Alarms and early warning

A camera shows you what happened. An alarm tells you something is happening right now. These are fundamentally different tools with different jobs, and confusing them is one of the most common security planning errors. A camera at the front door captures excellent footage of a break-in — and does nothing to interrupt it.

An alarm creates an audible deterrent, notifies you or a monitoring center within seconds, and forces the intruder to make a decision under time pressure. Both belong in a complete system. This page is about the alarm layer.

Sensor types

The sensor is the edge of the system — the component that detects the event. Different sensors detect different threat signatures. Using only one type leaves gaps.

PIR motion sensors

Passive infrared (PIR) sensors detect changes in infrared energy caused by a warm body moving through their field of view. A standard residential PIR covers a detection range of 20 to 40 feet (6 to 12 m) with a horizontal field of view of 90 to 110 degrees. They are the workhorse of interior intrusion detection.

Mount PIR sensors in the upper corners of rooms at 6 to 8 feet (1.8 to 2.4 m) height, aimed diagonally across the most likely approach paths. They are triggered by movement, not by presence — a person standing still in the detection zone may not activate the sensor after initial entry.

False trigger sources: pets, HVAC currents, direct sunlight shifting through a window, and heating vents. Dual-technology PIR sensors add a secondary microwave detection layer, requiring both technologies to confirm motion before triggering. They are worth the moderate cost premium in any environment with frequent false triggers from pets or HVAC.

Door and window contact sensors

Magnetic contact sensors are the most reliable and simplest sensor type. Two components — a magnet and a sensor body — mount on a door or window frame. When the gap opens beyond 3/8 inch (1 cm), the circuit breaks and the alarm triggers. They detect opening, not breaking.

Contact sensors are inexpensive per unit and should be installed on every exterior door, every accessible window, and any interior door protecting a high-value room. They integrate with every control panel and are the standard first layer of perimeter detection.

Glass-break sensors

Glass-break sensors use acoustic analysis to detect the specific sound signature of breaking glass: a sharp impact frequency followed by high-frequency shatter. Detection range is typically 15 to 25 feet (4.5 to 7.5 m) from the sensor. A single ceiling-mounted unit can cover all windows in an open-plan room.

They fill the gap contact sensors leave: a window that is broken and entered without being opened (smash-and-enter through closed glass) bypasses contact sensors entirely. Glass-break sensors catch this method.

Placement rule: mount on the ceiling or wall centered among the windows you're protecting. Avoid proximity to speakers, subwoofers, or TVs — sustained bass can cause false triggers. Test with the manufacturer's test button after installation and annually thereafter.

Vibration sensors

Vibration sensors mount on glass or structural surfaces and detect impact. They are useful for windows, glass doors, and safes. Detection range is local — each sensor covers only the surface it's mounted on, unlike glass-break sensors which cover a radius. Vibration sensors are more likely to false-trigger from nearby door slams or exterior construction but are more selective about the specific object being tested.

Wired vs. wireless

Both architectures are legitimate. The choice depends on your installation constraints, not on which is inherently superior.

Wired systems run low-voltage cable from each sensor to the control panel. The advantages are tamper resistance (cutting a wire triggers an alarm in a supervised circuit), no battery maintenance, and reliable signal regardless of radio frequency interference. The disadvantage is installation complexity — running wire through finished walls requires drilling, fishing cable, and patching. Wired systems are the right choice for new construction or major renovation, and for high-security applications.

Wireless systems communicate via encrypted radio frequency (typically 433 MHz, 868 MHz, or Z-Wave). Installation is fast and non-destructive. The disadvantages are battery maintenance (each sensor requires periodic battery replacement — expect 1–3 year battery life depending on sensor type and event frequency), and potential for RF interference, though modern encrypted protocols have made signal jamming attacks significantly more difficult than older unencrypted systems.

Unencrypted wireless systems

Older wireless alarm systems using unencrypted 433 MHz transmission are vulnerable to replay attacks — a device that records and replays the sensor's open signal, effectively telling the control panel the door is closed when it isn't. If your system uses unencrypted RF, upgrade the panel or replace sensors with encrypted alternatives. Systems certified to current ANSI/SIA CP-01 standards use rolling codes that defeat replay attacks.

Control panels

The control panel is the brain of the system. It receives sensor signals, manages zones, triggers the siren, and communicates with monitoring services or directly with you.

Entry-level DIY panels (SimpliSafe, Ring Alarm, Abode) are self-installable, integrate with smartphones, and support both self-monitoring and optional professional monitoring. They use cloud-based communication as their primary notification path, which means they depend on internet connectivity for remote alerts.

Professional-grade panels (DSC PowerSeries, Honeywell Vista, Qolsys) are installed by alarm contractors, support more sensors per zone, and use more robust communication architectures. They are generally required by central monitoring stations that offer UL-listed monitoring service.

For preparedness-focused installations, the critical control panel feature is communication redundancy — the ability to reach you or a monitoring center through multiple paths.

