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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple radio alarm system for car or premises security

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Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms

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Sometimes a simple system is enough for remote notification when guarding a garage or car. In this case, the proposed device may be useful, consisting of a radio transmitter operating at a fixed frequency of 26945 kHz and a narrow-band receiver.

The electrical circuit of the transmitter is shown in fig. 1. The high-frequency part consists of two stages on transistors VT1, VT2 and has a minimum number of tuning elements.

A simple radio alarm system for car or premises security
Pic. 1. Radio transmitter (click to enlarge)

This simplifies its manufacture and ensures the operation of the circuit without adjusting the transmitter in the frequency range of 26 ... 30 MHz when changing the quartz that sets the operating frequency.

Choke coils L1 and L2 are wound with PEL wire with a diameter of 0,12 mm on the body of the MLT-0,5 resistor with a nominal resistance of 1 ... 1.8 kOhm and contain 50 turns (the design is shown in Fig. 2.56). Coils L3, L4 and L5 are made on a dielectric frame with a diameter of 5 mm with a thread for screwing in a brass core with an M4 thread. They contain respectively 14, 14 and 15 turns of PEL wire with a diameter of 0.4 ... 0,5 mm. Coil L4 is placed horizontally on the circuit board. Brass screws can be used as a core (for this you will need to cut off the head and make a slot - a slot for a screwdriver). Before screwing in the cores, we lubricate them with any non-drying viscous sealant.

The circuit uses MLT resistors. non-polar capacitors K10-17 (with a minimum TKE), trimmer C10 type K4-236, electrolytic C4 - K52-1 for 22 V.

The modulating part of the transmitter is made on a single CMOS series digital microcircuit. On elements D1.2 and D1.3, a generator of low-frequency pulses with a frequency (about 1000 Hz) is assembled, which are switched using an electronic key on an element of the D1.4 chip, power to a high-frequency oscillator. The modulating frequency can be set to any in the range from 2 to 2 Hz by changing the elements C3, R300 and R2000.

When the F1 sensor circuit is closed, the generator does not work and the entire circuit in standby mode consumes microcurrent (no more than 0,05 mA). When F1 is opened, the transmitter is turned on. A working transmitter with 100% pulse modulation consumes no more than 100 mA.

The supply voltage of the transmitter circuit can be in the range of 9 ... 13 V. In this case, the output power of the transmitter in a pulse is not more than 0,8 W.

Setting up the circuit consists in obtaining, using the tuned cores of the coils, the maximum amplitude of the output RF signal. To do this, we first connect an active load equivalent to the antenna, fig. 2, and the core of the coils L3, L4 and the capacitor C10, we achieve resonance in the circuits of the P-filter.

The final adjustment is performed with the antenna connected to the electromagnetic field indicator using the ferrite core of the coil L5 and capacitor C11. The simplest scheme of a broadband field indicator is shown in fig. 3.

The transmitter antenna can be a metal pin (800...1200 mm) or any stretched wire approximately 1...2.5 m long. waves (up to 10 m), which increases the efficiency of signal emission.

With a portable version of the transmitter design, it is convenient to use a telescopic antenna as an antenna, from any household radio or TV. And to power the device, 8 batteries of the type NkHz-0,5 are suitable.

A simple radio alarm system for car or premises security
Rice. 2. Connecting a Dummy Antenna Load for Transmitter Tuning

A simple radio alarm system for car or premises security
Rice. 3. Broadband field indicator

All elements of the radio transmitter circuit are located on a printed circuit board 105x35 mm in size made of one-sided fiberglass 1 ... 2 mm thick, fig. four.

The high-frequency part of the receiver is made on an analog integrated circuit DA1 (K174XA2) according to a superheterodyne circuit, fig. 5. The internal local oscillator is frequency-stabilized with ZQ1 quartz (26480 kHz), which ensures reliable reception when the temperature and supply voltage change. The local oscillator frequency is chosen below the frequency of the received signal by 465 kHz. The intermediate frequency allocated by the internal mixer is amplified and fed to the VD2 detector. Diode VD1 improves the performance of the built-in automatic gain control system when receiving pulse-modulated
signals. That ensures the performance of the receiver and at a close distance from the transmitter.

The preamplifier of the high-frequency signal on the transistor VT1 allows you to increase the sensitivity of the receiver to 3 ... 5 μV (internal noise of the microcircuit limits a further increase in sensitivity). The input circuit L1-C2-C3 and the collector transistor VT1 (C5-L3) are tuned to the transmitter frequency using ferrite cores. The receiver antenna can be a hard wire pin 400 mm long.

