DIY Frequently Asked Questions / Home Area Alarm')">
Uses either ultrasonic waves or microwaves to detect motion. When it detects motion, it triggers an alarm.
Generally plugs into a standard electrical outlet, so they’re easy to install.
Ultrasonic detector waves only go as far as the wall of the room, while microwaves penetrate walls and windows.
Some ultrasonic alarms use a narrow sonic beam that must be bounced off a hard surface. Here, the intruder must break the beam to be detected.
Other ultrasonic alarms use a wide beam that fills the room and detects motion anywhere in the trap zone.
The most important consideration with this type of alarm is avoiding false alarms. These can be caused by air conditioners or drapery moving above a hot air register. Better systems have built in circuitry that does not respond to these types of signals.
Some systems have extra repeater alarms or satellites. These can be plugged in anywhere in the home to add extra noise and relay the warning to remote rooms.