How Do I Fix My Burglar Alarm? - Top Tips


How to Mend It - False Alarms

A blaring sounder on a security alarm is both annoying for you and those who live nearby. If you have good neighbors, they may keep an eye out for activity in the vicinity of your home when the alarm sounds. However If false alarms occur regularly, it can be like a scenario from the story "the boy who cried wolf" and they may just ignore it!
This hub explains the basics of how alarms work and how sensors are wired. It also covers the various faults which occur in sensors resulting in nuisance activation of your alarm.

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How Does a Burglar Alarm Work?

A basic, modern security alarm consists of an alarm panel with a display and keypad to which sensors are wired. Alternatively sensors may be wired to a control panel box which is hidden away out of reach of burglars, so that it can't be tampered with. The user then interacts with this box via a keypad mounted on a wall. A microcontroller (which is a type of microprocessor) on the circuit board in the alarm panel runs a software program which scans the sensors regularly. The program will generate an alarm if it thinks a sensor has been activated and an intruder has entered the building.

Block Diagram of a Basic Security Alarm System

A basic 3 zone alarm system. 1 or more sensors are connected in series to each zone
A basic 3 zone alarm system. 1 or more sensors are connected in series to each zone | Source

Components of an Alarm System


Sensors are electronic/electrical devices and in the context of security alarms, they either detect entry of an intruder into a building via a window or door, or directly detect the intruders presence. They are small modules which contain microswitches which are normally closed. The switch contacts are normally "volt free". This means that they are isolated from the electronics of the sensor, so that they can be connected to any external voltage source. When a sensor is activated, the microswitch opens and breaks a circuit. The alarm panel detects this and activates an external wall and internal sounder. The panel may autodial a phone number or send an SMS text message to a cell (mobile) phone. Some alarm systems are monitored by an alarm company to which a subscription is paid.

Typical sensors 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 sensors 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 Sensors These detect the body heat from an intruder as they walk past the sensor.
  • Microwave Sensors Like PIR sensors, they detect intruders but have certain advantages over them.
  • Pressure Mats Detect intruders stepping on a floor.

Alarm Panel

The alarm panel itself may have a rudimentary 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.
An alarm panel will also have a keypad for entering passwords and commands.
On some systems, wires from sensors are directly connected to the alarm panel. On other systems, the display and keypad module is kept separate from the incoming wiring from the sensors. The advantage of this is that the alarm panel can be smaller and less obtrusive, while the larger junction box with incoming cable, terminals, battery back up etc can be hidden out of view and away from tech savvy burglars who could hack into the system. Several remote auxiliary keypads without displays 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 a "countdown" of the alarm.


An alarm panel usually has several zones to which sensors are connected. The idea of separate zones is so that when arming the alarm, sections of the installation can be included/excluded from being armed. So for instance, exterior doors could be on zone 1, downstairs windows could be on zone 2, upstairs windows on zone 3, and PIR sensors on zone 4 in a basic setup. When an alarm is armed at night, zone 4 PIRs could be excluded to allow wandering around, and upstairs windows left open by disabling zone 3. When an alarm occurs or cannot be armed (for example due to a window left open), the panel indicates the problem zone.
An entry/exit zone is reserved for genuine entry to the building via a doorway. This zone has a delay associated with it before the sounder operates, allowing a password to be entered to disarm the panel.
More sophisticated entry alarms for larger buildings will have a greater number of zones and the ability to identify activation of individual sensors, possibly indicating the sensor location on a computer screen mimic, depicting the floor plan of the building.
On a wired 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. In less expensive systems, nickel metal hydride (NiMh) AA cells may be used. 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.


An external sounder operates when the alarm is triggered. Older systems used electromechanical bells. 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 6 core cable for connection to sensors, 1 pair for power, 1 pair for tamper and 1 pair for the microswitches in sensors which open when the sensor is triggered. Sensors are typically powered by 12 volts DC. If several sensors are used per loop, the microswitches are wired in series. One core of the cable travels outwards from the alarm panel to all the sensors, and another core of the cable returns to the panel to complete the circuit, similarly for the tamper circuit. The tamper circuit consists of a pair of tamper cores in the alarm cable and momentary tamper switches in sensors, the alarm panel, junction boxes and sounders. These switches are maintained in a closed position by lids/covers on sensors and other components of the system. 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 (but the exterior sounder may not activate). If the alarm is armed, the main sounder will activate.
Some sensors, e.g. door and window contacts don't have any integrated electronics or tamper switches and so only 2 cores of the alarm cable are required.

