Reducing the likelihood of false alarms. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Reducing the likelihood of false alarms. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Safety and security

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Security systems incorporating vibration sensors are highly prone to false positives. The proposed system is designed in such a way as to reduce the likelihood of false alarms and not needlessly disturb either the owner of the protected object or others.

A common drawback of most industrial and amateur radio burglar alarm devices is a high percentage of false positives. A vivid example of this is parked cars howling for no apparent reason.

An analysis of the causes of false alarms shows that for the most part these are short-term effects of natural factors on the protected object. So, in relation to car security systems with a piezoelectric sensor, this can be a gust of wind, a car that has passed close by, an electromagnetic impulse in a thunderstorm, etc. Malicious actions are usually longer, which makes it possible to effectively identify them.

This problem is not new, and some of its solutions have already been published in the journal. So, for example, Yu. Vinogradov in his article "Vibration sensor for a security device" ("Radio", 1994, No. 12, p. 38) proposed to apply a signal from the output of the shaping amplifier to a periodically reset binary pulse counter. The actuator is switched on by a signal from one of the digits of the counter, selected by the switch.

An alarm is given when the number of pulses from the sensor during the reset period exceeds the set limit. In other words, the device analyzes the situation not by the duration of the impact on the sensor, but by the number of generated pulses per unit time. If the sensor generates a large number of pulses in a short time (a kind of chatter), a false alarm is not excluded.

Schematic diagram of the proposed security alarm is shown in the figure. The BQ1 sensor converts the mechanical vibrations of the protected object into electrical impulses that are fed to the input of the amplifier-shaper with a high input resistance, assembled on transistors VT1, VT2. The tuning resistor R5 sets the sensitivity.

(click to enlarge)

Further, the signal, passing through the element DD1.2, is fed to the input of the time analyzer, collected on triggers DD2.1, DD2.2 and elements DD1.1, DD1.3. The R7C5 circuit generates an initial setting pulse at the moment of power-up, which, after being inverted by the elements DD1.3 and DD1.1, sets the triggers DD2.1 and DD2.2 to the zero state. At this time, the device is insensitive to the sensor signals and allows you to leave the protected object.

When the initial setting signal ends, the unit enters the standby mode. When pulses are received from the sensor, the DD2.1 trigger switches, and for the time specified by the R9C7 circuit, the device is blocked - the sensor pulses do not affect the state of the device. The owner, however, needs this time to turn off the device (if the switch is located inside the protected premises) upon returning to the object.

When the voltage on the capacitor C7 reaches half of the supply, the blocking is removed. Now the very first pulse that came from the sensor within 30 s (with the values \u8b\u6bof the elements R2.2, C3 indicated on the diagram) will switch the trigger DD10, power through the opened transistor VT8 will go to the alarm signal generator, and within the time specified by the circuit R2.2C2, an alarm will sound. In this case, a low level from the inverse output of the trigger DD6 prohibits the passage of pulses through the DDI.3 element in order to block the signals from the sensor for the duration of the alarm. The R1.2C1 circuit provides an additional small delay in opening the DD1 element. These measures reliably prevent acoustic feedback between the BAXNUMX driver and the BQXNUMX transducer.

With the ratings indicated on the diagram), the duration of the modes will approximately be as follows: initial setting - 30 s, dead mode - 3 s, alarm - 30 s.

The alarm sound signal generator consists of two generators and a 3-hour bridge amplifier loaded with a BA1 dynamic head with a power of at least 2 watts. On the elements DD3.1 and DD3.2, an infra-low frequency generator is made, which, with a period of about 3 s, smoothly changes by about an octave either in one direction or the other, the frequency of the audio frequency generator assembled on the element) DD3.3, DD3.4. Elements DD4.1, DD4.5 and diode VD5 form a 3H generator start circuit. Elements DD4.3, DD4.4 - buffer inverters; they can be excluded and used to expand the functionality of the device (for example, to control the light indication of the "alarm" mode).

The bridge amplifier is made on four composite transistors VT4VT6, VT10VT8, VT5VT7 and VT11VT9. In order to miniaturize the design, ready-made composite transistors of the KT972 and KT973 series can be used.

The described signaling device is used to protect the front door of non-residential premises. It is powered by a battery of galvanic cells. The device is mounted in the body of an old tape recorder, which retains the battery compartment and dynamic head. Power switch SA1 - TP1-2 or TV2-1 - secret, accessible from the outside. If the room is electrified, it is worth saving the power source of the tape recorder by connecting it with diode isolation in parallel with the battery. This will extend the life of the battery pack.

Sensor BQ1 is attached with screws near the lock and is masked. Instead of ZP-5, you can use other piezo emitters from the ZP series. The monophonic piezo head of the EPU pickup, pressed against the lock with an elastic plate, works well as a sensor. The end of the needle holder should be weighted with a pellet to increase sensitivity.

In a properly assembled device, you only need to set the required sensitivity level with resistor R5. If you still need to look for a malfunction, you should temporarily replace the resistors R7-R10 with others of lower resistance - this will shorten the duration of each mode and speed up the establishment. To check the health of the alarm driver, it is necessary to close the collector and emitter of the transistor VT3 when the power is on.

The time analyzer can also be used in other security systems to rid them of false positives.

Author: S.Kolinko, Sumy, Ukraine

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