Basic Introduction of Intrusion Detector

Intrusion detectors are devices composed of electronic and mechanical components used to detect the movement or other actions of intruders. They include active infrared intrusion detectors, passive infrared intrusion detectors, microwave intrusion detectors, microwave and passive infrared composite intrusion detectors, ultrasonic intrusion detectors, vibration intrusion detectors, acoustic intrusion detectors, magnetic switch intrusion detectors, ultrasonic and passive infrared composite intrusion detectors, etc.

1. Functional principle of intrusion detector

Each intrusion detector has a certain means of detecting the presence of people entering and leaving the security area. The component in the device that performs this task is called a detector or sensor.

An ideal intrusion detector only responds to the presence of people, not to the activities of animals such as dogs, cats and mice, nor to changes in the indoor environment, such as changes in temperature and humidity, wind, rain, sound and vibration. It is not easy to achieve this. Most devices not only respond to the presence of people, but also respond to the influence of some irrelevant factors. The selection and installation of the alarm should also consider making it non-responsive to irrelevant factors, and at the same time, the signal repeatability should be good.

When designing an alarm device, we must first understand and analyze the characteristics of various intrusion actions. When an intruder enters a room, he must first remove obstacles. He must open doors and windows, or make holes in walls, floors and ceilings. Therefore, some switch alarms can be installed to trigger the switch when the intruder starts to move. Another feature that should be considered is that light and infrared rays cannot penetrate the human body, so the method of installing photoelectric devices can be used to detect intrusion activities.

Another very important feature is that the normal body temperature of the human body can emit infrared rays, and the infrared sensor can detect the heat radiated by the human body. In addition, the intruder will inevitably make noise when stealing, and the use of voice-activated sensors can detect abnormal noises in the room. Ultrasonic and microwave intrusion detectors work based on the principle that the movement of the human body will interfere with ultrasonic or electromagnetic fields.

Various detectors have different working principles, and they each have their own advantages and disadvantages. In order for the detector to work effectively in any situation, it should be carefully selected and installed, and the protection measures for the detector should be taken into consideration as much as possible during installation.

Due to the different situations of families, shops, groups and enterprises, the intrusion detectors used are also different. In order to obtain the best security effect, it is usually necessary to tailor the alarm system according to the actual situation of the user, so that the detector can play a better role.

An alarm that is issued when there is no intrusion is called a false alarm. False alarms may be caused by component failure or some external influence. The consequences are unimaginable. The least serious consequence is that it adds a lot of unnecessary trouble and makes people feel annoyed, which greatly reduces the credibility of the alarm. It can be imagined that if store and warehouse managers are often woken up by false alarms, they will not be willing to use such alarm devices. The worst consequence is that it causes the police or security personnel to rush to the scene unnecessarily, thus endangering their own safety and the safety of people around them. Therefore, false alarms are the fatal weakness of alarms.

2. Various types of detectors

1) Microwave Doppler intrusion detector

It is often called a radar alarm because it is actually a Doppler radar. It is a device that uses the Doppler principle to radiate electromagnetic waves of a certain frequency, cover a certain range, and can detect human bodies moving within the range and generate alarm signals.

Technically speaking, it is generally required that the detector should be composed of one or more sensors and a signal processor, and the detector should have a method to change the detection range.

Microwave Doppler intrusion detectors are difficult to destroy if they are properly installed. Microwave detectors can also be used to protect more than two rooms with one device. Microwave intrusion detectors are very effective in catching hidden thieves. As long as the hidden person enters the security area, the alarm will be triggered.

The main disadvantage of microwave intrusion detectors is that they require high installation requirements. If they are not installed properly, microwave signals will penetrate walls with many windows and cause frequent false alarms. Another disadvantage is that they emit trace amounts of energy that are harmful to the human body, so the energy must be controlled to a level that is harmless to the human body. In addition, microwave alarm devices are interfered with by high-energy radars used by air traffic and defense departments.

2) Ultrasonic intrusion detector

Ultrasonic intrusion detectors work on the same principle as microwave intrusion detectors. They also use the Doppler principle to respond to ultrasonic waves reflected by moving human bodies, thereby causing an alarm. Ultrasonic intrusion detectors use ultrasonic beams to detect intrusions. Like microwave intrusion detectors, they are one of the most effective security facilities. Ultrasonic alarms must be very sensitive to small movements in the security area, while not being affected by airflow.

