a sound sensor. This simplified the connection of the sensor to the DA1 chip as much as possible. However, despite the small size of the dynamic microphone (several centimeters long), its installation and masking for for some it can be a tedious task. In this regard, I decided to modify the input circuit of the amplifier in such a way that it was possible to connect an electret microphone. These devices compare favorably with their small dimensions. For example, a CZN-15E type microphone, taken from an old imported cassette recorder, is only 10 mm in diameter and 6 mm high. Naturally, it is easier to install and disguise such a sensor. The figure shows the connection diagram of an electret microphone to an acoustic detector amplifier. The newly introduced resistors R14, R15 and capacitor C11 are installed on its board. The microphone must be connected with a shielded wire.
It is permissible to use other electret microphones in the design. I note that with such a replacement, the sensitivity of the detector increases, and therefore, in this embodiment, the distance between the microphone and the dynamic head BA1 can be increased. Literature
Author: I.Medvedev, Bryansk
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