Individual TV active antenna installed on roofs apartment buildings are often subject to abuse. Author proposes to apply the device, indicating what is happening.

Active towers with built-in plate amps are becoming more popular with the public. But they have a keen interest and the kidnappers, who consider them to be more expensive than other. In some regions it becomes a real disaster. In order somehow "protect" such an antenna, it is proposed that the security device, output audio signal with a substantial breach of the normal working mode antenna.

Schematic diagram of the device shown in Fig. 1. It is included in the gap conductors connecting the mains unit to the feeding unit voltage (usually it is connected to the antenna Jack of the TV) on cable.

Caretaker for active towers

In normal working condition of the antenna, lead-in cable and amplifier, the current consumed last, flows through the R2 resistor and creates a voltage drop, 1...2 V. the transistor VT1 is open, and with its collector to one of the inputs (pin 1) of the element DD1.1 goes to a high voltage level 1. Almost simultaneously the same level of impact and to the second input (pin 2) of that element. In the result at its output will be low voltage level of 0, preventing the pulse generator, assembled on the elements DD1.2, DD1.3.

In emergency case involving the kidnapping of the antenna or output her down, the device gives an alarm. There are two status at the end of the cable is cut (usually the robbers cut it) either Central conductor and braid are open or closed (short circuit). If they are open, the current through the resistor R2 flows and the transistor VT1 closed. Although on pin 2 of the element DD1.1 still comes a level 1 on it conclusion 1 directly affects the level 0 and at its output there appears a level 1, authorizing the operation of the pulse generator. The output buffer stage on the element DD1.4, which is connected to a sound projector, pulses of voltage a repetition rate of about 2 kHz and a beep sounds.

In the case where the conductors of the cable is shorted (short circuit), although at pin 1 of the element DD1.1 still level 1, level 0 is supplied to its pin 2 and the output also occurs level 1, authorizing the operation of the pulse generator. Again a beep sounds.

Consequently, in both a typical emergency alarm sounds.

All parts of the device are placed on the PCB of one-sided foil fiberglass, the conductor pattern formed of which is shown in Fig. 2. Moreover, a sound projector glued to her backside. The fee should to place in the body of insulating material.

The device can be applied, except indicated in the diagram, the transistors CTB-M, CTA-M, microchip CRL. Resistors - MLT, S2-33, capacitors - K10-17, CD, CT, a sound projector - ZP-1, ZP-3 or similar.

When creating a device set the desired frequency of the pulse generator at the maximum volume level of the sound projector selection resistor R4 or of the capacitor C2. Resistor R2 is selected under a particular instance of the amplifier antenna. During normal operation the voltage drop across the resistor must be less than 1 V, and when the circuit wires, connected to the node of the power amplifier cable, the voltage on the capacitor C1 should not be less than 4 V. If lower voltage chip may be unstable.

It should be noted that the resistor R2 at the same time provides a more easy mode the operation of the power supply, limiting the current in case of short circuit, the conductors of the cable.

Of course, we should also remember that the device will only work when included power supply.

In addition, when you use a power supply unit, which regulate the supply the voltage may be a problem with the selection of the resistor R2, since the current consumption of the amplifier may vary within a wide range.

Author: I. Nechaev, Kursk

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