Car alarm sensors. Free Technical Library article

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Car alarm sensors

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

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There are many different types of burglar alarm sensors. Due to the complexity of manufacturing at home, we will not consider most of them, but we will focus on the simplest ones.

So, the glass of a car can also be protected by an ordinary miniature button (MP-1) installed on a corner, fig. 1. It is attached to the glass with a corner clamp for a rubber seal.

In addition to buttons, hermetic reed contacts, which close when exposed to a magnet, are most often used as sensors. They have high reliability and small dimensions, which makes it possible to secretly place them in a recess on the frame of doors and window frames. In this case, a small magnet is attached to the moving part, for example with glue (Fig. 2).

Depending on the location of the magnet relative to the reed switch, the sensor can work to close or open the circuit when triggered (usually used to open).

The sensitivity of the reed switch contacts is sufficient for it to operate at a distance of 5...15 mm from the magnet.

Using only reed sensors and buttons, it will not be possible to provide reliable protection everywhere (windows can be broken).

The simplest way to signal when a window is broken is a thin and narrow strip of metal foil pasted around the perimeter of the glass, connected in series to the security loop circuit (see Fig. 3). It will break when glass is broken, but if a thief uses a glass cutter, it may not work.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

For protection on glasses, DIMK sensors manufactured by the industry are widely used (Fig. 4). Such a sensor is glued to the glass and responds to impacts or breaking due to the fact that the plate with the magnet attached to it does not have a rigid attachment and, upon impact, moves away from the reed switch, which breaks the circuit of the security loop (all sensors are connected in series).

A similar design is easy to make yourself or purchase.

For signaling shocks and vibrations, a B-2 type piezo signaling device (BQ1, Fig. 5) can also be used in conjunction with an amplifier circuit.

This sensor is designed to operate in the temperature range of ±60 °C and is glued to the glass with glue or pressed with a metal plate to the body of a car or garage (Fig. 6).

The power supply of the circuit can be from 5 to 15 V, and the sensitivity is set by the resistor R8 during tuning. When the signaling device is triggered, a zero level appears for a short time at the output of the circuit. The device can be connected directly to the security circuit (for example, shown in Fig. 7).

Rice. 5. Amplifier to the piezoelectric sensor

Rice. 6. Type of sensor mounting

On cars, to prevent the removal of wheels, sensors of mechanical vibrations of the body are often used. It is not difficult to make such a sensor from a springy metal plate with fastening at one end of the load (see Fig. 8), but it has low reliability and requires frequent mechanical sensitivity adjustment due to loosening, as well as changes in weather conditions.

The sensitivity of such a sensor depends on the unevenness of the ground at the parking lot, which often leads to false alarms or an uncontrolled decrease in sensitivity.

Rice. 7. Amplifier to the electromagnetic sensor

Rice. 8. Mechanical vibration sensor

All these shortcomings are devoid of a circuit with an electromagnetic sensor (Fig. 9). As a sensor sensitive to vibrations, you can use a small-sized milliammeter (for example, from household radio equipment, M4370 and many others), having finalized it. To do this, the case of the measuring device is opened and a small weight is fixed at the end of the arrow (you can take a piece of tubular solder with internal rosin). The rosin is removed with a needle, and the end of the arrow is inserted into the resulting hole and crimped with pliers.

When choosing a milliammeter, it does not matter where its initial position of the arrow is - under the influence of the weight of the load, it will take the middle position with the corresponding orientation of the device itself.

Rice. 9 The design of the electromagnetic sensor of mechanical vibrations

The sensitivity of the electromagnetic oscillation sensor does not depend on the unevenness of the place, and it can be triggered even by a light touch on the car, and its sensitivity can be easily changed with a resistor R5 of the SP5-2V type (for example, during rain it should be reduced to prevent false positives).

For ease of use, it is better to use two resistors R5 and switch them to pre-set sensitivities with a toggle switch. It is better to place the amplifier circuit to the sensor close to the sensor.

To protect a dark basement or cellar, you can use a light sensor (Fig. 10). It will allow you to detect the penetration of an outsider, who will surely use the light.

Rice. 10. Light sensor

Resistors marked with "*" in the diagrams must be selected during adjustment.

Publication: cxem.net

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