Alarm System Design
Please Note: This site has now moved!! Please Go to the New Sapphire Burglar Alarm Site
This is where you can start to think about designing your system.

Before you rush in and start buying goods it is wise to consider the following.

  • Security is always a compromise between cost, security level, and ease of use. There is an alarm system which will balance the three variables to suit your individual requirements.

  • Look carefully at your property (from an intruders point of view if you can) and identify the weak points such as;

    • Windows or doors which are not overlooked by neighbouring properties, or which at night are poorly lit, or where quick easy entrance could be made.
    • Consider potential access through first floor windows which are sited above single storey roofs e.g garages, outhouses.

  • Consider how you are going to use the alarm system.

    • If you intend to use the alarm only when the property is empty, then the wiring can be made simpler.
    • Or you may want to use all the system when the property is empty and just the ground floor at night.
    • Or you may want to be able to control every individual detector in your system.

Click on the Design Examples to start designing your system Example 1 | Example 2 | Example 3 | Example 4

Alarm System Basics

Every alarm system has three main parts as shown (right). In normal systems these are interconnected using multi-core cable.

Control Panels can range from very basic key operated models (which we do not supply) through to very sophisticated models with digital communicators. Control Panels can be supplied with an integral keypad, as a blank endstation for use with one or more keypads, or as a combination of the two.

Signalling units also range from simple external sounders (powered solely from the Control Panel) through self conttained units c/w sounder(s) and strobe(s)and an internal rechargeable battery such that even if the cable to the sounder unit is cut the sounder and strobe will still operate (these are the type of unit we supply).

There are a wide range of detectors available. In ordinary domestic installatios the two types commonly used are magnetic contacts and Passive Infra-reds (PIRs). These two types are what we would recommend for the DIY installer.

Control Panels

The Control Panel monitors the status of the detectors and processes any alarm condition in order to activate the signalling. All modern Control Panels have the following basic features.

  • Mains powered with provision for rechargeable battery back-up.

  • Integral or remote keypad for controlling the various Control Panel functions such as entering the alarmset/unset code, bell cut-off timer, zone functions.

  • A number of detection zones - which will include (or can be programmed to include):

    • Entry/Exit zone. This zone is a timed zone to which the detector on the final entry/exit door is connected. The settable time allows the code holder to enter and exit the premises within the preset time.

    • Intermediate (or inhibited) zone. Detectors connected to this zone are inhibited from causing an alarm during exit and during entry provided that the final exit door has already been opened. A typical use for this facility is where a Passive Infra-red (PIR) is situated in a hallway adjoining the final door. Connecting this PIR to the intermediate zone will allow accessthrough the hallway providing entry has been made through the final door (i.e. the door connected to the Entry/Exit zone). The intermediate zone behaves as an immediate zone if entry has been made through any other route.

    • Immediate zones. Detectors connected to these zones will give a full alarm condition when the detector is tripped. Most panels have typically 6 or more of these zones available. This allows flexibility in which detectors are connected to which zones such that for example all the first floor detectors can be connected to one of the zones and the ground floor detectors to another zone. This allows the zone to which the first floor detectors are connected to be omitted from the system at night. Another useful wiring scheme is to wire all the perimeter detectors to one of the zones which will allow the occupant to roam freely within the property but protect the perimeter from intruders - very useful for young babysitters and other vulnerable groups.

    • Personal Attack zone (not all control panels). Detectors wired to this zone will cause a full alarm whether the system is SET or UNSET - hence this is normally used for personal attack buttons.

    • Fire zone (not all control panels). Smoke detectors and other fire detection devices wired to this zone will cause an alarm condition whether the alarm is SET or UNSET. The fire zone is usually configured to produce a different sounding alarm.

    • Tamper Zone. This zone is constantly monitored whether the alarm is SET or UNSET. In a correctly installed system all the tamper zone is wired in one continuous loop through every detector in the system such that if any detectors or wiring is tampered with an alarm is signalled. In most cases, only the internal sounder operates when the alarm is UNSET, and the full alarm operates when the system is SET.

  • Non-Volatile Memory (NVM). This memory chip retains the user settings - alarm code, Entry/Exit times etc. - when all power is removed from the panel.

  • Other features such as chime, memory event log, NVM (non-volatile memory). Control panel specific fatures are given in the individual Control Panels details page.


These are the devices that signal an alarm condition. The most common signalling device is the sounder (bell in days gone by) and strobe. Other signalling devices are digital communicators - where the alarm information is sent to a monitoring station for verification prior to notifying the relevant authorities - Speech Diallers - which can be programmed to dial up to 4 different numbers and deliver different pre-recorded messages. The main types of signalling used are:

  • Internal Sounder (or basic external sounder). These sounders are powered solely from the Control Panel. This means that if the cable to the sounder is cut then that particular sounder will not operate. However if the sounder has had its tamper circuit connected then any other signalling devices will operate due to the tamper circuit being tripped. Internal sounders are a very good deterrent in that if an intruder does enter the property then an internal high volume sounder is very disorientating.

