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Forget-me-not in car protection. Encyclopedia of radio electronics and electrical engineering

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

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The "forget-me-not" principle described in the article "Project "Forget-Me-Not""(" Radio ", 1997, No. 10 pp. 6 - 9), can also be used to protect the car from being captured by intruders. The proposed article describes the nodes that need to be changed or added to an already published design so that the forget-me-not can work in car.

A security system that prevents the theft of a car from a parking lot, from a garage, etc., will not help much in case of a robbery attack on the driver along the way. But robbery loses its meaning if the robbers cannot use their acquisition. The idea of ​​​​the device is simple and well-known - to block the normal operation of the car's electrical equipment after capture, that is, everything works in the presence of the owner, and when he remains on the sidelines (along with a miniature radio transmitter) - the engine stalls.

Almost all elements of the device have already been described (see the article "Forget-Me-Not Project"). Here is a highly economical miniature radio transmitter that generates a pulsed radio signal, and a radio receiver that receives this signal only at a very short distance. It remains only to replace the electronic part of the radio receiver, which forms in "Forget-me-not" alarm acoustic signal, on the other - turning on and off the electromagnetic relay.This can be done, for example, as shown in Fig. 1.

If the radio receiver receives the signals of the transmitter, at its output (on the collector of the transistor VT1 not shown in Fig. 1), periodically repeating short pulses appear. These pulses regularly return the DD2 counter to the zero state, which corresponds to a low level at all its outputs. A high level occurs at the outputs of the elements DD1.3 and DD1.4, the transistor VT2 opens, relay K1 is activated and puts the car's electrical equipment into working condition.

Forget-me-not in car security

But if the distance between the receiver and the transmitter exceeds the limit (several meters), the pulses at the input R of the counter DD2 will disappear. The multivibrator on the elements DD1.1, DD1.2 (excited at a frequency of about 0,5 Hz) will begin to change the state of the counter DD2, and after 16 seconds a high level will appear at its output 9 (pin 11). Accordingly, the outputs of the inverters DD1.3 and DD1.4 will be set to a low level, relay K1 will turn off and the vehicle's electrical system will be blocked. Since further counting in DD2 is impossible (at its input CP is a high level), this state will persist until the first radio pulse appears.

+1 ... 5,5 V is removed from the zener diode VD6 to power the radio. It is desirable to display the HL1 LED on the dashboard of the car - it will inform the owner about the state of the security system.

How exactly the relay contacts will be used to block the electrical systems of the car depends on the capabilities and imagination of the owner.

In table. 1 shows the parameters of some relays. Suitable, of course, relays and other types. It is only important that the selected relay has a 12-volt winding with a resistance of at least 25 ohms.

Forget-me-not in car security

In the case of using a powerful relay or contactor capable of switching currents of tens of amperes, it is better to make an electronic key as shown in Fig. 2.

Forget-me-not in car security

Establishing a receiver with a relay output has no special features. The radio receiver and the executive relay are installed discreetly, in a hard-to-reach place. Placing the receiver itself can be tricky. And although it is equipped with a magnetic antenna, which has a reduced sensitivity to electrical interference from the same ignition system, it may be necessary to tidy up all the electrical equipment of the car: improve the wiring (place some in the screen), install filters.

Although the forget-me-not transmitter can work in this system without any modifications, it is better to make another one that is more convenient. Its circuit diagram (Fig. 3) almost does not differ from the prototype: only the RC circuit was removed, which lowers the supply voltage of the microcircuit, and the values ​​of some resistors and capacitors were changed. But this transmitter is powered by another source - a small-sized lithium cell (011,6x5,4 mm, voltage - 3 V, capacity - 130 mAh).

Forget-me-not in car security

The main changes are in the design and are associated with the desire to make the transmitter flatter. Hence the not quite usual configuration of the printed circuit board (Fig. 4), in the round cutout of which the power supply is placed.

Forget-me-not in car security

A standard quartz resonator and an oxide capacitor with dimensions of 14x18,5 mm are installed in a 6x12 mm cutout. A coil L1 - 35 ... 40 turns of wire PEVSHO-0,25 mm is wound on the curly ledge of the board. This is the "magnetic antenna" of the transmitter.

In three places, the board is "pierced" with jumpers a, b and c (when installing the microcircuit, you need to make sure that jumpers a and b do not touch its "bottom"). The connections of the terminals of resistors, capacitors, etc. with a common wire are shown as black squares. When soldering the terminals of resistors R1 - R3, capacitors C1 and C2 to the terminals of the microcircuit (on the side of the elements), it is recommended to use a miniature soldering iron with a tip with a diameter of no more than 2,5 mm, which has a thin inclined slot.

All resistors are MLT-0,125, ceramic capacitors are the smallest, better with side leads. All elements are mounted so that their elevation above the board does not exceed 4 mm. Transistors KT3102EM (plastic case) are placed with a cut segment directly on the foil. If these recommendations are followed, the height of the fully assembled board will not exceed 6...6,5 mm and, accordingly, the transmitter enclosed in a sufficiently strong case will be rather flat - no thicker than 8 mm.

The repetition rate of the radio pulses of the transmitter depends on the frequency of excitation of the multivibrator on the elements DD1.1, DD1.2 (see Fig. 3). With the ratings indicated on the diagram, this frequency is about 0,5 Hz. The duration of the emitted radio pulse tfh is less than ti, - the duration of a single pulse at the output of DD1.4, which allows the excitation of the transmitter. ti = 0.7R3C2 = 22 ms. In table. 2 shows the dependences of the current consumed by the transmitter (Icon) and tfh on Upit - the voltage of the power supply

Forget-me-not in car security

About the power switch. On the one hand, it is not really needed here, since the current consumed by the transmitter will allow you not to change the power source for more than six months (the element is soldered and forgotten about it for all this time). But with a switch, the same source can last even 10 years, since the self-discharge of a lithium cell is very small (in 10 years, its energy reserves decrease by only 10 ... 15%). But even if the switch is of very good quality (sealed, with gold-plated contacts, etc.), in this design it will be, perhaps, the most unreliable element. A kind of power switch can be a loop of thin wire, which breaks when the microtransmitter is forcibly removed from the driver. Successfully repairing it next to a freshly stolen stalled car seems unlikely.

Setting up the transmitter is reduced to the selection of a quartz resonator. The resonator may not fit in frequency either (as experience shows, the discrepancy between the frequency marked on the resonator case and the one at which it is actually excited can reach several kilohertz). The selection of a quartz resonator will be greatly simplified if not it itself is soldered to the board, but short "through" sockets from some suitable connector having an inner diameter of 1 mm, for example, 2PM.

You can verify the normal operation of the transmitter not only by the reaction of the radio receiver of the system itself, but also with the help of a nearby CB radio station. It must be set to channel 39 of the B grid of the European frequency scale (this is the frequency of 26945 kHz), and the transmitter must be switched to continuous radiation mode by connecting the inputs of the DD1.4 element to a common wire.

When repeating the device, it should be remembered that the operation of the device should not lead to an emergency.

Author: Yu.Vinogradov, Moscow

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