How Do I Fix My Security 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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What Causes False Alarms?
- Loose connections
- Loop resistance resistance outside the specified limits
- A worn out battery
- A misaligned or incorrectly spaced magnet
- Rodents, birds, bats, spiders or other small animals may be triggering sensors, especially in lofts or outbuildings
- Tamper strips on junction boxes may be tarnished and causing bad connections
- Tampers on some sensors are badly designed and may only barely close the tamper switch in the sensor when the lid is replaced. Consider replacing the sensor
- PIRs badly positioned and subject to nuisance trigger conditions
How Does a Burglar Alarm Work?
Alarm systems for homes come in two different formats:
- An alarm panel with an integrated display and keypad to which sensors are hardwired or linked by radio (known as wireless or radio frequency (RF) sensors)
- Alternatively sensors may be wired or connect wirelessly to a box without a display or keypad, hidden away out of reach of burglars, so that it can't be tampered with.
A second advantage of this is that wiring can be hidden out of view and a smaller more discrete keypad can be mounted on a wall. All the electronics, backup power, connection terminals, autodialler/GSM modules are kept inside this box. The user then interacts with the box via a remote wired or wireless keypad mounted on a wall.
A microcontroller (which is a type of microprocessor) on the circuit board in the alarm panel/alarm box 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.
What are the Components of an Alarm System?
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.
have a keypad for entering passwords and commands.
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.
Backup Battery 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.
These devices detect opening of windows or doors, body heat from intruders walking around a room or attempts to break glass in windows.
Either integrated into the alarm panel or a separate wired or wireless unit.
Block Diagram of a Basic Security Alarm System
What are Alarm Zones?
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. To set the alarm, a button on the panel corresponding to part arm or full arm is pressed by the user. Alternatively a series of buttons is pressed on the keypad (e.g. 0#4 or 0#3). There are normally three basic arming modes for a panel:
All zones are armed when a house is vacated.
The alarm is armed with some zones excluded. These zones can be preset when the alarm is programmed by the user/engineer so that they don't have to be picked when arming.
Consider this scenario:
- Exterior doors on zone 1
- Downstairs windows on zone 2
- Upstairs windows on zone 3
- PIR sensors to detect movement on zone 4
Part arm is used typically to set an alarm at night
In this setup, zone 4 PIRs could be excluded to allow an occupant to wander around the house, and zone 3 could be disabled to allow upstairs bedroom windows to be left open in hot weather.
The user chooses the zones to be excluded as they arm the panel.
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 can be connected in series to each zone, known as "daisy-chaining". The disadvantage of daisy-chaining is that if one sensor develops a fault and contacts stay stuck open, the zone has to be omitted during arming, making the other sensors useless until the fault is rectified.
Many modern alarm panels have zones which can be programmed to respond in a desired way to devices connected to their screw terminals on the circuit board. So a zone can be used as a standard alarm zone, a 24 hour tamper circuit, panic circuit, entry/exit zone etc.
When an alarm occurs or the panel cannot be armed (for example due to a window left open), the panel normally indicates the problem zone.
Tamper Circuits and Panic Buttons
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. Panels may have a single tamper circuit or a dedicated tamper circuit for each zone.
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.
How Burglar Alarm Sensors Work?
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.
What is a Tamper on an Alarm System?
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 can be 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.
A tamper or 24 hr 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 Zone Loop to Alarm Panel
Inside a Sensor
Wiring Two Sensors in Series
Wiring 2 Door/Window Contacts in Series
End of Line (EOL) Resistors and Single End of Line Wiring
Older alarm systems as described above had zone loops which were either closed circuit when no sensors were triggered or went open circuit when a sensor activated. This resulted in a low voltage or high voltage respectively at the control panel. The flaw in this system was that a burglar could short out zone wiring between panel and sensors, effectively bypassing them. Then at a later stage they could attempt a break in. Because the zone was shorted, when a door/window contact opened or a sensor activated, it would be undetected by the control panel. Newer alarm systems are made more secure by adding a resistor, typically about 5k, at the end of the loop. This is known as an end of line (EOL) resistor and adds supervision to the loop by detecting shorted wiring. The panel now has 3 voltages it can possibly measure, high voltage with the loop open (due to a broken/cut wire, or a sensor activating), low voltage if the zone wiring is shorted by a fault/burglar, or an intermediate voltage in a non fault/non triggered scenario.
EOL resistors must be fitted at the last sensor in a loop so that the panel can detect when a burglar shorts the two alarm cores in a cable "upstream", that is on the panel side of wiring to sensors. If resistors are fitted in series with the loop at the panel, they cannot detect that this has occurred.
If only one resistor is used at the end of the zone, this is called single end of line wiring.
Fully Supervised Loop (FSL) and Closed Circuit Loop (CSL) Wiring
Basic alarm systems use 2 cores for sensors, 2 cores for tamper and 2 cores if needed for powering sensors. The problem is that a global tamper is used and this loops through all the zones on the panel. So if a burglar cuts a cable, this deactivates the tamper for all zones and potentially puts the system out of action until the break can be pinpointed.