If all this sounds like gobbledygook have a look at the diagrams below and it should be clearer!

Wiring of Loop to Panel

Wiring of alarm contacts in sensors to the control panel (power and tamper not shown)
Wiring of alarm contacts in sensors to the control panel (power and tamper not shown) | Source

Inside a Sensor

This PIR sensor has 3 pairs of terminals. Alarm contacts are normally closed (n/c) and open on activation .Tamper contacts are n/c and open when lid is removed.
This PIR sensor has 3 pairs of terminals. Alarm contacts are normally closed (n/c) and open on activation .Tamper contacts are n/c and open when lid is removed. | Source

Wiring Two Sensors in Series

Sensors are normally wired in series to a zone. A core of the cable for both tamper and alarm contacts loops back from the last sensor in the zone to the alarm panel.
Sensors are normally wired in series to a zone. A core of the cable for both tamper and alarm contacts loops back from the last sensor in the zone to the alarm panel. | Source

Wiring 2 Door/Window Contacts in Series

Door/window contacts are connected in series. They don't require power and usually don't have tamper microswitches.
Door/window contacts are connected in series. They don't require power and usually don't have tamper microswitches. | Source

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.
Shock sensors and magnetic contacts are also available as a combined unit.

Window contact and magnet
Window contact and magnet | Source

PIR Sensors

These sensors use an element sensitive to human body heat. When someone walks in front of the sensor, electronics in the device opens a microswitch which triggers an alarm.
PIR sensors 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 sensors are sensitive over at least a 90 degree sector, but omnidirectional versions are available.

Shock Sensors

Shock sensors are bonded to glass in 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 Cause False Alarms

There are several causes of false alarms or sensors which fail to operate:

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

Remember you can use a shorting link at the terminals of the panel if you need to isolate any zones for testing purposes (e.g to test continuity of a loop) or to disable a zone. A short piece of insulated wire is ok. This allows the alarm to be used normally while testing.

Checking the Loop Resistance

Alarms are triggered when a normally closed (NC) microswitch in a sensor, or a tamper contact goes open circuit. The resistance of a loop circuit (consisting of sensor contacts and loop wiring all connected in series) must be below an upper limit with all switches closed. This is usually 5 to 10 kilo ohms, but depends on the panel. Also the resistance of the loop when a contact opens has a lower limit, in the range of 100 kilo ohms. To check the resistance of a loop, remove the two wires connected to the zone input at the alarm panel and connect the probes of a digital multimeter, set to the ohms range, to these two wires. Resistance should typically be less than 100 ohms, but can rise if sensors are giving trouble or if you have many sensors connected in series and long cable runs. If the resistance is excessively high, several hundred ohms or greater, further investigation is necessary. If you have an assistant, they can watch the meter and you can bridge the alarm contacts of each sensor in turn with a piece of wire. By a process of elimination, this will enable you to identify the problematic sensor. Alternatively, you can go to each sensor in turn with the meter and measure the resistance across the alarm contacts. Remember that the loop must be disconnected from the panel, otherwise voltage will be present on the contacts, giving a false reading. Also PIRs and other sensors requiring power must be powered up for the contacts to operate.

See this guide for instructions on how to use a multimeter:

How to use a digital multimeter (DMM)

Issues With PIR Sensors

PIR sensors are triggered by movement of humans walking perpendicular to the sensor through its sensitive zones. If a sensor is mounted outdoors, it can also be triggered by cats or other large animals. Sensors are available which are only sensitive to human movement. If a sensor is located in a shed/garage, it can be triggered by bats, birds or rodents which have made their way into the building.
Another cause of nuisance triggering is a badly placed PIR which points at a stove or radiator and picks up movement of warm air. It is also not recommended facing a sensor towards a window to avoid picking up variations in heat caused by the sun. Sensors can't "see" through glass and have dual element sensors to make them insensitive to overall changes in sunshine level. I'm speculating here, but large changes in IR intensity caused by cloud movement could trigger a sensor if it faces out a window.
Sensors should be placed so that intruders are likely to walk perpendicular to the device, through the sensitive zones. Also the sensor should be mounted in a suitable position in a room, at an appropriate height on a wall and angled in such a way that intruders cannot crouch down and pass through a blind zone (i.e low and close to the sensor). See diagram below. Instructions provided with sensors normally provide diagrams outlining the regions of sensitivity.

