Building security: intrusion sensors and access card readers

One-line orientation

Building security is a low-voltage electrical system; the exam tests which intrusion sensor detects which kind of breach, and the tradeoffs between access card reader technologies.

Key points

Intrusion / motion sensors:

• Magnetic (contact) sensor: magnet + switch detects a door or window opening. These contacts are commonly wired normally closed, but the exact device and wiring can vary.
• Mechanical motion detector: mechanical parts react to physical movement. Durable with few electronics, but prone to false alarms and limited in range/sensitivity.
• Passive infrared (PIR): detects the heat (infrared) radiating from a moving body. Covers a large area; same family as a lighting occupancy sensor. Can false-trigger and is affected by obstacles.
• Ultrasonic detector: emits high-frequency sound waves; motion changes the wave pattern.
• Microwave motion detector: like ultrasonic but uses microwave pulses; movement shifts the reflected frequency. It needs power and can penetrate some obstacles; exact performance varies by detector.
• Acoustic / glass-break detector: senses the sound or vibration of breaking glass; an early warning for window breaches; needs careful placement.

Access card readers:

• Magnetic stripe reader: reads encoded data on the card’s stripe. Widely used, cheap — but wears with use, is easy to clone (lower security), and needs protective coating outdoors.
• Proximity (prox) reader: RFID, contactless — card only needs to be near the reader. Convenient and fast, but credentials can be intercepted or copied if the system lacks strong security.

Confusions / comparison

Security: match the sensor to the breach (three types)

Each band pairs one breach with the sensors that catch it; card readers sit below the rule.

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PIR reads heat the same way a lighting occupancy sensor does; ultrasonic and microwave instead emit a wave and read what comes back.

Device	Detects	Operating principle	Watch-out
Magnetic contact	Door/window opening	Magnet + contact; commonly normally closed	Exact wiring varies
PIR	Moving body (heat)	Infrared radiation change	False alarms; blocked by obstacles
Ultrasonic	Motion	High-frequency sound-wave shift	Sensitive to room acoustics
Microwave	Motion	Powered microwave-pulse frequency shift	Performance varies by device
Acoustic (glass-break)	Breaking glass	Sound/vibration signature	Placement-sensitive; similar sounds false-trigger
Magnetic stripe reader	Credential (contact)	Reads magnetic stripe	Cloneable; wears; poor outdoors
Proximity reader	Credential (contactless)	RFID	Security depends on credential and system

Related

→ Lighting: Lighting controls (PIR / ultrasonic occupancy sensors share PIR’s heat-detection principle) · Electrical: Service & distribution (low-voltage vs line-voltage systems) · Fire classes & detection (PIR-style detection vs smoke/heat detection — different goals).