Car security systems on the PIC12F629 microcontroller. Encyclopedia of radio electronics and electrical engineering

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

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In continuation of the topic of car security systems based on MICROCHIP microcontrollers [1], [2], two more schemes of car security systems based on PIC12F629 are proposed, modernized in order to increase the reliability of operation both in adverse climatic conditions and in car break-in conditions, when trying to building a car security system.

The scheme in fig. 1 is a modernized version of the AOS car security system [1] using a new element base. The modernization touched upon the correction of the security system operation algorithm for the PIC12F629 microcontroller, changes in the input circuits for receiving signals from door lighting switches, hood and trunk limit switches, and changes in the LED connection scheme.

Fig.1. Schematic diagram of a car security system (click to enlarge)

The AOSM car security system is based on the PIC12F629 microcontroller from MICROCHIP with non-volatile memory. The presence of non-volatile memory allows you to save the current state of the AOSM during a normal or deliberate power outage and switch to it when the power is restored. Changes in the AOS operation algorithm affected mainly the PREPARATION mode [1]. In the new version, the AOSM operation algorithm in this mode will be as follows:

PREPARATION - after disembarking passengers and closing all doors, visually checking the condition of the hood and trunk, the driver, sitting in the cab, turns on the hidden power switch SA1 (Fig. 1) of the AOCM unit and, if the power supply of the AOCM system was turned off by the driver in the DISARM mode, then the VD1 LED will light up and will burn continuously (Fig. 1).

Note: if the power of the AOSM system was turned off in the ARMED mode, then when the power is restored, the AOSM system will immediately switch to the ARMED mode.

If the power supply of the AOSM system was turned off in the ARMED mode after the burglary, then when the power is restored, the AOSM system will immediately switch to the ARMED mode with ignition blocking.

If the power supply of the AOSM unit was turned off during the HARM state, then when power is applied, the AOSM system will immediately switch to the HARM mode with a corresponding sound alarm. And only after working out this mode and switching to the ARMED mode, the DISARMING mode is possible.

Thus, attempts to deliberately disable the AOSM system by opening the hood and removing the wires from the battery terminals and then connecting them will be accompanied by an audible alarm every time the wires are connected to the battery in the HACKING mode.

The subsequent algorithm of the modernized automobile security system AOSM coincides with the algorithm of the AOS system [1].

The change in the input circuits for receiving signals from end sensors is caused by the need to increase the reliability of the security system in conditions of high humidity and in winter conditions. When one of the limit switches is triggered, the cathode VD3 or VD4 (Fig. 1) [1] is closed to the housing. The voltage at one of the inputs of the PIC controller decreases from 4...5 V to 0,5...0,7 V due to the voltage drop across the diode. In this case, the microcontroller will work in accordance with the operation algorithm given in [1].

With careless connection of wires to the limit switches, with oxidation of the contacts of the switches due to long-term operation, with the use of diodes with an increased voltage drop at low temperatures, this voltage increases and may exceed the value of 0,8 V - the threshold of the microcontroller.

As a result, the security system will not arm or will not respond to the operation of any switches. To exclude such situations, the input circuits of the security system were finalized.

The LED connection scheme has also been changed in case the wires connecting to the LED become available to the hacker, and he tries to disable the AOSM by applying a voltage of, for example, 100 V to the wire connected to the LED cathode. In this case, the resistor R12 will burn out , but the performance of the car security system will not be affected. A method for manufacturing a shock sensor, other technical details are given in the article [1].

The scheme in fig. 2 is one of the options for the technical implementation of the immobilizer - a car security system with two channels for blocking the operation of the engine.

Fig.2. Wiring diagram (click to enlarge)

The immobilizer prevents the engine from starting when the ignition is turned on. Arming and disarming is carried out in a non-contact way using an IR remote control. The scheme of the key fob and a description of its operation are given in [2]. To transmit commands from the key fob to the immobilizer unit, a 32-bit code is used, individual for each sample of the car security system, and pulse-phase modulation of infrared rays.

The SA1 switch is located in a hard-to-reach place and is designed for emergency shutdown of the immobilizer unit when it fails. The block itself, together with a photodetector and a relay, are located behind the dashboard. The photodetector is attached from the inside to the dashboard in any way, having previously drilled a hole with a diameter of 1 ... 3 mm at the attachment point for the passage of IR rays. Although you can first check the operation of the immobilizer without drilling a hole. Perhaps the radiation power of the key fob is enough to overcome the obstacle in the form of a dashboard wall.

The 12 V power supply to the immobilizer unit is supplied from the car's electrical circuit, on which voltage appears when the ignition is turned on. When the ignition is turned off, the immobilizer is de-energized and does not consume current from the battery.

The algorithm of the immobilizer is as follows:

1. DISABLED - the power supply of the unit is turned off, and the security system in this mode does not affect the electrical equipment of the car.
   PREPARATION - during the initial power supply to the immobilizer unit, the VD1 LED lights up and lights up continuously. Relays K1 and K2 are disabled. The immobilizer remembers this current state of the system in its non-volatile memory and returns to this state upon subsequent power off and power on. The immobilizer also enters the same state upon command from the key fob when disarming.
2. ARMING - with the ignition on in the PREPARATION state, point the key fob at the photodetector and press the key fob button once. The system then switches to the LOCK mode.
3. BLOCKING - in this mode, the VD1 LED (Fig. 2) flashes at a frequency of 2 Hz, the relays K1 and K2 turn on and their contacts K1.1 and K2.1 break the selected blocking circuits. The immobilizer remembers this current state of the system in its non-volatile memory and, upon subsequent switching off and on of the ignition, returns to this state, blocking the operation of the engine.
4. DISARMING - with the ignition on in the LOCK mode, point the key fob at the photodetector and press the key fob button once. The immobilizer will then enter the PREPARATION mode.

During operation in the BLOCKED mode, the microcontroller constantly monitors the presence of a signal from the photosensor. If the signal from the photo sensor fails, the blinking frequency of the LED decreases from 2 Hz to 0,5 Hz, but the engine block is not removed when the ignition is turned off and on. To unlock the engine, it is necessary to eliminate the malfunction or turn off the power to the immobilizer using the SA1 switch (Fig. 2). When a signal from the photosensor appears, the blinking frequency of the LED returns to the original frequency.

When the power is turned on and the PREPARATION mode is switched on, the microcontroller also monitors the presence of a signal from the photo sensor, and if it is absent, the LED starts flashing even less often at a frequency of 0,2 Hz, indicating a malfunction, but not blocking the engine. In this case, it is also necessary to quickly eliminate the malfunction that has arisen.

Photodetector DA1 type ILMS5360 can be replaced with photodetectors SFH506-36, TFMS5360, etc. Switch SA1 can be of any type, preferably small.

Relays K1 and K2 - any 12 V relays with a permissible current through contacts of 8 ... 15 A or more, depending on the switched load.

Other technical details of the operation of the remote control on IR beams are given in the article [2].

For advice on the operation of the described schemes and with your wishes and suggestions, contact the author of the article.

Literature

1. "A simple car security system on PIC12C508A", - Radio amateur, 2002, No. 2.
2. "A simple car security system based on PIC12F629 with IR remote control", - Radio Amateur, 2003, No. 5.

Author: N. Kupreev, Minsk; Publication: radioradar.net

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