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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal radio signaling. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling The developed universal VHF radio signaling allows you to protect various objects: apartments, cottages, stalls, garages, as well as cars from unauthorized access. VHF radio signaling operates in the permitted frequency range of 40.. 48 MHz and does not interfere with television and radio receivers. The radio signaling range can be up to 10 km. When using various sensors (photo sensors, thermal sensors, capacitive and acoustic sensors), the radio alarm can work with any kind of impact, and perform the functions of not only security, but also fire alarms. Thus, the device has a wide range of capabilities that can satisfy both beginners and experienced radio amateurs. The circuit is extremely simple and has good characteristics, does not contain scarce parts, is easy to manufacture and adjust. The principle of operation of radio signaling The radio signaling consists of a transmitter and a receiver spaced apart from each other at a distance of up to 10 km. The electrical circuit diagram of the transmitter is shown in fig. 1.
The transmitter consists of a sensor, a crystal oscillator, a frequency multiplier and a power amplifier. The basis of the receiver (Fig. 2) is the DA1 TDA7021 chip, which is a superheterodyne with one frequency conversion and a sound generator based on the DD1 K561LA7 chip.
The triggered sensor G1 (Fig. 1) (the door was opened) starts the crystal oscillator, assembled on the transistor VT1 according to the capacitive three-point circuit and operating at the main frequency of the quartz. From the quartz oscillator, the signal goes to the frequency multiplier, made on the transistor VT2. The signal from the frequency multiplier circuit through the coupling coil L5 is fed to the input of a power amplifier made on the transistor VT3. The frequency multiplier and power amplifier operate with high efficiency in class C mode. Then the signal from the power amplifier goes to the output P-loop, which matches the output impedance of the transistor with the radio signaling antenna and filters the harmonics of the output signal. Even if the sensor returned to its original state (the door was closed). The RF signal will still go on the air for some time (this time depends on the capacitance of the capacitor C1). The signal from the antenna of the radio signaling receiver (Fig. 2) enters through the selective circuit L2, C14 to an external UHF receiver, made on the transistor VT1 KT368. The amplified high-frequency signal and the local oscillator signal, the circuit of which is the inductor L1 and capacitor C5, are fed to the internal mixer of the DA1 chip. The IF signal (about 70 kHz) from the mixer output is selected by band-pass filters, the correction elements of which are capacitors C7 and C8, and is fed to the input of the limiting amplifier. The amplified and clipped IF signal is fed to the FM detector. The demodulated signal, passing through the low-frequency correction filter, the external element of which is the capacitor C3, is fed to the noiseless tuning device (SNR). The BSHN system of the receiver reacts to the presence of the carrier frequency, which starts the sound generator on the DD1 chip. The external capacitor C4 sets the response time constant of the BSHN system. Thus, a call occurs in the receiver signaling the penetration of the object into the protected area. Technical characteristics of radio signaling:
Radio alarm setting This scheme, in the absence of errors in installation and the use of high-quality element base, works at the first start. It should be noted that the transmitter must be switched on for the first time with a 51 Ohm (1 W) load non-inductive resistor connected between the transmitter output and the common bus. Before starting measurements close the sensor G1. The operation of the master oscillator is controlled by an RF voltmeter based on the transistor VT2. In this case, the resistor R1 achieves optimal operation of the generator. After that, by controlling the RF oscillations based on the transistor VT3, tune the frequency multiplier to the second harmonic of quartz by adjusting the circuit C8, L4. At higher harmonics, quartz should not be excited, since with an increase in the harmonic, the power of the radio signaling transmitter decreases. Then the output stage is tuned by adjusting the P-loop L7, C9, C10, controlling the RF oscillations on the load resistor to the maximum voltage. The receiver is tuned to the transmitter frequency by tuning the local oscillator circuit L1. Next, the selective circuit L2, C14 is tuned to the frequency of the transmitter and, by adjusting the extension coil L3, the maximum sensitivity of the receiver is achieved. By adjusting the resistance R3, a reliable operation of the sound generator on the DD1 chip is achieved when the transmitter is turned on. By adjusting the resistance R2, the desired frequency of turning on the sound generator is selected, and by adjusting the resistance R1, it is generated at the frequency of the mechanical resonance of the piezoelectric BF1, which will affect the volume of its sound. Elements marked with "". are selected during adjustment. This completes the radio signaling setup. Details and design of radio signaling It is better to use a quartz resonator imported at a frequency of 20-24 MHz. You should pay attention to the fact that quartz is suitable for the circuit only with the values of the fundamental frequency, and not the frequency of the mechanical harmonic. The TDA7021 chip can be replaced with its domestic counterpart K174XA34. But it should be noted that domestic analogues work unstably in this range. Chip K561LA7 can be replaced by K176LA7. The KT368 transistor can be replaced with any RF transistor with a cutoff frequency of at least 500 MHz. The KT645 transistor can be replaced with KT603. Transistor KT610, in extreme cases, can be replaced by KT646. The piezo emitter in the receiver can be used ZP-1, ZP-3 or imported. Chokes are used with any inductance greater than 20 μH. Transmitter coils L4, L7 and receiver L1, L2 contain 5…6 turns of PEV wire with a diameter of 0,6 mm, wound on a frame with a diameter of 4…5 mm with a brass or ferrite trimmer. For coils L4 and L2, the tap is made from the middle of the winding. The transmitter coil L5 is wound over the coil L4 and contains 3 turns of the same wire. The number of turns of the extension coil of the L3 receiver is selected experimentally, since its inductance depends on the length of the antenna used in the receiver. The capacitance of the capacitor C1 is chosen in the range of 500 ... 4700 microfarads. To power the transmitter, you can use a stabilized 12 V power supply rated for a current of at least 400 ... 500 mA. As a G1 sensor, it is better to use a reed switch or a switch of any design. The type and design of the sensor depends on the application of this radio signaling. The antenna in the base uses an external whip antenna with counterweights, which is fixed on the roof of the protected facility. To protect the car, you can use its standard antenna or install a pin about 170 cm long, and the body will serve as a counterweight for it. True, the range in this embodiment will decrease to 3.5 km. If we completely abandon the external transmitter antenna and use the built-in telescopic antenna, we will get a radio signal with a range of up to 1 km. Various designs of outdoor antennas for the range of 40 ... 48 MHz can be found in the relevant literature or obtained from the author. Printed circuit boards must be manufactured in compliance with the specifics of the construction of RF devices, as this has a greater effect on the raceability of the design as a whole. The radio signaling communication range largely depends on the height of the suspension and the design of the antenna, as well as on the signaling settings, and can reach 10 km. Literature
Author: A. Shumilov, Bobruisk, Mogilev region
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