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Beacon to protect children from abduction

Beacon to protect children from abduction

This device has been tested in the laboratory of the journal "Rad and Oator " . When setting up the circuit, not every radio amateur will have the devices listed in the circuit. If they are absent, the circuit can be adjusted using an oscilloscope with a band of up to 100   MHz, a tester, a power supply, two radios, a simple indicator of field strength. A 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 by two radios located at different distances from the transmitter, and an indicator of field strength. The receiver antenna is best placed on the windowsill.

The small-sized electronic device is designed to provide a timely alarm in the event of an attempted attack, theft or robbery. This device is placed in a pocket and, if necessary, is switched on 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 leaving to detain criminals.

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

The whole structure is located on the printed a board for a specific case, which can be purchased by a radio amateur. In fig.   1 shows one of the design options - a 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, SA 2 mode switch , at the bottom is a steel wire antenna with a length of 300   mm in isolation.

The power source is a Krona battery, a 9 V battery or a miniature battery for powering the car alarm on / off system. The detector operates in the FM band (88 ... 108   MHz) or VHF (66 ... 74   MHz), in free space covers a distance of at least 500   m when the sensitivity of the radio is not worse than 10   μV. Since the main mode of operation is “Standby” , which turns on for 2 s after 30 ... 60   s , the battery can be used 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 a frequency of 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, the transistor VT 1 is turned on for light signaling during an alarm. To generate the signal “Standby mode” a generator of intermittent generation of sound frequency is assembled on a transistor VT 2. The frequency of generation is determined by the inductance L 6 and circuit capacitances. In the process of tuning the generator, the on and pause times can be changed within wide limits. In fig. 3 shows the timing diagrams of the generator.

A power amplifier is assembled on the transistor VT 4, to which an antenna is connected through a Collins filter ( P-loop ).

Details in the circuit are best used miniature, imported, after checking their quality. All resistors type OMLT-0.125; capacitors C 6 ... C8 type CT, CT5 type K50-35, the rest type KM. Transistors VT 1 ... VT 3 of the type КТ315Б (КТ315Г, КТ312Б, КТ342Б), VT4 - 2Т371А (КТ367А, КТ372Б, КТ382Б), diode VD 1 of the type D9B (D 2 , D18, D310), LED HL1 of the type AL336K, AL306, А30 AL102B), Zener diode VD 2 type 2S156A, switches SA1, SA2 - PD9-2, microcircuit DD1 type K561LA7 (564LA7). Choke L6 unified - miniature impulse transformer TIM-170. The scheme of its connection is shown in Fig. 4. If it is absent, it is necessary to wind the winding with PEV-1 wire Ø 0.1 mm on the M2000K 12x8x3 ferrite ring. Contour coils are wound with PEV-2 wire 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 mounting (Fig. 5) , foil- coated getinax was used. The printed conductors of the RF generator should be tinned to exclude their inductance and made wider. To install TIM-170, 8 holes 8 0.5 mm should be drilled in the board. When using a homemade inductor, the winding must be wrapped with fluoroplastic insulation, the conclusions drawn from the wire and MGTF- 0.07 mm . Since this device is wearable and may fall to the ground, it is necessary to carefully solder all connection points. After adjustment, all loop coils with the corresponding capacitors ( C 6- L 1, C 9- L 2, etc.) must be filled with paraffin to moisture-proof the circuits and give them rigidity. Failure to comply with these conditions may lead to malfunction of the beacon. To fasten the battery, two tinned bronze racks with a thickness of 0.2- solder to the appropriate points on the board 0.3 mm . The battery is attached to the case with a spring, the contacts are soldered into the board. At the end of all work, it is useful to cover the entire board, except for switches and batteries, with varnish UR-231 to protect it from rain, snow and corrosion.

Schema setup. To do this, the following devices are needed: an adjustable power supply unit with a power of at least 2 W, a tester, an oscilloscope with a band of up to 100 MHz, GIR, a wave meter, a field strength meter, a lamp voltmeter, and a control radio.

To check the node on the transistor VT 2 " Warning", it is necessary to unsolder the wire from the switch SA1, which goes to the power of the RF generator circuit, and from the power supply 9 V. Switch SA 2 switch to position "1" . When setting up this unit, the following should be borne in mind: the generator generates intermittent sinusoidal oscillations, the capacitors C16-C18 are in the feedback circuit and serve to start the generator. Together with the L 6 coil, they determine the pitch of the sound in the control radio. The selection of the ratings of these capacitors affects the operating mode of the generator. The capacitance of the capacitor C16 affects the frequency of the generator.