Zones of Sensitivity for a PIR Sensor

A PIR is most sensitive to intruders walking perpendicular to the zones
A PIR is most sensitive to intruders walking perpendicular to the zones | Source

Loose Connection Repair

Loose connections are always a cause of problems with any electrical or electronic device. When installing sensors, screws should be screwed down tightly on the cores of alarm cable, and ideally boot lace ferrules should be used to keep the strands of wires together. Ferrules are crimped onto the ends of wire and prevent the fine strands of wire of from being damaged by screws of terminals. They also make it easier to remove and replace wires from terminals, and have a shroud to prevent inadvertent contact between loose strands of wire and adjacent terminals.
Connections can also become corroded over time, especially in damp environments.

Boot lace ferrule
Boot lace ferrule | Source

False Triggering of Shock Sensors

This type of sensor can be subject to false alarms caused by hailstones hitting windows, or more likely skylights, birds seeing their reflections and banging on glass or even impact of heavy traffic on roads close to exterior walls. There may be a setting inside the sensor to reduce the sensitivity. Also the number of impacts required to trigger an alarm can usually be set and this may need to be increased.

Alarm Contacts in Sensors Becoming Faulty

Over time, the resistance of microswitches in sensors can increase. Ideally the resistance of a closed switch should be zero ohms, but this can become higher as switches age. If doors and windows are rarely opened, the reed switches in magnetic contact sensors can become "sticky" and fail to open, preventing the alarm from activating if an intruder breaks in. PIR or microwave sensors if installed, act as a backup in this scenario.
To check the resistance, set your DMM to the ohms range and measure resistance between the screw terminals. The loop should be disconnected at the alarm panel to remove voltage from the terminals, and of course in the case of magnetic contacts, the magnet on the window or door should be adjacent to the contacts to keep the reed switch closed.

Testing Magnetic Contacts

Set the DMM to the ohms range and measure resistance between screw terminals of contacts. The loop should be disconnected at the alarm panel to remove voltage from the contacts.
Set the DMM to the ohms range and measure resistance between screw terminals of contacts. The loop should be disconnected at the alarm panel to remove voltage from the contacts. | Source

Tamper Switches Becoming Faulty

Tamper switches consisting of a spring operating a microswitch are used to detect someone removing the lid of a sensor or other components of an alarm system. An alternate style of switch consists of springy, nickel coated metal strips, pushed together when a lid is in place. These strips can tarnish over time, contributing to an increase in the loop resistance. This can produce false alarms as contacts expand and contract, and move relative to each other during hot or cold weather. Contacts can be cleaned with a piece of fine wire / steel wool and then wiped with rubbing alcohol / IPA (Isopropyl Alcohol). Don't overdo it because the coating (It's either nickel or chrome) could be removed.

Tamper Contacts in a Junction Box

Tamper contacts in a junction box (the outer casing has been removed)
Tamper contacts in a junction box (the outer casing has been removed) | Source
Clean with fine wire wool and IPA
Clean with fine wire wool and IPA | Source

Voltage Spikes on Supply

Voltage spikes on your mains supply are caused by disturbances such as heavy loads being switched on and off in the locality, generators coming on and going off line, switching activity in substations and lightning strikes. These spikes can trigger false alarms. Your alarm is likely to be powered directly via a cable from the electrical panel in your home or via a spur from a cable via a fused connection unit. A surge filter may give some protection from false alarms caused by spikes injected into the power supply of the alarm panel.

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 of 3 to 5 years. 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.

Badly Placed or Installed Wiring

If wiring is run adjacent to power cables, voltage spikes can be coupled directly into the alarm cables. During installation, staples or clips may have cut through alarm cable. This can cause problems later as cores get shorted out. Also if you have had workers in your home doing renovations, make sure they haven't dislodged or damaged sensors, cables etc.

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. See this article: How to Repair a Keypad or Remote Control With Kitchen Foil

The keypad of an alarm is similar to this membrane keypad from a remote control
The keypad of an alarm is similar to this membrane keypad from a remote control | Source
The conductive pads on the membrane make contact with copper contact pads on the PCB
The conductive pads on the membrane make contact with copper contact pads on the PCB | Source

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Comments 11 comments

Faith Reaper profile image

Faith Reaper 5 months ago from southern USA

Hi Eugene,

This is certainly a detailed article here on burglar alarms. I think you've covered all the bases.

It seems my house alarm will just go off while we are still in the house for some odd reason every now and then. As far as I know, no one was attempting to break in. It actually usually happens during the day on the weekends. To my knowledge, it has never gone off while we were away.

The other weekend when I had the small children up at the house, I think one of them may have attempted to go out the backdoor but it was still dead bolted and possible hit the door hard. The alarm did go off. Can something like that make the alarm go off?