Communication paths

Cellular backup: The panel maintains a dedicated cellular communicator independent of your home internet. If the broadband connection is cut, the cellular path continues. This is the minimum standard for a monitored system. Monthly monitoring fees for professionally monitored systems typically run $25–$50/month — that fee is the decision point between monitored and self-monitored systems, and it buys 24/7 human response dispatch, which you cannot replicate yourself while asleep.
Internet/WiFi: Fast and reliable under normal conditions. Single point of failure if your router or ISP goes down. Should not be the sole communication path.
Landline: Traditional POTS (plain old telephone system) landlines are increasingly uncommon. VOIP lines do not provide the same reliability as POTS. If you have a true copper landline, it can be a secondary path but should not be primary in new installations.

Internet-only systems (some smart home integrations) are not appropriate as your sole alarm communication path. They fail silently when internet connectivity is disrupted — exactly the condition that may accompany a break-in in which the attacker cuts your cable line first.

Sirens

The siren serves two functions: audible deterrent and neighborhood alert. Most residential alarm systems use interior sirens; exterior sirens provide significantly greater deterrent value and neighborhood notification.

The minimum effective siren output for residential use is 85 dB at the measurement point. Most residential sirens operate between 100 and 115 dB at 1 meter (3.3 ft). At 110 dB sustained output, the sound is painful at close range and audible at significant distance. An exterior siren mounted under the eaves is audible throughout most residential lots and to neighbors up to several hundred feet away.

Interior sirens are adequate for self-monitoring setups where you are likely to be home and the goal is to wake you and startle the intruder. Exterior sirens add the neighbor-notification layer and increase deterrent value because they signal visibly to passers-by that an alarm is active.

Siren placement: avoid pointing exterior sirens directly at neighboring bedrooms. Mount at the eaves line on the most exposed side of the home, angled slightly downward to project toward the street.

Battery backup

A system with no battery backup is disabled the moment power is cut. Every control panel should have an internal backup battery, and the key question is how long it lasts.

The minimum practical backup is 4 hours — sufficient for a brief power outage. The recommended standard for preparedness is 24 hours of battery backup under typical sensor polling conditions. Some professional-grade panels achieve this with a standard 12V 7Ah sealed lead-acid battery. Upgrading to a larger battery (12V 18Ah) is an inexpensive modification on most panels that extends backup to 48–72 hours.

Inspect the backup battery annually and replace it every 3–5 years. Aged batteries fail silently — the system reports "AC power present" but the battery has insufficient capacity to actually maintain the panel under outage.

Off-grid and no-internet configurations

Standard consumer alarm systems assume grid power and internet connectivity. For off-grid properties, cabins, or preparedness-focused installations:

Use a panel with cellular communication as the primary (not backup) communication path
Size the battery backup to cover extended outages — a 100 Ah LiFePO4 battery with a small solar trickle charge can maintain a low-power panel indefinitely
Self-monitoring via cellular SMS or push notification eliminates the monthly monitoring fee but places response responsibility on you entirely
Local-only configurations (alarm sounds but sends no notification) provide deterrent value without any communication infrastructure

For properties visited periodically rather than occupied continuously, trail camera integrations and cellular-connected motion sensors can provide a lighter-weight remote monitoring capability without a full alarm system.

False alarm management

False alarms are the primary reason alarm systems get disabled. The UL estimates that more than 90% of alarm dispatches are false alarms. Municipalities increasingly charge fines after repeat false responses. Managing your false alarm rate is both a cost issue and a systems reliability issue.

Causes and mitigations:

User error: The most common cause. Require everyone in the household to know the arming and disarming procedure before the system goes live. Post the code on the inside of a cabinet, not the keypad.
Pet-triggered PIR: Use pet-immune PIR sensors rated for animals under 40–80 lbs (18–36 kg). These detect humans at normal height while ignoring thermal signatures low to the ground.
Environmental triggers: HVAC drafts, balloons, hanging plants, and direct sunlight. Walk the interior with the system armed and identify any sensor that triggers from HVAC activity.
Low battery: A sensor with a low battery can trigger spontaneously. Respond to low-battery warnings promptly.

A well-designed system with appropriate sensor selection and household training should produce fewer than one false trigger per year.

Field note

Set a 30–60 second entry delay on your entry door sensor, but keep motion sensors in interior areas on zero delay once the system is armed. This means someone who enters without the code will trigger the siren before they've reached your valuables — rather than having time to search for a keypad first.

Integration with the broader security system

An alarm system performs best when integrated with your other layers: exterior cameras that capture the approach, perimeter security that provides lighting and physical barriers, and a safe room that gives you a hardened retreat point if the alarm doesn't deter entry. The alarm's job is to interrupt the sequence — to make staying in your home the wrong choice for an intruder. It does this best when they encounter lighting, sensors, and audible alert simultaneously within the first few seconds of any approach.

The alarm also connects directly to your situational awareness practice: a system you trust and understand is one you'll actually monitor and respond to. A system you've stopped trusting because of false alarms is worse than no system — it has trained you to ignore alerts.

Also review displacement security for portable alarm options that extend your detection layer when you're away from home.