Fully Supervised Loop wiring provides a separate tamper for each zone, so if a cable is cut or a loose connection occurs in the tamper circuit, only that zone will be out of action and also it's easier to trace the fault. An EOL resistor is used at the last detector plus in addition a resistor across the contacts of each sensor in the loop. This is called dual end of line wiring. The system can then differentiate between a no alarm condition, a tamper, or an alarm condition, using just 2 cores of cable for both contacts and tamper. A third core is needed if multiple sensors are daisy chained together.
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 additional sensors added to the loop 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.
Magnets should be placed as close as possible to the contact part, within the specified spacing. If the gap is too large, a contact may stay closed ok for a period, but open if there are any vibrations, causing spurious alarms. Sometimes there is an arrow on the contact indicating which side should face the magnet.
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 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:
- Badly positioned 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.
What Causes False Triggering of 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.
False Triggering can be caused by:
- bats, birds, rodents, spiders or other small animals, especially if a PIR is located in an outbuilding
- warm air currents if the sensor is pointing towards a radiator
- draughts coming under doors
- a sensor facing towards a window and 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. However large changes in IR intensity caused by cloud movement could trigger a sensor if it faces out a window.
How Should I Position a PIR Sensor?
Ideally place sensors in the corner of a room, facing away from a window and any heat sources. Also it should be located so that intruders are likely to walk perpendicular to the device, through the sensitive zones, rather than towards it. It should also be mounted high enough 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
Are Some PIR Sensors Better Than Others?
Newer sensors are less likely to cause false alarms. Consider upgrading to pet immune, dual-tech, dual or quad PIR types to lessen the chances of this happening.
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.
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.
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 (or around 5k if an EOL resistor is fitted at the last sensor), 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:
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. Another possibility is that the magnet can become weak, failing to keep the contact closed, especially if it wasn't placed close enough during installation. This can cause nuisance triggering, when e.g. vibration from wind or heavy nearby traffic is sufficient to shake the contact open.
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
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.
Sometimes sensors have tamper microswitches that are closed by a projection on the lid (often a small round piece of rubber) of the sensor when it is replaced. It can happen with badly made sensors that the rubber piece on the lid gets squashed over time and loses its springiness, so it doesn't put enough pressure on the microswitch. The symptoms are a sensor that generates a tamper when knocked (and possibly may do so if there is nearby heavy traffic causing vibration).
Alarm panel lids can also cause tamper faults, so check they are seated properly when replacing.
Tamper Contacts in a Junction Box
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
How Often Should Burglar Alarms Be Serviced?
Generally once a year. However if you get a discount on your home insurance, there may be further stipulations, so check your policy.
Did You Fix Your Alarm Using the Info in This Article?
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
- Helpful 17
When I key my code into the keypad and leave the house by the front door, the alarm sounds for 20 seconds and then turns off. But now, the alarm goes off indefinitely. Once I open the door to enter the house, the alarm goes off, and I can turn it off as usual. What is the problem? Where is the 12V battery?
The 12-volt battery should be in an alarm panel box where the wires from all the zones feed into, or in a combined keypad/alarm panel. If the exit tone goes off indefinitely, that means the contact on the door may be working intermittently and should be replaced.
The external bell on my alarm is not working, but the internal one is. How do you fix the external bell?
It could be a fault with the wiring going to the bell if it's wired. However, it's more likely to be due to a fault in the bell (possibly caused by moisture ingress). If the bell is wireless and solar powered, there could be an issue with the battery/charging circuitry.Helpful 1
I got message code 403 'Kitchen open' on my alarm panel. The PIR sensor also keeps flicking. Also, when I try to set the alarm, it will not set as the code message appears. Can you guide me on how to fix this?
You need to source the manual for the specific brand of control panel to see what the "403" code refers to. If the sensor is activating, it could be due to faulty wiring/loose connections, or a faulty sensor if there was no problem previously. Another possibility is a low battery if the system is wireless, but it should tell you this.Helpful 4
This morning my Honeywell Accent G4 alarm went off but read no faults on the keypad, so I had to get out my ladders and disconnect the battery in the siren, the battery in the main panel and pull the fuse. On closer inspection of the panel the only normal resistor has burnt out, all others are resistors. Any idea why this resistor would have burnt out?
Sounds like there was a short somewhere on the board. Logic circuitry/the microcontroller on a board will work on 5 or 3 volts, powered by a voltage regulator. Wired sensors run on 12 volts, again sourced by a voltage regulator which would probably source 1 or 2 amps.
The resistor here seems to be in the power /supply battery section. If the resistor has burnt out, chances are some of the other electronics became faulty and caused this to happen, so even if you replace it, the it may blow again.Helpful 3
© 2014 Eugene Brennan