Alarm Systems Need to ‘Speak’ to Users
A commercial customer has recently had an alarm system installed to protect his business. The system is quite modern and, in fact, is designed with a built-in basic proximity card access system. There are front and back entrances, with most of the daily employees entering from the rear. At each entrance an interior keypad has been installed, with one programmed for delayed entry. The front keypad is used most of the time for arming/disarming.
For the most part the system has operated reliably, but occasionally an alarm sounds at opening time. Apparently, the problem is sometimes employees unfamiliar with and not normally part of daily arm/disarm procedures swipe their cards at the back door only to discover the system had not yet been disarmed by regular staff for daytime operation. With no advance warning of the armed status at the perimeter, a breach was detected and triggered the alarm. Since it was not a regular employee disarming the system, he/she did not know or remember the password and a false alarm dispatch was generated.
While there may not be anything sophisticated about this story, it does point out a basic performance task fundamentally lacking in the design of this and many similar alarm systems. What design function is missing?
In another situation, a store cashier is in the process of being robbed and has had an opportunity to trip a silent alarm. A manager in the back office starts to head out to make a routine floor check and walks in on the robbery in progress. He startles the robber, and is shot and killed. Here again, what could have been done to help avoid this type of lethal accident?
Any engineering student is taught early on that there is a particular key element needed when designing any control system. It is an element that, if not designed into the system and implemented properly, will cause the system to have substandard and unreliable performance. That important element or function is known as feedback. Let’s take a closer look at the importance of alarm systems to “speak” to users.
How Visual and Audio Cues Help
I know for many this sounds simplistic, but the practice of providing proper feedback in an alarm system is often overlooked or not specified. The reason could be simple oversight or adding extra cost to the installation. However, in the long run, even basic, properly designed and implemented feedback devices and techniques will make the system more user-friendly, and reduce the opportunity for false alarm dispatches.
In the case of our open/close alarm system example above, if the system had either a visual or audible indicator on the perimeter that conveyed the system was not yet disarmed then the individual would have simply been warned to wait until someone of authority arrived.
In this case, there were a couple of options for feedback. The system had a local siren and a LED/audible indicator in the reader. Depending on the programming flexibility of the alarm control, either device may have been able to be programmed to give an indication on the perimeter that the system had not yet been disarmed, before the door was opened.
In the case of the holdup alarm scenario, the placement of a discreet red flashing indicator in the manager’s office could alert the staff that a silent holdup was in progress. This could easily come from the programmable alarm outputs of most modern alarm control panels. Most alarm control panels have programmable transistor open-collector output terminals that employ either current sourcing (+12V) or sinking (GND) outputs.
Remember, this is typically a direct transistor output and, depending on the manufacturer, will have limited current output in the milliamp range. Usually, this will be fine for something like light-emitting diode (LED) annunciators. However, if you control anything with large current demands, such as a siren, you will have to interface the limited output with a control relay. Also, don’t forget that LEDs need a current-limiting resistor. If you buy a preconfigured LED indicator it should already have the current-limiting resistor.
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