The effectiveness of ultrasonic alarms depends on the energy being reflected multiple times within the security area. Hard surfaces like walls, tables and filing cabinets are good reflectors of sound waves, while soft materials like carpets, curtains and cloth are poor reflectors of sound waves. Therefore, a small area with a hard reflective surface like a wall needs fewer sensors than an office with tapestries and many curtains. Areas filled with soft materials are best protected by other methods.
In addition, if the room is well ventilated or a part of the room is heated, causing a large air flow, it will cause a false alarm to be set off by an ultrasonic alarm installed opposite. This is because when the air flow is large, if the signal is transmitted downwind, the ultrasonic wave will reach the receiver faster than when it is stationary, so the standing wave waveform will be destroyed, thus triggering the alarm.

3) Active infrared intrusion detector

A detection device that generates an alarm state when the infrared radiation beam between the transmitter and the receiver is completely blocked or blocked by a percentage greater than a given percentage.

Active infrared intrusion detectors are generally composed of a separate transmitter and receiver, which are installed separately. The performance requires that the infrared radiation spectrum of the transmitter should be outside the visible light spectrum. In order to prevent external interference, the infrared radiation emitted by the transmitter must be modulated, so that when the receiver receives a signal with a different modulation frequency close to the radiation wavelength, or a signal without modulation, it will not affect the generation of the alarm state and the alarm state generated by interference.

4) Passive infrared intrusion detector

When a person moves within the detection range, the received infrared radiation level changes, which can generate an alarm.
The sensitivity requirement is that when a person is dressed normally, moves horizontally within the detection range at a speed of one step per second, and walks less than 3 meters continuously, the detector will generate an alarm.

Passive infrared intrusion detectors use pyroelectric infrared detection elements to detect moving targets on the road. As long as the temperature of an object is higher than absolute zero, it will continuously radiate infrared rays to the surroundings, and use the infrared rays radiated by the moving target (such as people, animals, and vehicles) itself for detection.

Compared with other types of security equipment, passive infrared intrusion detectors have the following characteristics:
1. There is no need to install any equipment in the security area, and remote control can be achieved;
2. Since it works passively, it does not generate any type of radiation, has strong confidentiality, and can effectively perform security tasks;
3. There is no need to consider illumination conditions, it can be used day and night, and is particularly suitable for working at night or in dark conditions;
4. Since there is no energy emission and no moving parts that are easily worn, it has low power consumption, a solid structure, a long life, easy maintenance, and high reliability.

5) Microwave and passive infrared composite intrusion detector

A device that combines microwave and passive infrared units into one unit and only sounds an alarm when both are in alarm state. This composite detector consists of a microwave unit, a passive infrared unit and a signal processor, and is installed in the same housing. The microwave and infrared detection ranges are of similar size and overlap, and there is a device in the housing to adjust the overlap between the two.

6) Electromechanical detectors

The simplest intrusion detector consists of a closed circuit surrounding the protected area. Once an intruder enters the area, the circuit will be destroyed and an alarm will be triggered.

Electromechanical detectors include:
1. Metal foil detectors. The most commonly used electromechanical detectors are metal foil or metal strips installed on doors and windows to form a component of the detection circuit. When the metal foil is damaged due to intrusion, an alarm will be triggered;
2. Door and window switches. Door and window switches can be used for electromechanical intrusion alarms;
3. Glass break detectors. In various theft cases, a considerable proportion of cases are caused by criminals breaking glass doors and windows by violent means to break into the room, so glass break detectors have great use value in anti-theft alarms;
4. Tilt and vibration switches. As the name suggests, the so-called tilt switch or vibration switch is a device that can be sensitive to tilt or vibration and open and close.
The most basic advantage of electromechanical detectors is that their working principle is simple, there are few circuit components, and therefore the reliability is relatively high. As long as it is properly installed and maintained, and a backup concealed switch is installed, the alarm can have good security performance, and it can serve as an excellent backup system for higher-level alarm systems; in addition, because the electromechanical detector can be seen and easily identified, it has a certain deterrent effect on most "amateur" thieves and vandals, and also has a certain confusing effect on habitual criminals. For example, when he finds the electromechanical alarm, he will confidently try to damage the alarm first, and then start to commit crimes with confidence, at which time he may trigger a more advanced alarm system.

