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Radio beacon to protect children from abduction

Radio beacon to protect children from abduction

This device has been tested in the laboratory of the magazine "Rad i oamator " . When setting up the scheme, not every radio amateur will find the devices listed in the scheme. With their absence, the circuit can be adjusted using an oscilloscope with a band up to 100   MHz, tester, power supply, two radios, the simplest indicator of field strength. Properly assembled circuit starts working immediately. The oscilloscope controls the operating mode of the low-frequency generators according to the diagrams in fig.   3. Next, adjust the RF cascade, controlling the operation of the transmitter with two radio receivers located at different distances from the transmitter, and an indicator of field strength. The antenna of the receiver is better placed on the windowsill.

Small-sized electronic device is designed to timely alarm in case of attempted assault, theft or robbery. This device is placed in a pocket and, if necessary, is included in the "Alarm" mode. Children who play near the house, parents will be able to provide the necessary assistance. At home, there is a control radio tuned to the appropriate FM or VHF frequency , which gives an intermittent alarm that resembles the sound of a police siren going to arrest criminals.

When leaving for a walk, the child turns the device switch to the “1” position ( “Standby mode” ). In this mode, from the dynamics of the control radio located in the apartment, rare sounds are heard (once in 30-60   c) quiet sounds - a signal that the device is in the on state. If necessary, the switch is turned to the position “2” ( “Alarm” ), and intermittent signals are heard from the speaker, the LED flashes on the beacon.

The whole structure is located on the printed motherboard for a specific case, which can be purchased by a radio amateur. In fig.   1 shows one of the variants of the design - the radio beacon in the marker, which has a clip for mounting, so it is convenient for children to use it. At the top is the HL 1 LED , on the side is the SA 1 power switch, the SA 2 mode switch , at the bottom is a steel wire antenna 300   mm in isolation.

The power source is a Krona battery, a 9 V battery or a miniature battery to power the car alarm on / off system. The indicator works in the FM band (88 ... 108   MHz) or VHF (66 ... 74   MHz), in free space covers the distance of at least 500   m when the sensitivity of the radio is not worse than 10   µV Since the main mode of operation - "Duty" , which is activated for 2 s after 30 ... 60   c , the battery can be operated for a long time.

The schematic diagram of the device is shown in Fig.   2. Elements DD 1.1 and DD 1.2 generate a signal with frequency 1   Hz, which controls the operation of the generator on the elements DD 1.3 and DD 1 4, generating a frequency of about 2   kHz At the output of the element DD1.2 transistor VT 1 is turned on for light signaling in case of alarm. To generate the "standby" signal , a generator of intermittent sound frequency generation at the transistor VT 2 is assembled . The generation frequency is determined by the inductance L 6 and circuit capacitances. In the process of setting up the generator, the turn-on time and pause can be changed within wide limits. In fig. 3 shows the timing diagram of the generator.

On the transistor VT 4 assembled power amplifier to which an antenna is connected through a Collins filter ( P-circuit ).

Details in the scheme is better to use miniature, imported, having previously checked their quality. All resistors such as OMLT-0,125; C 6 ... C8 capacitors of type KT, CT5 of type K50-35, the rest of type KM. Transistors 1 ... VT VT type 3 KT315B (KT315G, KT312B, KT342B), VT4 - 2T371A (KT367A, KT372B, KT382B) diode VD D9B type 1 (D 2, D18, D310), the LED HL1 type AL336K (AL307B, AL102B), Zener diode VD 2 type 2S156A, switches SA1, SA2 - PD9-2, DD1 chip of type K561LA7 (564LA7). L6 Uniform Choke - Pulse Transformer Miniature TIM-170. The scheme of its connection is shown in fig. 4. In case of its absence, on the ferrite ring M2000K 12x8x3, wind the winding of the FEV-1 wire Ø 0.1 mm to the filling to fill it. Contour coils are wound with wire PEV-2 Ø 0.71 mm on a mandrel Ø 5 mm. Coils L 1 and L 2 have 5 turns, L 3 and L 5 have 7 turns, and L 4 have 4 turns.

For installation (Fig. 5) foil getinaks are used. The printed conductors of the RF generator should be tinned to exclude their inductance and be made wider. To install TIM-170, 8 holes of Ø 0.5 mm should be drilled in the plate. When using an improvised choke, the winding should be wrapped with fluoroplastic insulation, the conclusions should be drawn from wire a MGTF- 0.07 mm . Since this device is wearable and may fall to the ground, all connection points must be carefully soldered. After adjustment, all loop coils with appropriate capacitors ( C 6- L 1, C 9- L 2, etc.) should be filled with paraffin to dampen the circuits in order to provide water protection and rigidity. Failure to comply with these conditions may result in malfunction of the beacon. To fasten the battery, solder two tinned bronze posts with a thickness of 0.2 to the corresponding points on the board. 0.3 mm . The battery is attached to the body with a spring, the contacts are soldered to the board. After all the work is complete, it is useful to cover the entire board, except for the switches and the battery, with UR-231 varnish to protect it from rain, snow and corrosion.

Scheme setting. The following instruments are needed for this: an adjustable power supply with a minimum power of 2 W, a tester, an oscilloscope with a bandwidth of up to 100 MHz, a GIR, a wave meter, a field strength meter, a lamp voltmeter, a control radio.

