Antenna watchdog. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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Recently, a new type of theft has become popular - the theft of expensive individual industrial-made antennas with antenna amplifiers installed on the roofs of multi-storey buildings. This usually happens at the moment when the programs are not being watched, and their owners will only find out about the absence of the antenna by turning on the TV.

The burglar alarm device I propose sounds an alarm when you try to turn off the television antenna or break the power cable. It is distinguished by reliability, simple design, a wide range of supply voltages, small size, does not require scarce parts and can be assembled even by a novice radio amateur.

The device is powered by a power supply included in the antenna amplifier kit, consuming current in rest mode no more than 3 mA, and in alarm mode - less than 1 mA. This is especially important, since the current consumed directly by the antenna amplifier is more than 30 mA, and the transformer, which is part of the power supply, operates in the maximum load mode.

The diagram of the device is shown in the figure. It consists of a key on transistors VT1 and VT2 and a tone generator on a DD1 chip. The device is included in the break of the negative power supply circuit of the antenna amplifier and operates as follows. When the power supply is connected to the network, the transistor VT1, thanks to the resistor R1, opens, and voltage is supplied to the antenna amplifier. The current flowing through the amplifier creates a voltage drop sufficient to open the transistor VT2. In this case, the voltage on its collector is less than 1 V, which is perceived by the input of the element DD1.1 as a logical "0", and the generator does not work.

When the antenna cable is broken or disconnected, the supply voltage is removed from the transistor VT1. Transistor VT2 closes. Logic "1" voltage appears on its collector. In this case, the generator turns on and gives an alarm signal.

The tone generator is made according to the multivibrator circuit on two logic elements - DD1.1 and DD1.2. Element DD1.3 is a buffer. The generator is loaded on a piezoelectric emitter from an electronic clock. The design of the device allows the use of any low-power germanium transistors of the p-p-n structure, which have a lower collector-emitter junction resistance in the open state and have a lower bias voltage. It is allowed to use transistors of the p-p-p structure, however, this will require a change in the polarity of the device being connected to the power supply circuit of the antenna amplifier and the use of an additional inverter before DD1.1.

The DD1 chip can be replaced with a similar one from the K1561 or 564 series. Resistors and capacitors can be of any type, preferably small ones. The device is mounted in the housing of the power supply unit of the antenna amplifier. In this case, the piezoelectric emitter is brought out and glued to the side wall of the housing through a thin foam pad.

Checking the operation of the device is as follows.

Connect the power supply of the antenna amplifier to the network. From the antenna plug leading to the TV, disconnect the central core of the cable (it is screwed on with a screw). An alarm tone should sound when this happens. If necessary, the tone frequency can be changed by selecting capacitance C1.

This watchdog can be used to protect against theft of satellite dishes and other products powered by a two-wire line.

Author: S. Rybchinsky, Valdai-5, Novgorod region; Publication: N. Bolshakov, rf.atnn.ru

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