Simple anti-theft devices. Free Technical Library article

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Simple anti-theft devices

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

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The simplest car watchdog described in the literature is shown in Fig. 1. The circuit consists of only one diode installed between the voltage generator and the battery, as well as the SA1 toggle switch.

Rice. 1. The simplest anti-theft device

The device is armed with a toggle switch and starts to manifest itself only when you try to start the engine - the car horn will start working, but it is more reliable if it is an additionally installed howler. Before starting the engine, the owner must turn off the watchdog with a discreetly installed toggle switch.

Any diode can be used for direct current of at least 50 A and reverse voltage of at least 20 V.

Such a scheme, although attractive in its simplicity, has a number of disadvantages: by the nature of the appearance of the sound, it is easy to understand the connection point and the principle of operation of the signaling device, and when the engine is not running, calmly turn it off.

On fig. Figure 2 shows a more advanced circuit using the same idea. The additionally installed relay K1 self-locks with its contacts and allows you to fix the power of the howler in the on state, regardless of the further actions of the hijacker. The circuit will remain in this state until it is turned off by the SA1 toggle switch or the battery is discharged, which, given its large capacity and relatively low energy consumption by the sounder (0,2 ... 0,3 A), will not happen soon.

If the SA1 toggle switch is used with two groups of switching contacts, for example, type TZ, then the free group can be used to break the ignition switch circuit to block the engine from starting.

Diodes VD2 and VD3 are suitable for any current of at least 1 A.

Relay K1 is best used in a sealed version, for example, type RES48A, passport RS4.590.202, or similar, for an operating voltage of 12 V.

It is possible to ensure the fixation of the operation of the howler without the help of a relay, fig. 2. In this case, an electronic switch is used - a thyristor VS1. When triggered once, it will stay open until the howler chain is broken. In this circuit, you cannot use the sound signaling device found in the car, due to the fact that the principle of its operation is similar to a buzzer - an oscillating membrane is connected to a group of contacts that break the coil circuit. Due to the discontinuity of the current through the horn coil, the thyristor turns off when the signal on the control electrode ends.

Rice. 2. Anti-theft device with continuous operation howler

Rice. 3. Electronic method of fixing the alarm

The circuit can be made even simpler if you use the K1 current relay, fig. 4. In addition, this circuit allows you to connect additional sensors that respond to a short circuit (F1, F2). They can be installed on the hood and in the trunk. The appearance of current in the circuit where the winding K1 is turned on will lead to the operation of the group of contacts K1.1, which will turn on the thyristor VS1.

It is easy to make a current relay on the basis of a reed switch, fig. 5. The reed switch will require a little more than the most common KEM-1. It can be obtained, for example, by disassembling the reed switch from the RPG-8 series. A paper frame is glued onto a glass cylinder and 20 turns are wound in two layers with PEV wire with a diameter of 1,8 ... 2,5 mm. Depending on the type of reed switch used, it will operate at a current of more than 1,6 ... 4,6 A (reed switches with one group of normally open contacts are the most sensitive). At the same time, the internal resistance of the current relay is very small and does not have any effect on the operation of electrical equipment.

Rice. 4. Using a current relay in an alarm circuit

Rice. 5. The design of the current relay using a reed switch

Another option for turning on the security alarm is shown in Fig. 6. The base circuit of the transistor is connected so that when the contacts of the ignition switch are closed, the capacitor C1 starts charging through the diode and resistor R1. Transistor VT1 opens, and the thyristor VS1 is activated, including the buzzer.

Rice. 6. Option to turn on the alarm

Such anti-theft devices can be used both separately and as part of a multifunctional alarm system. In addition, due to their simplicity, they are easy to install on a motorcycle or any other vehicle where there is no need (or space) for a more complex security system. In this case, all parts of the device should be placed in a closed case made of dielectric material in order to prevent the circuit from shorting to the case and causing a fire.

As a general disadvantage of the above schemes, a limited number of places can be canceled where a toggle switch for disarming can be installed. This increases the likelihood of its rapid detection.

Publication: cxem.net

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