However, electromechanical detectors cannot protect all possible access to protected areas. Even if all doors and windows are equipped with such detectors, intruders can still penetrate the walls, ceilings or floors and enter the room. Another disadvantage of electromechanical detectors is its installation. If you lack imagination and installation experience, it is difficult to achieve good results. Its sensitive components are very exposed and can easily be handled by criminals and become ineffective.

7) Photodetector

Photoelectric detectors use the characteristic of light propagation in a straight line, so they are suitable for detecting entrances and exits or relatively open areas where there are no objects blocking the light beam. If the area is large, mirrors can be used to reflect the light. The main disadvantage of photoelectric detectors is that they are not suitable for short and crooked passages. If they are used in short and crooked passages, multiple mirrors are required, and each mirror will cause false alarms if it is not installed in the correct position or is contaminated by dirt. In addition, intruders may also use mirrors to reflect the light beam so that the light beam is not blocked and sneak into the security area without being detected.

8) Light detector

A light detector is a light detector that is not driven by a light source. This device can automatically measure the light intensity in the security area and respond to sudden changes.

9) Infrared body temperature detector

Infrared body temperature detectors are another form of optical detectors that are triggered by the heat energy emitted by an intruder's body. This detector does not respond to changes in room temperature, whether it rises or falls. However, when a target with a temperature approximately equal to human body temperature (such as an intruder) enters a non-sensitive area from a sensitive area, the alarm can detect the difference in radiation and trigger an alarm. Infrared body temperature detectors are highly sensitive and not easily damaged. However, if the intruder's body temperature is the same as the temperature of the indoor environment, the alarm will fail, which is difficult to achieve in practice.

10) Proximity detector

A proximity detector is a detection device that triggers an alarm when an intruder approaches it (but has not yet touched it). False alarms are very likely to occur when using proximity detectors outdoors, and special measures must be taken when using them. The most common influences are changes in temperature and humidity. When it rains, the influence is even greater. High-grade insulating materials should be used to support the sensing wires to minimize the influence of rain.

Proximity detectors are more suitable for indoor use, such as providing protection for special objects such as desks and filing cabinets. The objects to be protected are usually metal and can actually form part of the protection circuit. The sensitive wire is connected to the frame of the cabinet and acts as a plate of the capacitor in the sensitive circuit.

Proximity detectors are very suitable for protecting specific objects. Its most prominent advantage is that it can easily treat the protected object as part of the circuit, so as long as someone tries to damage the system, the alarm will be triggered immediately.
The main advantages of the proximity detection system are versatility and universality. It can be used to protect almost any object and will not be triggered by interference from a few meters away. Once someone approaches a jewelry box or a filing cabinet to steal, the alarm will be triggered, but normal business work nearby can proceed as usual.

The main disadvantage of proximity detection systems is that they are too sensitive. If the sensitivity is adjusted too high to suit a certain application, it is easy to cause frequent false alarms. Unlike other systems, it is impossible to plug in the power plug and the system will work properly. Certain adjustments must be made to minimize the probability of false alarms.

11) Audio intrusion detector

In addition to being used for entrance control of portals, it can also be used to monitor areas where intruders appear, but at this time the guards must always listen for sounds of intrusion. On the other hand, intruders generally try not to make any noise, especially when a guard has to monitor several different areas. Adding a trigger circuit can overcome the above shortcomings. There
are many limitations to audio intrusion detection equipment. Under normal conditions, when the background noise varies over a wide range, this detector is prone to false alarms. It is also not suitable for occasions where thick curtains are hung on doors and windows and thick carpets are laid on the floor. In addition, some departments' mechanical equipment automatically connects and disconnects day and night, which will produce constant sounds, and the above detectors should not be used at this time. The outstanding advantage of the audio intrusion detector is that it can be used to identify the cause of the alarm.

12) Vibration intrusion detector

Vibration detectors are essentially the same as acoustic intrusion detectors. The sensor of the vibration system is a vibration detector, which must have mechanical displacement to generate a signal; vibration detection equipment is most suitable for the protection of valuable, confidential and special objects such as filing cabinets and safes, and is also suitable for use in combination with other systems to prevent thieves from breaking into the wall. The effectiveness of vibration detectors has a lot to do with whether they are used correctly. It is often used to provide protection for special objects in certain protected areas where people are usually active.