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Car alarm. 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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Simple and reliable alarm. Of the controls - only one switch. Intrusion sensors are ordinary switches that, when working with a circuit, fulfill their intended purpose, for example, buttons on doors. It is enough for one of them to close for a short time and the circuit will work. The scheme gives itself out only after 10 seconds. During this time, the owner can turn off the alarm, and the burglar think that there is none.

In armed mode, the device consumes a current that depends on the parameters of the parts used: in my case, it was not possible to measure it due to its small value.

This scheme was developed a very long time ago and was assembled from improvised parts.

Car alarm. Scheme

Operating principle

Elements SA2-SAn - penetration sensors. Diodes VD5-VDn serve to isolate the sensors if they are used for other purposes. In some cases, diodes can be excluded.

The supply voltage supplied from the closed sensor is supplied to VD1 through R1 C1. The R1 C1 circuit creates a short current pulse even if the sensor is left closed. Capacitor C2 prevents the alarm from triggering when SA1 is turned on.

On elements C4, R4, R5, VT2, K1, a multivibrator and an output key are assembled. The duration of K1 being in the on position is selected by the resistor R5, and in the off position - R4. The overall pulse frequency is set by C4. This part of the circuit requires careful tuning. I got a frequency of 2 Hz.

C3, VD3, VD4 - a node that generates an alarm response delay when the sensor is closed. This is necessary to turn off the alarm when the door is opened. The duration of the delay is set by the capacitor C3. Resistor R3 ensures that the capacitor discharges when the power is turned off.

The unit that turns off the alarm some time after the operation was not developed, since there are no false short circuits of sensors on the doors, trunk and hood of the car.


The circuit consists of a small number of available parts. VD1 - any low-power trinistor, for example KU101. You just need to select C1 (increase if it does not work when the sensor is closed), R2 (decrease if it does not work, but increase if possible) and C2 (increase if it works when the power is turned on). Diodes - any low-power. Relay K1 - RES55A, however, it can switch the load with a current of not more than 1A. If you use a more powerful relay, then you will need to greatly increase the capacitances of the capacitors C3 and C4 (and, consequently, the dimensions of the device). Therefore, it is better to connect a powerful relay to the RES55A output. Transistors - also any, with the appropriate structure, and VT2 must withstand the relay turn-on current. SA1 - any small switch.

The types of capacitors C2, C3, C4 depend on the climatic conditions in which the alarm will be operated. For cold winters, it is better to choose them from the K53 series. If capacitors of the K50 series are used, then it is better to install the device in the passenger compartment. However, an experimental sample of the alarm on the K50 was installed under the hood and operated for more than two years (removed due to the transition to a new alarm system) in cold climatic conditions (the car was in the open), and not a single malfunction was noted.


1. Turn on SA1 with the sensor closed (door open). In this position, the scheme can be indefinitely long.

2. Open the sensor (close the door).


1. Open the door (sensor closes).

2. Within 10 sec. turn off SA1.

The printed circuit board for the device was not developed. The installation was carried out by a hinged method on a sheet of hard cardboard impregnated with varnish. A finished metal box was used for the case.

Author: Azanov Marat, Ufa; Publication: N. Bolshakov,

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