11.2 Microwave, Ultrasonic, and Dual-Technology Sensors

Microwave, ultrasonic, and dual-technology sensors fill the rest of the eight-hour motion block. The DOS sitting will mix technologies. Independent OpenExamPrep teaching for this exam keeps three ideas separate: what energy the device emits, what moving target it can see, and which false-alarm sources belong to which physics. A PIR never becomes a microwave unit because someone labels the plastic housing motion detector.

Microwave: active Doppler

A microwave motion detector is active. It transmits microwave energy into the protected space and listens to what returns. Motion is detected by the Doppler effect: a reflector moving toward or away from the unit shifts the frequency of the return. Processing treats that shift as motion. Typical security products operate in the microwave bands allocated for this kind of sensing. Installers meet X-band and K-band families on data sheets. New York does not put a required gigahertz number in 19 NYCRR 196.8; the exam cares that the unit emits energy and uses Doppler, not that you memorize a catalog frequency.

Because the energy is electromagnetic, it can penetrate some materials that stop a PIR's optical path: many plastics, some drywall, glass, and similar non-metallic partitions, to a degree that depends on thickness, moisture, and the product. Metal reflects and can block. That penetration is a double-edged tool. It can catch motion on the far side of a lightweight partition. It can also see a hallway, a restroom, or street traffic the owner did not intend to protect, or see a rodent in a wall cavity. Range that leaks through a window or thin wall is a classic false-alarm and privacy problem. Aiming and range adjustment are part of listing-compliant application, not optional extras.

Microwave units generally draw more current than PIR-only devices. On a wireless job that matters for battery life; on a hardwired job it matters for auxiliary-power budgeting (Chapter 7 and Chapter 9).

Reflective and environmental false alarms

Microwave Doppler reports moving reflectors, not people only. Exam-style sources include:

• Fans and blower motors — moving blades are excellent Doppler targets
• Vibrating metal — loose duct, hanging signs, oscillating machines
• Fluorescent fixtures, especially older magnetic-ballast types that can disturb the microwave field
• Moving water in some pipes, or swaying blinds if they present a changing reflector
• Traffic or pedestrians outside a window if energy leaves the room

A PIR might ignore a spinning fan that is the same temperature as the room; a microwave often will not. That contrast is why dual-technology exists.

Ultrasonic: Doppler in air

An ultrasonic motion detector is also active, but it uses sound above the usual human hearing range instead of microwave energy. It fills a volume with ultrasonic energy and looks for a Doppler shift as a target moves through air. Sound does not punch through walls the way microwave energy can. It does couple strongly to air motion. Drafts from HVAC, open windows, hissing, and moving drapes are textbook ultrasonic problems. Telephone bells and some hissing appliances were historic nuisances on older ultrasonic jobs.

Ultrasonic space protection is less common on new security installations than it was a generation ago. PIR, microwave, and dual-tech displaced most of it. It remains in the Module 3 motion hours because the physics still shows up on test items, in existing systems you may service under 195.1(e), and as a distractor against PIR and microwave.

Dual-technology: AND logic to cut false alarms

A dual-technology (dual-tech) motion detector combines two principles in one housing, most often PIR plus microwave. The usual security reason to specify it is false-alarm reduction. AND logic (coincidence processing) requires both channels to detect motion inside a defined time window before the unit opens an alarm output. A stimulus that fools only one technology — a hot-air draft for PIR, a fan for microwave — should not report.

AND logic is a catch versus nuisance trade. It is not a New York Uniform Code mandate that every dwelling bedroom use dual-tech. Some products offer an OR or sequential mode that alarms if either channel trips; that raises catch probability and usually raises false alarms. When a stem says reduce false alarms and names dual-tech, think AND / coincidence, not extra range and not sees through walls.

Dual-tech does not turn the PIR channel into a through-wall sensor. The PIR still needs an infrared optical path. If only the microwave channel sees motion on the far side of drywall, AND logic should not alarm — which is often the desired behavior next to a corridor. If a unit is set to OR, that same leak can become a false alarm.

Failure modes when one technology is blocked or masked

AND logic fails closed (no alarm) when only one channel can see the target:

• PIR masked: cardboard, a sticker, paint, or spray on the window; a coat hung on the unit; a plant stuffed in front of the lens. Microwave may still Doppler the walk-test. AND does not alarm. Catch is lost.
• Microwave blocked: large metal surfaces, foil-backed insulation, a filing cabinet pushed in front of the unit, or range turned down so far the microwave never confirms. PIR may still see the walk-test. AND does not alarm.
• One channel dead: failed pyroelectric, failed microwave oscillator, or a jumper left in a single-tech service mode. Without supervision, the unit can look normal on the zone while it has become a single-tech device — or a dead device.

Higher-security and many commercial units add anti-mask or channel supervision so a blocked window or dead microwave reports trouble instead of silent failure. Residential commodity dual-tech may not. After any repair, you walk-test both technologies if the instructions give a method (PIR-only walk, microwave-only walk, then both). Replacing a dual-tech with a PIR-only unit because it was false alarming, without fixing the environment, trades one problem for another and still requires listing-compliant placement.

Pet-immune ratings are manufacturer listings

Pet immunity is a listing and instruction claim for a given model, not a number in 19 NYCRR 196.8 and not a statewide pound limit. Data sheets may state a weight, a height of animal, or language about one pet or two pets. Those numbers belong to that product's evaluation. Do not tell an examiner that New York allows pets under 40 pounds, 80 pounds, or any other invented code figure.

Pet-immune processing tries to ignore small, low, floor-level signatures while still catching an adult walk. It is not a promise. Stairs, furniture that lets a pet walk at lens height, and aiming down a staircase all make a small animal look larger (more zones, closer range). Dual-tech pet models still need the listed mounting height and a walk-test that includes the owner's actual animals if the contract promises immunity.

Licensed work, not a lighting gadget

Placing, connecting, inspecting, testing, and replacing these sensors on a security or fire alarm system is installation, maintaining, or servicing under 195.1(c)–(e). A microwave occupancy sensor that only switches warehouse lights is not this chapter. A dual-tech unit tied to a burglar control is. Module 3's eight hours exist because false alarms, catch holes, and technology mix-ups are how interior protection succeeds or fails — and because GBL §69-vvv later makes false-alarm discipline a business problem (Chapter 12).

A practical Rochester warehouse example: a PIR-only unit stares at a west dock door and false-alarms when afternoon sun hits the floor. Swapping to dual-tech AND logic often stops the sun events because microwave does not see the thermal contrast. The same swap fails the next week if a box fan is left blowing in the microwave field and someone has taped the PIR window to stop the cat — AND logic then sees only one channel and will not catch a walk-test. Technology choice and application travel together.