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

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Encyclopedia of radio electronics and electrical engineering / infrared technology

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The fact that information can have a very high value today is no surprise to anyone. But if earlier only a limited circle of people could really fear the leakage of information, today almost everyone can face this. The first thing that usually comes to mind is radio microphones. They are widespread, because. it is not difficult to assemble a "bug" according to the description in amateur radio literature. The author even knows of a case where students passed exams successfully with the help of a radio microphone. However, such radio microphones can be detected without much difficulty, one has only to assemble a simple field detector.

However, there is another way to extract information. It is known that sound waves in a room cause micro-vibrations of window panes. If you direct an IR stream onto the glass, then most of it will pass through the glass inward, but there will also be a reflection. In this case, the reflected stream will be modulated by speech information. In order to assess the real possibilities of stealing information in this way and find an effective way to counter it, the author developed an experimental scheme for a listening device. It consists of two relatively independent parts: an IR transmitter and an IR receiver.

The schematic diagram of the IR transmitter is shown in Figure 1. The transmitter is based on a rectangular pulse generator on the D1 chip. The output signal of the generator with a frequency of 35 kHz is fed to the base of the transistor VT1, which, together with VT2, forms a composite Darlington transistor. Using this transistor, the IR LED VD1 is switched.

IR listening device
Ris.1

The reflected signal is fed to the input of the receiver, the circuit of which is shown in Figure 2.

IR listening device
Fig.2 (click to enlarge)

The signal received by the VD1 photodiode is fed to the input of the amplifier assembled at the A1.1 op amp, here the entire band of received frequencies is amplified twice, and the photodiode is also matched with subsequent stages. An active band-pass filter (L.1.2) was assembled on the op-amp A1, tuned to a frequency of 34,67 kHz, i.e. to the carrier frequency of the transmitter. The stage gain is 100, the ripple bandwidth is 6,8db - 1.2 kHz, this provides selective amplification of the carrier and sidebands. Such a construction of the circuit allows you to maximally weaken the effect of interference and parasitic background from lighting devices. From the output of A1.3, the signal goes to the amplitude detector, built according to the classical scheme, which does not require explanation. An ULF was built on the A1 op-amp and transistors VT2 and VT2, the load of which is high-resistance telephones TM-1A or similar. Decoupling of circuit nodes for power supply is carried out by circuits R1C14, R9C15, R8CXNUMX.

Establishment A properly assembled circuit is reduced to adjusting the frequency of the transmitter with resistor R1 until the maximum signal amplitude is obtained at the receiver output.

Op-amp K1401UD4 does not have a direct replacement among domestic microcircuits, but instead of A1.1 and A1.2, you can use any op-amp with field-effect transistors at the input and a unity gain frequency of at least 2,5 MHz. A1.3 can be replaced by any general purpose op amp. The author tested this option: KR574UD2B and K140UD708. It is possible to noticeably improve the characteristics of the receiver if low-noise op amps TLE2074CN and TLE2144CN from Texas Instruments are used. The pinout of these microcircuits completely coincides with the pinout of K1401UD4. LED and photodiode can be taken of foreign production for remote control systems

In the author's version, the circuit with K1401UD4 ensured reliable reading of information from a distance of 5-10 meters, the version with TLE2074CN provided information reading from a distance of up to 15-20 meters, in addition, this version, due to the lower noise level, made it possible to confidently parse quiet words even against a background of loud music.

The sensitivity of the device can be increased by additional IR LEDs connected in parallel with the VD1 transmitter (through their limiting resistors). You can also increase the gain of the receiver by adding a cascade similar to the cascade on A1.2, for this you can use the free op-amp of the A1 chip.

Structurally, the LED and the photodiode are located so as to exclude the direct hit of the IR radiation of the LED on the photodiode, but confidently receive the reflected radiation. It is not excluded the use of optical systems, for example, such as in L.2. The receiver is powered by two Krona batteries, the transmitter is powered by four R20 cells with a total voltage of 6V (1,5V each).

In conclusion, it should be recalled that the use of this device is in some cases prohibited by the legislation of the Russian Federation and may lead to administrative or criminal liability.

Literature:

1. Graf R.F., Schnit W. Encyclopedia of electronic circuits. Volume 7, part 2. - M.: DMK, 2000, p.44.
2. Vinogradov Yu. IR communication line in burglar alarms. Radio, 1998 No. 2, pp. 50-51.

Author: Uvarov A. S.; Publication: N. Bolshakov, rf.atnn.ru

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