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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Car guard with a small number of parts. 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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This simple autoguard is made on a single chip. Its characteristic feature is the minimum of parts used.

The main technical characteristics of the device:

  • Time to switch to armed mode, min.......1
  • Alarm delay time, s.......5
  • Alarm duration, s.......20
  • Frequency of alarm interruptions, Hz.......1
  • Current consumption in security mode, not more than, uA.......50
  • The size of the board of a simple guard, mm.......25x40

Schematic diagram of the autoguard is shown in Fig.1. Standard door switches are used as contact sensors. Similar switches can be installed for the hood and trunk lid by connecting them in parallel with the door switches.

Car caretaker with a small number of parts
Ris.1

To transfer the guard to the armed mode, you must turn on the SA1 toggle switch, located in a secret place in the car's saloon. The time interval between the moments when the power is turned on and when the auto-guard switches off the armed mode is about one minute. During this time, it is necessary to close all doors, the hood and the trunk lid (if contact sensors SBn are installed there).

When opening any door, hood, trunk lid, the contacts of the push-button switches SB1-SBn are closed and the circuit goes into alarm mode. If the owner of the car does not have time to turn off the alarm in 5 seconds using the secret toggle switch SA1, an audible alarm sounds. In alarm mode, an intermittent sound signal is heard for 20 seconds. After 20 seconds, the watchman automatically switches to armed mode.

The autoguard works as follows. When the watchdog is turned on, capacitor C1 is charged through the resistor R5 for about 4 min. At the same time, there is zero potential at pin 11 of the OD1,5 element, and the multivibrator on the elements DD1.5 and DD1.6 is blocked. After 1 minute, the circuit switches to the protection mode.

When any contact sensor SB1 - SBn is closed, zero potential is supplied to pin 1 of the DD1.1 element through the diode VD1. As a result, a zero potential appears at pin 4 of the DD1.2 element, which leads to the appearance of a positive potential at pin 6 of the DD1.3 element, which, through the delay circuit R4, C3, enters the cathode of the VD3 diode. The diode closes, which allows the operation of the multivibrator (DD1.5 and DD1.6). Circuit R4, C3 generates a 5 s alarm delay. The multivibrator generates rectangular pulses with a repetition rate of 1 Hz, which, through the resistor R9, are fed to the key on transistors VT1 and VT2, in the collector circuit of which the winding of the car horn relay K1 is turned on. The sounding time of the signal is determined by the time constant of the circuit R3, C2. After charging the capacitor C2 (after about 20 s), a zero potential is set at the output 11 of the multivibrator (element DD1.5) and it is blocked. The circuit goes into protection mode. If any contact sensor SB1-SBn is closed, the alarm sounds continuously.

When using door switches as sensors, a situation may arise when voltage is supplied to the inputs of the microcircuit (pin 1 DD1) of the CMOS structure on which the device is made, while it will be absent in the power circuit (the guard is turned off). This can lead to the failure of the microcircuit. To protect the microcircuit, a circuit is used, consisting of a diode VD1 and resistors R1, R2, R7.

In the device, instead of the K561LN2 chip, you can use the K564LN2 (in this case, you will have to change the pattern of the printed circuit board). The KT315 transistor can be replaced with KT342, KT3102, and KT815 with KT817 or KT819. Capacitances of all capacitors may differ by ± 50% from the ratings shown in the diagram, however this will change the selected time intervals. They can be adjusted by selecting the appropriate resistors. It is desirable to use electrolytic capacitors with an operating voltage of 16 V and a small leakage current, this is especially important for capacitors C2 and C4, which, with a significant capacity, are paired with high-resistance resistors. Otherwise, a large leakage current may render the circuit unusable.

The device is mounted on a printed circuit board made of one-sided foil textolite (Fig. 2). With a properly assembled circuit and serviceable elements, the device does not need to be adjusted. The setting consists in setting the desired delay time intervals by selecting the resistances of the resistors R3, R4 and R5.

Car caretaker with a small number of parts
Ris.2

The guard is mounted in a miniature plastic case, for example, from children's counting sticks, and filled with epoxy to prevent moisture. It can be installed in an inconspicuous place under the dashboard, and the SA1 toggle switch can be installed in a convenient hidden place in the car interior. The watchman can work with other mechanical swing sensors. At the same time, it is important that when triggered, their output is at zero potential. The connection of the autoguard to the elements of the car is carried out by the contacts indicated in the diagram X1.1 -X1.4.

See other articles Section Automobile. Security devices and alarms.

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