The duration of the generation is determined by the resistors R 6, R 7. Increasing the capacity of C16 increases the pause and saves battery consumption. Decreasing the resistances R6 and R7 increases the frequency of switching on the generator . To control the robots of this generator , an oscilloscope should be connected to the base of the transistor VT 3, and through a capacitor with a capacity of 510   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. If there is no oscillation, select C17, C18 or increase the inductance of the L6 coil. The required timbre of sound is determined mainly by the magnitude of the inductance: the larger it is, the lower the frequency of the sound.

Then, the switch SA2 is moved to position “2” “Alarm” . The HL 1 LED starts flashing immediately , bursts of rectangular pulses are visible on the oscilloscope screen . The resistor R 1 regulates the pulse duration 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. When finalizing the beacon, you should select the capacities C3 and C14 to prevent overmodulation , and also so that the generator does not "stall" .

To check the HF generator on the transistor VT3, you should unsolder the capacitors C3, C14, restore the power wire coming from SA1. Connect the oscilloscope to the VT3 collector through a 10 pF capacitor. Instead of R 8 and R 12, turn on 100 kOhm potentiometers with a 1 kOhm limiting resistor, and instead of R 9, turn on a 3 kOhm potentiometer and put it in the 300 Ohm position. By adjusting R8 to achieve the appearance of generation on the oscilloscope screen, sometimes selection C 7 is required. By adjusting R8 and R9, find the net and maximum voltage amplitudes. Then reduce the supply voltage to 6 V and fine-tune R8, R9 to find the maximum voltage for this supply voltage, and then set the average value to 6 ... 9 V.

After that, connect a real antenna and check the operation of the power amplifier on the transistor VT 4. In the circuit, a microwave transistor is used, so even minor changes in the capacitance of the capacitor C 9 affect the power output and the operating frequency (the frequency decreases with increasing this capacity, and the output power increases). To adjust instead of C9, solder a tuning capacitor of 1.9 / 20 pF, and connect the oscilloscope to the VT4 collector. Instead of C10 and C11, solder variable capacitors with capacities up to 150 pF. To adjust the C 9 you need a screwdriver made of plexiglass, textolite, etc. When the master oscillator is working, adjusting C 9 and R12 to achieve maximum voltage on the VT4 collector. The collector current measured by the tester must not exceed 18 mA at 9 V supply. By adjusting C 9 , the operating frequency should be controlled with the help of GIR and a wave meter so that it does not go beyond the working range of the control radio receiver. In accordance with the current GOST, in the range 88 ... 108 MHz, radio stations operate above 100 MHz, therefore, the beacon should be located below 100 MHz. In the domestic range, the beacon frequency should be located above 70 MHz.

The next step is to adjust the maximum output power of the cascade to VT4 and adjust the P-filter , which allows you to match the antenna length to the maximum output power, and also suppresses harmonics. The adjustment is carried out mainly by changing the capacitance C10 and C11 with a fixed inductance value of L 4. For the filter to be correctly configured, solder the capacitors C3 and C14 and again on the VT4 collector check the voltage value and the shape of the curve, and adjust them if necessary. The ultimate goal of tuning the filter is to obtain the maximum power output, and the radius of the beacon depends on this. At point B there should be a maximum voltage. An oscilloscope must be connected here. By adjusting C10 and C11, to achieve maximum voltage. It is also monitored by a distance field meter 1m (disable the oscilloscope!). A C 9 trim may also be required. If the sound in the control radio is unclean, select C3 and C14. Setting up this filter is a rather troublesome task, and its setting is described in more detail in [1]. In the author's version, the introduction of an inductor L 5 was required : without it, the power output was 40% less. Other hams may not need it.

After setting, the selected elements should be replaced by constant values ​​of a similar value and insert the board into the case along with the battery; the frequency will shift down. If necessary, squeezing or pushing the turns of L 1, adjust the frequency. Thus it is necessary to adjust and L 3.

Then the operation 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. Place the beacon in your jacket pocket, place the antenna down. Turn on SA1, SA2 put in position "1" . The assistant in the apartment finds the best position for the antenna, turning it in different directions, as well as the place in the apartment where the signal sounds louder. Then check the mode "2" . Changing the position of the beacon in relation to the radio, you can get a complete picture of the use: go to the maximum distance, go around the corner of the building, etc.

The final stage is the conduct of mechanical tests. Turn on the control radio, find the beacon signal. When turned on, the beacon should be dropped from a height 200 mm on a wooden table, first flat, then on the side rib, and then on the upper rib; 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 .