A number of product types and techniques can detect motion; many are used in applications such as opening shop doors, turning on security lights and video recorders, or sounding alarms. Whether as standalone units or parts of systems or computerized networks, detectors fall into two common categories: area sensors and local sensors. Area sensors scan broad fields on properties, using techniques such as infrared (IR) or ultrasonic fields. Local sensors occupy rooms and interiors, using techniques such as lasers and light beams.
Sometimes called perimeter and space sensors, both types employ active and passive sensing techniques. Active sensing emits a constant field, while passive sensing waits in standby for an event to trigger a threshold setting. These techniques can electronically sense light, sound, heat, or vibration.
Active sensors detect motion by dispersing a continuous fan-shaped field, for example, of ultrasonic waves. The field remains static until something enters it and disturbs the reflected pattern. A disturbance may fall within calibrated tolerances or it may exceed them and trigger the control unit and alarm.
Passive sensors remain inactive until an event, such as motion or sound, exceeds a preset level. Active and passive techniques both have limited ranges that may weaken with distance. Hybrid products combine sensor types for greater effectiveness, pairing infrared with ultrasonic, for example.
Area sensors commonly rely on two methods to detect motion: passive infrared (PIR) and microwave sensors. PIR types detect interruptions to an invisible laser beam, as with automatic doors. Microwave sensors use radio waves.
Other types include ultrasonic, which relies on sound wave ripples, and video, which detects lighting changes and may activate a computer or digital cassette video recording device. Vibration sensors monitor manufacturing equipment for early detection of mechanical problems. These may rely on accelerometer technology, using gyroscopic or three-axis orientation circuitry.
Local sensors sometimes rely on disruptions to beams of IR, laser, or visible light. They can also detect motion with components that measure tilt, proximity, or strain. Pressure pads detect walking traffic, while camera detectors activate video or lights only in the presence of motion, saving power and memory. Microwave types occur primarily in security industries.
Detecting motion often requires the use of an emitter and a sensor: for example, a photodiode that responds to light, or a transducer that responds to ultrasound. A photodiode might catch photons and amplify them into an electronic signal. Fluctuations or disruptions in the emitted fields are registered and electronically responded to. These detectors are typically designed to react to large changes in their fields and beams, rather than gradual changes such as weather and temperature variations. The human element, however, is very easily mapped in infrared light and physical space, making it virtually impossible to thwart these sensors even with very slow movements.