Radio emission sensor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio emission sensor. 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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Installation of various kinds of electronic listening devices in homes and offices, unfortunately, has become commonplace. Fans of detective and spy movies may remember entire episodes in which characters plant button-sized listening devices in rooms.

This situation is not far from reality. However, detecting electronic listening devices ("bugs") is not as easy a task as it might at first seem, even if the presence of a "bug" in the room is known. The reason for the difficulty of detection lies in the very small size of the "bugs", as well as their similarity to everyday items (pens, tees, etc.).

By assembling a relatively simple device described in this article, it will be possible to provide reliable protection against extraneous eavesdropping.

Electronic bugs* are subminiature low-power radio frequency transmitters placed in the listening room. Since most bugs operate in the radio range, any wideband receiver can be used to detect them. The described detector operates in the frequency range of 1 ... 1 MHz, which quite enough for practical purposes Fig. 000 shows a circuit diagram of the radio emission sensor.

(click to enlarge)

Consider the operation of the device

When an electronic eavesdropping device starts to work, it emits an electromagnetic field into space, which is received by the WA1 antenna and converted into an alternating radio frequency electric current. Next, the signal enters the base of the transistor VT1 through a high-pass filter formed by the elements C1. C3. R1. At low frequencies, the reactance of the capacitors is very large, thereby attenuating the level of the low-frequency signal supplied to the transistor VT1. The HPF elements are selected in such a way as to provide effective suppression of the signal with the frequency of the household network, i.e. 50Hz, which is the main source of interference. Transistor VT1 is included in the usual scheme with a common emitter and is used to amplify signals in a wide frequency range. The cascade gain on VT1 is about 10 dB in the range of 1 ... 1000 MHz. Resistors R3 ... R5 provide a given mode of operation of the transistor for direct current. The amplified signal is taken from the collector VT1 and through the coupling capacitor C6 is fed to the diode VD1, which acts as an amplitude detector.

It should be noted that the use of a diode instead of the type indicated in the diagram can significantly reduce the operating range of the radio emission sensor. To expand the frequency range of the amplitude detector on VD1, a circuit consisting of elements R12 is used. R20. C7. A conventional diode cannot instantly go from open to closed, and a reverse current occurs. This property limits the frequency range of the detector. Variable resistor R10 can achieve the highest sensitivity of the device. The operational amplifier DA1.1 serves as a DC amplifier with a high transfer coefficient, determined by the ratings of the elements R21, R20, C9. If there is no signal at the input, the potential of the common wire will be present at its output. If there is a source of radio emission in the room in the range of 1 ... 1000 MHz, the signal is received by the antenna, amplified and inverted by the cascade on VT1. which knocks on the diode VD1. Next, the voltage is applied to the inverting input DA1.1, which amplifies and inverts the signal.

The following stages after the op amp DA1.1 form a low-frequency voltage-controlled oscillator. Operational amplifiers DA1.2, DA1.3 form a VCO operating in the audio frequency range. From the VCO output (pin 8 DA1.3), the audio frequency signal is amplified by the voltage follower on DA1.3 and the cascade on the transistor VT3. An ordinary low-power speaker with a resistance of 8 ohms is used as a sound emitter.

Design

The device is assembled on a printed circuit board made of foil fiberglass.

First, a pane is installed on the printed circuit board under the IC DA1. Then solder all passive components (resistors, capacitors). In this case, it is necessary to control the correctness of their inclusion according to the electrical circuit diagram. Then semiconductor elements are installed - the VD1 diode, transistors and ICs. When soldering active elements, overheating of the diode and transistors should be avoided. For soldering, it is advisable to use a soldering iron with a power of no more than 25 watts. After installing all the elements, check the installation for errors.

Setting up the sensor is not very difficult. Install a radio emission source (for example, a simple quartz oscillator operating at a frequency of 20 ... 100 MHz) in the middle of the room. By rotating the slider of the variable resistor R19, the minimum sound level in the BA1 speaker should be achieved. In this case, the radio emission detector is coordinated with the electromagnetic environment inside the room. To detect the source of radio waves, it is necessary, by changing the position of the sensor in space, to achieve maximum sound in the speaker. In this case, the radiation source will be in close proximity to the sensor.

The sensor is powered by a 9 V battery.

Author: S.Romashin, Kazan

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