Components of an Alarm System


Detectors are electronic devices and in the context of security alarms, they detect entry into a building via a window or door, or directly detect the intruders presence. When a detector is activated, the microswitch opens and breaks a circuit. The alarm panel detects this and activates an external bell and internal sounder. The panel may have a monitoring device installed and dial a phone number to notify the recipient that the alarm has been activated.

Typical detectors are:

  • Contacts on windows and doors These contain a tiny reed switch enclosed in a small glass tube within the body of the sensor. The switch is kept closed by a nearby magnet.
  • Shock detectors Used for detecting someone attempting to break glass or otherwise using impact force to attempt to gain entry. These may also incorporate magnetic contacts.
  • PIR detectors These detect the movement from an intruder as they walk past the sensor.
  • Pressure Mats Detect intruders stepping on a floor.

Alarm Panel

The alarm panel itself may have a display consisting simply of LEDs, or a more fancy LCD display may be provided which gives textual information about the status of the alarm, which zone an alarm occurred in, error codes etc.

Several remote auxiliary keypads may also be provided for arming/disarming the alarm in the vicinity of additional exterior doorways. An attempt at entering an alarm code at the panel (if the sensors have been bypassed) will also trigger an activation of the alarm.


An alarm panel usually has zones to which the detectors are connected. The idea of separate zones is so that when arming the alarm, sections of the installation can be included/omited from being armed. For example when an alarm is armed at night, a zone could be excluded to allow wandering around upstairs.
An entry/exit zone is reserved for genuine entry to the building via the entrance. This zone has a delay associated with it before the sounder operates, allowing a password to be entered to disarm the panel.

On a wired alarm system, one or more sensors are connected in series to each zone. The alarm cable is then routed back to the control panel.
A tamper circuit detects an intruder interfering with alarm system wiring even when it is not armed. This is sometimes called a 24 hour circuit.
A panic button circuit and panic buttons may be included. When a panic button is pressed, the external sounder activates. Panic buttons can be located near doorways, in bedrooms etc.

Backup Power

An alarm panel is usually provided with backup power by a 12 volt lead acid battery. The backup battery maintains power to the alarm panel, sensors and sounder in the event of a power cut or when an intruder cuts the mains power to the panel.

Audible Devices

An external sounder operates when the alarm is triggered. Most modern systems use electronic piezoelectric transducers in the sounder. For added security, a sounder may have a backup battery. This allows it to operate even if the cable connecting it to the alarm panel is cut or power to the alarm panel is removed.

Alarm Wiring

A basic alarm system uses cable for connection to sensors. If anyone removes a lid while the alarm is unarmed, or cuts a cable, (cutting through the pair of tamper wires) a warning sounder will indicate this situation. If the alarm is armed, the main sounder will activate.

Door / Window Contacts

These come as two parts, the contact part and a magnet. The contact part consists of a small plastic module containing a reed switch (a miniature switch enclosed in a thin glass tube) which is mounted on the door jamb or window frame. The magnet part is fixed to the door, or window sash / casement so that it is close to the contact part when the door or window is closed. This keeps the reed switch in a closed state. When a window is opened, the magnet moves away from the contact and the reed switch opens.
Contacts don’t require power and only 2 cores of a cable are required, however if 6 core cable is used, 2 unused cores in the cable can be used for powering sensors added to the system at a later stage. Usually they don’t have tamper contacts either, however 2 of the cores can be wired to tamper contacts in junction boxes or sensors, during modifications/upgrade to the system.

PIR Detectors

These sensors use an element sensitive to movement. When someone walks in front of the sensor, electronics in the device opens a microswitch which triggers an alarm.
PIR detectors have varying ranges and detection profiles over which they are sensitive. Usually they have near, far, and possibly intermediate zones through which an intruder must pass before triggering an alarm. Normally detectors are sensitive over at least a 90 degree range.

Shock Detectors

Shock detectors are bonded to a door or window or fixed to the frame. During setup, the sensitivity of the sensor can be set and the number of impacts which trigger an alarm. Some sensors are “intelligent” and can detect the sound of breaking glass.
Shock and magnetic contact sensors can be combined into one unit.

Troubleshooting Sensors Which Don’t Work or That Cause False Alarms

We are often asked to investigate faults on alarm systems, we find that there are several causes of false alarms or detectors which fail to operate:

  • Issues with PIR detectors
  • False triggering of shock detectors
  • Loose connections
  • Alarm contacts in sensors becoming faulty
  • Tamper switches becoming faulty
  • Voltage spikes on supply
  • Backup battery problems
  • Badly placed or damaged wiring

We are able to repair the majority of faults, simply get in touch, we are always happy to help.

Backup Battery Problems

A lead acid or NiMh battery is used to keep an alarm alive in the event of power failure due to an interruption of your supply or deliberate cutting of power by an intruder. These batteries have a limited lifespan. As a battery ages, its voltage can fluctuate, injecting noise spikes into the system.
When a battery nears the end of its life, its capacity decreases and the length of time it can maintain backup decreases.

Regular servicing of your alarm system will ensure the battery is capable of ensuring the system is always working.

Buttons Not Working on the Keypad

Although older alarm panels may have keys which are actually push buttons (like what used to be used on computer keyboards), newer keypads are usually membrane type. These have “contacts” printed as pads onto a PCB, and conductive rubber pads on a moulded flexible membrane. When a key is pressed, the rubber pads press against the PCB and complete a circuit. This type of keypad is also used on TV remote controls. Over time, the conductive rubber pads lose their conductivity, however they can be repaired.

If you are having trouble arming or disarming your system please get in touch.


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Reviews & Testimonials


Tidy no wires very neat did not leave any mess feeling so much more secure. Good to know the home is alarmed up. Need a new house alarm system CTB ticked all my boxes.

Barry Sharp

Fantastic service – cannot praise this company enough. Four hours following a power cut, the internal panel started sounding and I couldn’t silence it. 15 Minutes after my call, Katie phoned me back and – despite the fact that CTB didn’t install the system or maintain it – talked me through various things to do to silence it (as a temporary fix) until it we stopped it and was very kind and caring as I suffer from anxiety I have booked a return visit to resolve it permanently. and the price quoted seems very reasonable to me. I wish I could award more stars!

Karen Fenton

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