Will share on Pinterest and Twitter.

eugbug profile image

eugbug 5 months ago from Ireland Author

Thanks Faith Reaper! Transient problems with an alarm going off can be really annoying because they are so difficult to troubleshoot. Its much easier to sort out a problem where part of the alarm doesn't work at all. Sometimes a loose connection in a sensor can cause the alarm to be triggered by a shock (banging the door as you suggested) or a change in temperature (causing metal to expand/contract with resultant movement of metal surfaces over each other). The sensor fitted to your back door could be a combined contact/shock sensor which would trigger if the door is opened, but also if the door is banged.

Faith Reaper profile image

Faith Reaper 5 months ago from southern USA

Oh, thank you, Eugene.

That makes a lot of sense about the loose connection in a sensor and/or it being a contact/shock sensor on the door because it did receive contact or a bit of a shock so-to-speak.

You are certainly knowledgeable in many areas it seems from glancing over the topics of the hubs you write about here.

Bret Schaller 3 months ago

We lost a transformer and a single leg of the 3 phase power coming into our building. I also had a PC power supply fail along with the alarm system going blank. The AC/AC transformer power the alarm no longer works. Where this power supply attaches to the alarm board - there is zero resistance across the two contacts. I'm sure connecting a replacement power supply will short the power supply. What's your advice? Is it time to upgrade my alarm system?

eugbug profile image

eugbug 3 months ago from Ireland Author

Hi Bret. If you measure resistance with the transformer still connected, you will get very low resistance (because the transformer secondary has such a low resistance), so you would need to disconnect the input wire to the board to measure the resistance. If the transformer got an overvoltage from the line failure, the chances are that a fuse built into the windings of the transformer may have blown, and these are not replaceable. Check also for blown fuses on the circuit board of the alarm.

eugbug profile image

eugbug 3 months ago from Ireland Author

You could try replacing the transformer (which would have to have the same VA rating, input and output voltages as the original), but there's no guarantee the board isn't "fried", or damaged and potentially a fire risk.

Doug 2 months ago

Some of our window contacts have been remover by window fitters leaving lengths of cable which they've tied up. Is it safe to just cut these off?

eugbug profile image

eugbug 2 months ago from Ireland Author

If the cables are still attached to the alarm panel, one the cores may carry 12 volts (although only 2 cores without a voltage supply is necessary for contacts so there may be no 12 volts present). If you cut through the cable, it may possibly short out the 12 volts momentarily and blow a fuse at the alarm panel. If you can trace the wiring back, the best thing to do is de-energise the panel by powering down the mains supply to it (remove the fuse in a fused connection unit or turn off the MCB for the panel). Disconnect the backup battery, and then disconnect the 12 volt core in the alarm cable/cables which travel out to the windows. Are you going to fit the contacts to the new windows?

George 6 days ago

My front door has dropped slightly creating a gap between the two halves of the contact. Can I remove the bottom contact from the door and move it up a little? How is it attached to the door? I assume there's a cover that slides off revealing screws through the back part of the plastic.

Thank you kindly!

eugbug profile image

eugbug 6 days ago from Ireland Author

Hi George,

The cover is probably like a box which you should be able to prise off from underneath with a screwdriver (prise up one side a little then the other side until you get it off).

If you're lucky, the fixing holes in the magnet will be slots, allowing some adjustment. However if they aren't, you'll either have to relocate the two halves of the sensor, or you could try removing the screws and gluing the magnet to the door with a "no more nails" type adhesive product. (there is a double sided tape version of this also).

(These might be a little thick though and a thin layer of glue would be better)

If the door hasn't dropped much, there shouldn't be an issue with the contact not working. The range is usually about 1/2 an inch or so before a contact opens when the two halves are separated. However it can be borderline and you may be getting spurious false alarms. Over time also, contacts can get "sticky" and fail to open, especially if they are fixed to windows which are never opened (so the contacts don't get "exercised"). So if you have any more, it's worth checking them out. When installing an alarm system, it's wise to have e.g. PIR sensors included as "belt and braces" should this happen.

Hope this helps George!

eugbug profile image

eugbug 6 days ago from Ireland Author

Before you try using glue, check whether it's possible to move the magnet upwards (assuming it's at the top edge of the door), and re-drive the screws adjacent to the existing holes.

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    Eugene Brennan (eugbug)311 Followers
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    Eugene is a qualified control/instrumentation engineer Bsc(Eng) and has worked as a developer of electronics and software for SCADA systems

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