  • Self-contained External Sounders. These are the type of unit we supply for external use.The units comprise one or more sounders (approx 115dB), one or more strobe lights, a rechargeable battery circuitry to control the sounder functions. All these components are fitted into a strong watertight bell box. The bell box is fitted with one or more tamper switches to prevent the box being opened or removed from the wall. Self contained sounders incorporate a bell cut-off timer (the law requires that the bell should be shut off after a maximum of 20 minutes, but note that the strobe light(s) is normally allowed to continue) and either static or flashing comfort LEDs.

  • Speech Diallers. These units are used to dial user specified telephone numbers and deliver pre-recorded messages. Most units can dial up to 4 different telephone numbers and deliver different messages to each dialled number. The number dialled can be specified on the type of alarm occurring. The units can thus be set up for intruder, fire, personal attack or whatever. NB: it is illegal to configure these units to dial any of the emergency services (Fire, Police or Ambulance).

  • Digital Communicators. These units are used to connect to a central monitoring station. The central station receives notification of the alarm and attempts to verify that an intrusion has occurred. Once verified, the relevant authorities can be notified. The charges levied for such a service can be expensive.


These are the devices that are used to detect an intrusion into the protected area. There are a lot of different types of detector, some of which are quite specialised, but for domestic situations the following three types will cover most situations.

  • Magnetic Contacts. These detectors are present in nearly every installation. The detector is supplied in two parts - a reed switch and an operating magnet. In a typical application (say fitted to a door or window) the reed switch (fitted to the door frame) is held closed by the adjacent magnet (fitted to the door). This is known as a Normally Closed (NC) circuit and is the normal method of wiring detectors. There are two main types of magnetic contact - 'flush' and 'surface'. There is a third type - the user operated Personal Attack Button.

    • Flush Magnetic Contact. These are designed to be fitted to wooden doors and frames. The operating magnet is fitted into the edge of the door and the reed switch into the door frame exactly opposite the operating magnet such that when the door is closed, the detector cannot be seen nor tampered with.


      1. Low cost

      2. Neat and hidden from view when fitted correctly.

      3. Good false alarm immunity


      1. Can be difficult to install neatly - especially on windows

      2. They only protect against the opening of a door or window and are useless if a panel is removed from a door or the glass taken out of a window. However it is always good practice to fit magnetic contacts to the front and rear doors of the property.

    • Surface Magnetic Contacts. These perform the same function as the Flush Magnetic Contacts, but are fitted to the surface of the door and frame. Surface magnetic contacts are available in different designs for fitting to aluminium doors, up-and-over doors and roller-shutter doors.


      1. Low cost

      2. Easy to fit - but can be time-consuming if wiring several in series.

      3. Good false alarm immunity


      1. Unsightly wiring - especially on windows

      2. Can be accidentally damaged

      3. They only protect against the opening of a door or window and are useless if a panel is removed from a door or the glass taken out of a window. However it is always good practice to fit magnetic contacts to the front and rear doors of the property.

  • Passive Infra-red (PIR). PIRs are one of the most useful detectors. They are easy and neat to fit, extremely reliable, and low cost.

    The working principle of the PIR detector is its ability to detect the radiated heat of an object. If the object is moving then the PIR detects the change in the radiated heat and signals an alarm.

    PIR detectors typically have a 90 degree coverage over 10 to 15 metres. PIRs can also be fitted with curtain and pet lenses.


    1. Low cost

    2. Easy to fit

    3. Good false alarm immunity in normal conditions


    1. Large pets can compromise security - even if pet lenses are fitted.

    2. Crawling insects can cause false alarms.

    3. Can be subject to false alarms or poor detection efficiency in high ambient temperatures - e.g. conservatories.

  • Dual Technology Detectors. These detectors use a combination of PIR and Microwave technology. The PIR is used to detect the perceived change in temperature and the Microwave to detect motion or a change in the protected space e.g. a door or window opening. The unit will only signal an alarm if both the PIR and the Microwave detectors are tripped.

    Dual Technology detectors typically have a 90 degree coverage over 10 to 15 metres.


    1. Easy to fit

    2. High false alarm immunity


    1. Relatively expensive.

  • Inertia/Vibration. Inertia sensors are another very useful detector. The detectors are designed to detect a shock or sudden movement when say fitted to a window frame, wall, etc. They are very reliable and have adjustable sensitivity.


    1. High reliability - can be used in presence of pets.

    2. Good overall coverage - e.g. when fitted to a window with say two opening lights and several non-opening lights, an inertia sensor will protect all forms of attack. Similarly when fitted to a door, however in this case it is probably wise to have a magnetic contact fitted too.

    3. Good false alarm Immunity.


    1. Can be expensive if there are a lot of entry points.