Watchman for an active television antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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Individual active television antennas installed on the roofs of apartment buildings often become the object of encroachment. The author proposes to use a device that signals what is happening.

Active TV antennas with built-in plate amplifiers are becoming more and more popular among the population. But at the same time, kidnappers are also showing increased interest in them, who consider them more expensive than others. In some regions, this becomes a real disaster. In order to somehow "guard" such an antenna, a security device is proposed that gives an audible signal in case of a significant violation of the normal operating mode of the antenna.

A schematic diagram of such a device is shown in fig. 1. It is included in the break of the conductors connecting the power supply unit to the voltage supply unit (usually it is connected to the antenna jack of the TV) to the drop cable.

In the normal operating condition of the antenna, drop cable and amplifier, the current consumed by the latter flows through the resistor R2 and creates a voltage drop of 1 ... 2 V on it. In this case, the transistor VT1 is open, and from its collector to one of the inputs (pin 1 ) element DD1.1 receives a high voltage level 1. At the same time, almost the same level affects the second input (pin 2) of this element. As a result, its output will be a low voltage level of 0, which prohibits the operation of the pulse generator assembled on the elements DD1.2, DD1.3.

In the event of an emergency associated with the theft of the antenna or its failure, the device gives an alarm. In this case, two states are possible at the end of the drop cable (usually robbers cut it): either its central conductor and shielding braid are open or closed (short circuit). If they are open, then the current through the resistor R2 does not flow and the transistor VT1 is closed. Although level 2 still comes to pin 1.1 of the DD1 element, level 1 immediately affects its pin 0 and level 1 appears at its output, allowing the pulse generator to work. At the output of the buffer stage on the element DD1.4, to which the sound emitter is connected, voltage pulses are formed with a repetition rate of about 2 kHz and an audible signal is heard.

In the case when the cable conductors are closed (short circuit), although level 1 is still present at pin 1.1 of the DD1 element, level 0 goes to its pin 2 and level 1 also appears at its output, allowing the pulse generator to work. The beep sounds again.

Therefore, in both typical emergency situations, the alarm sounds.

All parts of the device are placed on a printed circuit board made of one-sided foil fiberglass, the conductor pattern of which is shown in Fig. 2. Moreover, the sound emitter is glued to its reverse side. The board must be placed in a housing made of insulating material.

The device can be used, in addition to those indicated in the diagram, transistors KT208B-M, KT209A-M, chip KR1561LA7. Resistors - MLT, S2-33, capacitors - K10-17, KD, KT, sound emitter - ZP-1, ZP-3 or similar.

When setting up the device, the required frequency of the pulse generator is set according to the maximum volume of the sound emitter signal by selecting resistor R4 or capacitor C2. Resistor R2 is selected for a specific instance of the antenna amplifier. During normal operation, the voltage drop across the resistor must be at least 1 V, and when the wires leading to the power supply unit of the amplifier are shorted to the drop cable, the voltage across capacitor C1 must be at least 4 V. At a lower voltage, the microcircuit may work unstable.

It should be noted that the resistor R2 simultaneously provides an easier mode of operation of the power supply, limiting its current in the event of a short circuit of the cable conductors.

Of course, you should also not forget that the device will only work when the power supply is on.

In addition, when using a power supply in which the supply voltage is regulated, there may be a problem with the selection of the resistor R2, since the current consumed by the amplifier may vary widely.

Author: I. Nechaev, Kursk

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