The detector in the security system plays a similar role to a sentry or lookout in the army: its task is to provide information. Of course, the data collected by a lookout in the army are quite different by their nature, but the idea is similar. The lookout provides information to the corresponding command post so that suitable measures can be taken. The detector in the security system supplies information to the control panel, which is equivalent to the command post in the army. If anything happens in the detector surroundings, which can be recognized as a threat according to the alarm control panel program, the control panel will give orders to the respective units to take specified actions. Departing from these military references, but still trying to figuratively describe the essence of detector operation, you can say the detectors play the part of senses of the alarm system. The stimuli which reach the alarm control panel (i.e. the brain) through the detectors enable reactions and actions of the system. Depending on the type of detector, various stimuli can be used.
There are many types of detectors but, as we are primarily concerned with those used in the burglary and panic systems, the following types can be named: infrared (active and passive), ultrasonic, microwave, vibration, inertia, glass-break, magnetic, as well as combinations of these techniques. We will focus on discussing just a few examples of detector types, which we rather arbitrarily consider to be the most popular ones: passive infrared, glass-break, magnetic, and dual detectors. They can satisfactorily protect the building against burglary. The burglary is often preceded by breaking the window / door glass pane. The event of this type will be recorded by the glass-break detectors. Most of the commercially available glass-break detectors are microphone detectors. It is known that glass breaking is accompanied by a sound. The detector microphone registers the sound and triggers the alarm. Unfortunately, as we live in the world full of various sounds, the detector might respond to signals which have nothing in common with glass breaking, e.g. telephone rings or music. This is why the more advanced detectors are able to analyze the incoming sounds for the occurrence of one having a strictly defined nature. Only in situations when a (high-frequency) glass breaking sound was preceded by a (low-frequency) impact sound is suitable information sent to the alarm control panel.
The burglar can enter the protected building without breaking any glass, but in such a case it will be difficult for him to avoid opening any window or door. The control panel will be informed about opening of the window or door by the magnetic detectors (contacts), which are sometimes colloquially called reed relays. The magnetic detectors are always composed of two elements: magnet and reed relay. The principle of their operation is simple. As long as the reed relay remains in the magnetic field, it is either closed or open. When it is outside of the magnetic field, i.e. the magnet is moved away from it, the reed relay opens (or closes). This information is transmitted to the control panel and suitably interpreted. If the burglar somehow succeeds in entering the protected premises neither breaking the glass, nor opening the windows or doors, he will not avoid moving around. And then the control panel will be informed about it by movement detectors, the next line of defense of the alarm system.
The movement detectors include the passive infrared detectors, or the so-called PIR (Passive Infra Red) detectors. They notify the control panel of moving and heat-emitting objects, which have appeared within the area protected by them. The concept of operation of this detector type is based on measurement of heat radiation. Some basic physical principles should be recalled here. Each physical body has its temperature, hence it emits heat radiation. Such radiation is invisible to the human eye, until the object temperature reaches 500 deg. Celsius. This part of heat radiation which is invisible to the human eye is called infrared radiation. It is essential for the burglary and panic alarm system to be able to detect within the supervised area the presence of people who should not be there at the given moment. Naturally, the power of man-emitted radiation is insignificant, and it may be hardly detectable against the surrounding background which is a heat emission source itself. Therefore, the PIR detectors focus on picking up rapid temperature changes within its field of view. Exactly such changes are caused by the movement of people. Of course, the analysis of signals that reach the detector is much more complex, however we hope we have managed to at least briefly outline the working principles of this type of detectors.
Approximate distance at which the detector 'sees' the moving object
If there is a suspicion that the PIR detector might be exposed to rapid temperature changes (e.g. because an oven or air conditioner is situated nearby), a dual detector can be used. Generally, this term denotes any detector in which two different types of detection are used. In this way a better performance can be reached and potentially false alarms can be eliminated. An example of the dual detector is the one that combines infrared and microwave detection. What does the microwave detection consist in? It operates in much the same way as the commonly known radar. The transmitter sends microwaves which are reflected by objects and sent to the receiver. Based on signal analysis, the transmitter determines which of them are moving (this is possible owing to the Doppler effect, which some may remember from the physics lessons and which, for example, makes the approaching car sound different than the one driving away) and only these are interesting from the aspect of the alarm system. Consequently, in case of the dual detector, the suitable information can only be transmitted to the control panel when a rapid change is found, based on infrared detection, and the movement is simultaneously confirmed by microwave detection. Thus triggering of a false alarm, e.g. by a quickly heating up oven, will be impossible.