A simple radio alarm system for car or premises security
Rice. 4. Topology of the printed circuit board and the location of the elements of the radio transmitter

A simple radio alarm system for car or premises security
Rice. 5. High-frequency part of the receiver (click to enlarge)

Low-frequency pulses after the VD2 detector are fed to an amplifier assembled on transistors VT2 ... VT3, fig. 6. The value of the resistors R13 and R18 is selected so that with an input low-frequency signal with an amplitude of 20 mV (to set up a sinusoidal signal, send a sinusoidal signal from the generator) - the output has a symmetrical amplitude limitation.

In order for the receiver to give an alert signal only when receiving its own (against the background of other signals and interference), a narrow-band filter at a frequency of approximately 26 Hz is assembled on elements C28 ... C7, L1000. The filter bandwidth is 200 Hz. In the event that a frequency receiver appears at the output of the detector in this range with a level of more than 20 mV, short pulses will appear at the output of the logic element DD1.2/8. They charge the capacitor C30 to the level of the log. "one". In this case, a log will appear at the output of the inverter DD1 / 1.3. "12". Diode VD0 is locked, which allows the operation of the sound oscillator on DD4, DD1.4. The oscillator frequency can be adjusted using resistor R1.5 so as to obtain the maximum volume of the ZGI 23 (ZP-8) piezo emitter. Usually this frequency is about 25 kHz (internal resonance of the radiator).

The topology of the single-sided printed circuit board of the receiver is shown in fig. 7. Elements R22, R23 and C31 are located above the DD1 chip. To obtain a high mounting density, most resistors are mounted vertically on the board.

During installation, constant resistors of the C2-23 type, trimmer R18 of the SPZ-19a type, capacitors of the K10-17 and KM-4 types, polar C9, C12 ... C14, C20 of the K50-35 type for 22 V were used. The ZGI 8 piezoelectric radiator can be replaced on ZP-25. Diodes KD521 are replaced by any pulse.

Coils L1 and L3 are made on a frame with a diameter of 5 mm with a PEV-2 wire with a diameter of 0,23 mm and contain 14 turns each. Coil L2 is designed for horizontal board mounting. It contains in the windings: 1-12 turns, 2-3 turns over the primary winding, wire with a diameter of 0,4 mm. Any high-frequency ferrite core is used for tuning.

The design of the coils of the intermediate frequency circuits L4 ... L6 can be used ready-made, from miniature radios, or - in the presence of all incoming nodes - they are performed independently with a PEL wire with a diameter of 0,1 mm and contain 80 turns each.

For the manufacture of the filter coil L7, two armored ferrite (600 ... 2000NM) cups of size B14 (without a tuning core) were used. The winding is wound with PEL wire with a diameter of 0,08 mm until the dielectric frame is filled and is located inside the ferrite cups. The resonant frequency of the circuit L7-C27 (1000 Hz) may differ from the specified. In this case, you will need to set the same modulation frequency in the transmitter during tuning.

We start setting up the receiver with the decoder when the circuit is powered by a voltage of 7,5 V. By supplying a sinusoidal signal from the low-frequency generator (15 ... 20 mV) to the input of the decoder, resistors R13 and R18 achieve a symmetrical signal limit on the resistor R19 when the supply voltage changes.

A simple radio alarm system for car or premises security
Rice. 6. Receiver decoder (click to enlarge)

A simple radio alarm system for car or premises security
Rice. 7. a) Topology of the printed circuit board of the receiver

A simple radio alarm system for car or premises security
Rice. 7. b) Arrangement of elements

After that, we determine the resonant frequency of the filter (measure it).

Establishing the high-frequency part of the receiver comes down mainly to tuning the circuits using ferrite cores. Why do you need a high frequency generator.

The receiver must remain operational when the voltage changes in the range of 6,6 ... 9 V.

The current consumed by the circuit is no more than 12 mA. If six D-0.26D batteries are used to power the receiver, continuous autonomous operation can be 20 hours.

The design of the receiver housing is similar to that shown for the electroshock device. Batteries are placed in glasses glued from cardboard. The second printed circuit board is mounted on the side walls made of plexiglass 4...5 mm thick (the same board provides the electrical connection between the batteries). The frame formed from two boards is wrapped in cardboard and glued (it should be easily removed). After that, a decorative film in the color of wood will help to give a pleasant look to the body (it is more convenient if it is self-adhesive).

Publication: cxem.net

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