To test the node on the transistor VT 2 “ Warning Signal ”, it is necessary to unsolder the wire from the switch SA1, supplied to the power supply of the RF generator circuit, and supply 9 V from the power supply unit. Switch SA 2 to the position “1” . When setting up this node, the following should be kept in mind: the generator produces intermittent sine-wave oscillations, the capacitors C16-C18 are in the feedback circuit and serve to start the generator. Together with the coil L 6 they determine the tone of the sound in the control radio. The selection of the values ​​of these capacitors affects the mode of operation of the generator. The capacitance of the capacitor C16 affects the frequency of switching on the generator.

The duration of generation is determined by resistors R 6, R 7. Increasing the capacitance C16 increases the pause and saves battery consumption. Reducing the resistances of R6 and R7 increases the frequency of switching on the generator . To control, the robots of this generator should be connected to the base of transistor VT 3 by an oscilloscope, and through a 510 capacitor   pF - headphones. During normal operation of the generator, bursts of a sine wave are visible on the screen, and a musical tone is heard in the headphones. In the absence of oscillations, you should choose C17, C18 or increase the inductance of the coil L6. The required timbre of the sound is determined mainly by the value of inductance: the larger it is, the lower the frequency of the sound.

Then switch SA2 is transferred to position “2” “Alarm” . The HL 1 LED starts flashing immediately , bursts of rectangular pulses are visible on the oscilloscope screen . Resistor R 1 regulate the duration of the pulses and amplitude: the greater the resistance R1, the shorter the duration and vice versa, and R 4 and C 2 determine the frequency of addition of the generator. In the final adjustment of the beacon, it is necessary to select the C3 and C14 capacities in order to exclude overmodulation , and also not to cause the generator to "stall" .

To check the RF generator on the transistor VT3, you must unsolder the capacitors C3, C14, restore the power wire coming from SA1. Connect a oscilloscope to a VT3 collector through a 10 pF capacitor. Instead of R 8 and R 12, turn on potentiometers per 100 with a limiting resistor of 1 , and instead of R 9, a potentiometer on 3 and place it in the position of 300 Ohms. By adjusting the R8 to achieve the appearance of generation on the oscilloscope screen, sometimes it is necessary to select C 7 . Adjusting R8 and R9, find the net and maximum voltage amplitude. Then reduce the supply voltage to 6 V and adjust the R8, R9 to find the maximum voltage for this supply voltage, and then set an average value of 6 ... 9 V.

After that, connect a real antenna and check the operation of the power amplifier at the transistor VT 4. The circuit uses a microwave transistor, so even minor changes in the capacitance of the C 9 affect the output power and the operating frequency (as this capacity increases, the frequency drops and the output power rises). To adjust instead of C 9, solder a 1.9 / 20 pF trimmer , and connect the oscilloscope to the VT4 collector. Instead of C10 and C11, solder variable capacitors with a capacity of up to 150 pF. To adjust the C 9 need a screwdriver made of plexiglass, PCB, etc. With the master oscillator running, adjusting C 9 and R12, achieve maximum voltage at the collector VT4. The collector current measured by the tester should not exceed 18 mA at 9 V supply. Adjusting С 9 , it is necessary to control the working frequency with the help of the GIR and the wavemeter so that it does not go beyond the operating range of the control radio. In accordance with the current GOST, on the range 88 ... 108 MHz, radio stations operate above 100 MHz, therefore, the radio beacon should be located below 100 MHz. On the domestic band should be located the frequency of the beacon above 70 MHz.

The next stage is to adjust the maximum power output of the cascade on VT4 and adjust the U-filter , which allows you to adjust the antenna length to the maximum power output, and also suppresses harmonics. Tuning is mainly carried out by changing the capacitance C10 and C11 with a fixed value of inductance L 4. To properly adjust the filter, you should solder the capacitors C3 and C14 to the circuit and again check the voltage and shape of the curve on the collector VT4, and adjust if necessary. The ultimate goal of the filter setting is to get the maximum output power, and the beacon's radius of action depends on it. At point B there should be a maximum voltage. Here you need to connect an oscilloscope. Adjusting C10 and C11, achieve maximum voltage. It is also controlled by a field strength meter located at a distance 1m (turn off the oscilloscope!). You may also need to adjust the C 9 . If the control radio receiver sound unclean, you should pick up C3 and C14. Setting up this filter is quite troublesome, and its configuration is described in more detail in [1]. In the author's version, it was necessary to introduce an inductance L 5: without it, the power output was 40% less. Other amateurs may not need it.

After adjustment, the selected elements should be replaced with close ones and insert the board into the case along with the battery; the frequency will shift down. If necessary, squeezing or pushing the coils L 1, adjust the frequency. It is necessary to adjust and L 3.

Then the work of the beacon is checked in real conditions. The control radio should be placed on the window of the apartment from the street where the user of the beacon will be located, fully extend the antenna of the radio receiver. The radio beacon is placed in the jacket pocket, the antenna is placed down. Turn on SA1, SA2 to put in position "1" . An assistant in the apartment finds the best position of the antenna, turning it in different directions, as well as a place in the apartment where the signal sounds louder. Then check mode "2" . Changing the position of the radio beacon in relation to the radio, you can make a complete picture of use: go to the limiting distance, go around the corner of the building, etc.

The final stage - carrying out mechanical tests. Turn on the control radio, find the beacon signal. When switched on, the beacon should be dropped from a height. 200 mm on the wooden table, first flat, then on the side edge, and then on the top edge; when testing on three planes, the beacon should work normally, and its setting should be stable.


1. Wojciechowski J. Remote control models. - M .: Communication, 1977. - 432 p .