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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Car alarm with IR control. 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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Recently, the interest of motorists in electronic systems for opening and closing doors (central locking) has begun to increase. This is due to the increased theft of radio tape recorders and other valuables from car interiors. As a rule, setting only a simple alarm does not give the desired effect, since the thief has time to remove the radio and leave the car in a few minutes. Therefore, car owners began to install alarms with central locking drives.

This article is about a homemade central locking system based on the Atmel AT90S2313 microcontroller. For a remote command transmission system, infrared (IR) radiation is used. This choice is based on the fact that most radio amateurs do not have the equipment for setting up high-frequency transceivers, as well as the experience of assembling them. The use of IR beams is facilitated by the fact that ready-made IR receivers for household equipment are commercially available, containing the entire necessary set of amplifiers and detectors and having an output with logic levels suitable for direct connection to a microcontroller.

Consider now the scheme and logic of the device.

For transmission, a device based on the AT90S2313 microcontroller is used. In this scheme, there are two buttons, depending on the state of which one or another command is transmitted. The transmitter firmware is a software encoder of information in the RC-5 standard. This standard is widely used in consumer equipment, so the description of the encoding is not given here (see, for example, the article "Using the RC-5 code" in this issue of the magazine - ed.). In order to prevent the transmitted information from being intercepted by a scanner (which can be a microcomputer with an IrDA port), it was decided to use a dynamic authentication system. In this case, a counter of button presses is used with an index selection of values ​​from three tables of 256 bytes. This method was chosen due to the fact that in each case it is easier to modify the tables (which should be the same in the receiver and transmitter) than to rewrite the mathematical function. Another argument was that the function can be selected by several values ​​of the counter and result, and 768 bytes of the table are much more difficult to select. To create a table, you can use a program to generate random numbers.

Thus, the information package of the transmitter consists of the following parts (see Table 1).

Table 1

1 2 3 4
Identification byte Command Byte (00, 01, 02) Counter value (4 bytes) Function value

The diagram shows two versions of the transmitter unit. The first option (Fig. 1) has a constant power supply of 3 V, the microcontroller in this case operates in Sleep mode and exits it only by an interrupt caused by pressing a button. The power consumption of the transmitter in standby mode in this case is about 40 μA.

Car alarm with IR control. Device diagram option 1

Device diagram option 1

The second option (Fig. 2) does not consume energy when the buttons are not pressed, but contains two diodes and more resistors.

Car alarm with IR control. Device diagram option 2

Device diagram option 2

When assembling the transmitter, it is advisable to use SMD components, and the microcontroller in a SOIC package. The LED is applicable to any infrared range. Time delays in the program are given for a 4,19 MHz quartz resonator. When using SMD components, the transmitter is placed in a key fob from a standard car alarm.

The heart of the stationary unit (Fig. 3) is also the AT90S2313 microcontroller. The microcontroller program is an RC-5 decoder, an authentication system, and also contains control blocks for lock drives.

Car alarm with IR control. microcontroller

Microcontroller AT90S2313

To receive the control signal, a ready-made RC-btype receiver HS0038 or similar is used. However, it must be taken into account that the receivers are designed for different carrier frequencies (from 32 to 40 kHz) and it may be necessary to correct the time delays in the receiver and transmitter programs. It is also necessary to ensure that the receiver is installed in a location protected from direct sunlight.

One annoying feature of the AT90S2313 is the possible destruction of information in the EEPROM during power failures. To bypass this feature, you must use an external reset circuit (for example, MAX709, MAX809, MAX811 or similar), and when powered from the on-board network, use a good-quality stabilized 5 V power supply, since the on-board network is a source of a large amount of interference and high-voltage surges, which if you use a low-quality stabilizer, it can lead to unstable operation and even damage to the device.

Consider further the operation of the device. The system is designed to work with two-wire lock drives where the polarity reversal is controlled by an internal motor. Upon the arrival of the "open door" command, the microcontroller outputs a logical "1" to the PB0 output, while the RL4 relay is activated and a potential of +2 V appears at the Lock(12) output relative to the Lock(1) output. Upon the arrival of the "close the door" command, the relay RL3 is activated and the potentials at the outputs are reversed.

When connecting the relay, it must be taken into account that the normally closed contact must be connected to ground, and the normally open contact must be connected to +12 V.

The process of opening and closing the doors is accompanied by the inclusion of headlights, for which relay RL1 serves.

The remaining outputs (siren, LED) are reserved and are not supported in this version of the program.

In this design, it is necessary to use a relay with a response voltage of 3 - 5 V, while the transistors are selected to provide the required current through the relay. However, it must be taken into account that the current consumed by the lock drives reaches 10 A per pulse, which requires the use of a relay of the appropriate type. The frequency of the quartz resonator for the above program is 8 MHz. Diodes can be used any.

In conclusion, we can say that the described system can easily be expanded to a full-fledged car alarm by adding software blocks for arming and disarming, as well as shock detection and door opening, especially since there is already a siren output, as well as an LED for indicating operating modes.

All that remains is to add an input circuit for sensors and connect it, for example, to the PD3Int1 input. In this case, an interrupt will be called if the corresponding sensor is triggered.

The software required for the operation of the device is available at: platan.ru/shem.

Author: Vladimir Zimin, phantom@tversu.ru; Publication: radioradar.net

See other articles Section Automobile. Security devices and alarms.

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