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A simple car watchman. 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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You can buy a ready-made burglar alarm for your car. Having made the watchman on your own, you will save not only money. Practice has shown that a home-made device, unlike a branded one, often significantly complicates the "work" of a hijacker. The author offers a description of a simple but quite reliable security device that can be assembled in three to four evenings.

This signaling device gives disturbing sound signals at any attempt of unauthorized penetration into the car or dismantle of its knots. The device is powered from the on-board network with a voltage of 11,5 ... 14,4 V and consumes no more than 3 mA in standby mode. Alarm signal - sound pulses with a repetition rate of 1 Hz.

The circuit diagram of the autoguard is shown in fig. one.

Simple car caretaker
(click to enlarge)

When the power is turned on with the toggle switch SA1, the capacitor C3 begins to slowly charge through the resistor R2. A high voltage level at the input R of flip-flops DD2.1 and DD2.2 (point A in the diagram) will set them to zero. A high level from the inverted output of the trigger DD2.1 will quickly charge the capacitors C3 and C4.

The node, assembled on the elements DD3.3 and DD3.4, will generate a pulse with a duration of 400 ms, which will pass through the diode VD5 to the input of the current amplifier, assembled on transistors VT1, VT2. For this time period, relay K1 will work, which serves as a load of a powerful transistor VT2. and the BF1 siren will give a single short beep to indicate that the guard is powered.

After about 6 seconds, the capacitor C2 will be charged, the voltage at point A will decrease to the switching threshold of the CMOS elements, and the autoguard will be set to standby mode. The inhibited generator, made on the elements DD3.1 and DD3.2, will start operating at a frequency of about 1 Hz. The HL1 LED will flash, indicating that the autoguard is armed.

Before the expiration of this time delay tt, the driver must get out of the car and close the doors, bringing the security contacts SF1 and SF2 to the position shown in the diagram.

When the car doors are opened, the SF2 contacts will close. A high level will appear at the output of the DD1.1 element. This positive voltage drop will switch the DD2.1 trigger to a single state, a low level will be set at its inverted output and capacitors C3 and C4 will slowly discharge. As soon as the voltage on the capacitor C4 reaches the threshold level, the generator assembled on the elements DD1.3 and DD1.4 will turn on. and will begin to generate pulses with a frequency of about 2 Hz.

The pulses will go to the trigger DD2.2, included by a frequency divider by 2, and then through the diode VD4 to the input of the amplifier on transistors VT1. VT2. As a result, relay K1 will operate and release the armature after 0,5 s through pauses of the same duration. The sound siren, activated by contacts K 1.1 of relay K1, will begin to reproduce alarming sound pulses with a repetition rate of 1 Hz.

The VD7R11 circuit contributes to a clearer inclusion of the generator on the elements DD1.3. DD1.4. After their first switching, a low level occurs at the output of the DD1.4 element, the capacitor C4 is quickly discharged through the diode VD7 and the resistor R11 and does not affect the further operation of the generator.

While the capacitor C4 is discharging (time t2-5 s), the owner, entering the salon, must have time to turn off the power of the watchdog, otherwise it will work and turn on the alarm signal.

In the event that the watchman has tripped, the alarm will sound until then. until capacitor C3 is discharged. As soon as this happens, the DDI.2 element will switch to a single state and, similarly to the one described above, will set the device back to standby mode, provided that the SF2 contacts are open by this moment. If they remain closed, the alarms will cycle. The duration t3 of this cycle is about 35 s.

All three time delays t1 - t3 can be corrected by the appropriate choice of capacitors C2, C4 and C3.

Diode VD8 protects the signaling device from erroneous switching on of the supply voltage in reverse polarity. Capacitor C6 smooths out the ripple of the supply voltage that occurs during the operation of the watchman.

Security sensors SF2 - contacts installed on the doors of the car interior (conventionally, the diagram shows one pair of contacts; it is actually possible to connect up to ten pairs in parallel, including the swing sensor). If the existing door interior lighting switches are used as sensors, they must be "uncoupled" by separating diodes VD1 and VD2 (Fig. 2).

Simple car caretaker

A group of sensors SF1 (of which there may also be several, but connected in series) are mounted on the interior windows. Structurally, glass sensors "can be different - from ready-made microswitches to home-made foil or wire (0,05 mm in diameter or less) stickers on the glass surface.

Most parts of the device are mounted on a 1,5 mm thick foil fiberglass printed circuit board. The drawing of the board is shown in fig. 3. The board has places for mounting isolation diodes (VD1, VD2 in Fig. 2). The board is reinforced in a durable duralumin or steel box.

Simple car caretaker

Transistors KT315A and KT815A can be replaced by KT3102A and KT817A, respectively. A powerful transistor VT2 must be installed on a heat sink with a cooling area of ​​at least 6 cm.

Oxide capacitors - K50-35; the rest - K73-17. Diodes VD1 -VD7 - any of the KD521 and KD522 series. Relay K1 - RES22, passport RF4.500.129 or automotive relay RS527.

As a BF1 siren, the sound signal (horn) available on the car is suitable, but it is better to use a siren from one of the imported alarm devices. Sirens come in different capacities - 5.10 and 20 watts. At high power, the signal of the guarded car sounds very loud, but the battery, of course, discharges faster. It is impossible not to take into account the fact that excessively loud sounds disturb the neighbors. Therefore, the choice of the optimal power of the siren should be given due attention. Recommended siren types - BS37-PO, SNEETAN-CH119.

The right approach to choosing the power of the siren in a number of specific cases makes it possible to increase the reliability of car protection by using an alternative power source for the watchman - an autonomous small-sized battery recharged from the on-board network while driving.

Many consider it expedient to supplement the watchdog with a light signaling. It is easy to implement by using free relay contacts or by connecting lamps in parallel with the siren, you only need to take into account the load capacity of the contacts. As a last resort, we can recommend installing two relays in parallel instead of one. In this case, the resistance of the resistor R8 must be reduced by a factor of three.

Author: O. Tsitsersky, Lviv, Ukraine

Refinement of this device

See other articles Section Automobile. Security devices and alarms.

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