Getting Started

An electronic device capable of converting the movements it senses into electrical signals, it detects motion either by emitting a stimulus and monitoring any changes in its reflection or by picking up signals emitted by the moving object itself. Some will alert when the ‘normal’ (i.e., unmoving) state has altered due to the intrusion of a human or other object, while others will also alert when the normal state has been re-established after intrusion.

Security systems around the world rely ubiquitously on motion sensors to activate alarms and/or switch on automatic lighting, and they are frequently positioned at points of relatively easy ingress to a building, such as windows and main doorways.

The technology varies from one type of motion sensor to another. Some employ infrared radiation; others use sonic pulses and function similarly to radar, while others are based on fluctuations in vibration.

There are two broad categories of motion sensor, each of which include a similar range of different technologies. Local motion sensing focuses on a designated area and creates a form of “electrical fence” by sending a beam to a detector. Should a human, object or animal stray into this beam, it becomes interrupted and the fence is compromised. The detector’s output immediately alters, and passes through electrical circuitry to activate visible or infrared lighting and/or an alarm.

Area motion sensing employs a “field of view” (FOV) and will register any movement that enters this arena. The sensors included in this category are video camera monitoring of an area, microwave Doppler sensors, ultrasound motion detection, footstep sensing and Active Infrared (AIR) or Passive Infrared (PIR) motion sensors. Of the last two, PIR sensors are by far the most common, and are considered passive because, unlike AIR sensors, they do not emit a beam of radiation and measure its disruption, but simply receive IR rays in the form of body heat.

The four primary technologies utilised by motion sensors are; PIR, which emits no energy of its own, Active Ultrasonic, which measures the altering reflections of ultrasonic wave pulses as they bounce off a moving object, Active Tomographic Motion Sensors that measure disruptions to radio waves over a wide areas and detect motion through solid obstructions such as doors and walls, and, finally, Active Microwave, which like the ultrasonic version, measures alterations to pulses of microwaves as they are reflected back by a moving object or person.

Technical aspects

A major drawback of using a single technology in motion sensor devices is the danger of false triggering. Increasingly, contemporary motion sensors combine at least two different technologies, although even this is not without disadvantages. If, for example, PIR and active microwave technologies are combined in a single device, false triggering is for much of the time significantly reduced: changes in the intensity of ambient light may falsely trigger the PIR but will not affect the microwave, while the swaying branch of a tree may activate the microwave but will not affect the PIR at all. Both technologies must trip simultaneously if true motion is to be detected. The disadvantage, however, is that such devices may underestimate human ingenuity - a burglar may “fool” one or other of the technologies and ensure that the alarm does not activate, even though a real intrusion is taking place.

PIR is, however, widely used as one of the technologies in dual-tech devices. To save energy, microwave sensors can be calibrated in such a way that they only send out a pulse of waves for monitoring if initially activated by the PIR. If they detect movement as well, the alarm will be sounded, but no security activation will occur if the microwave fails to confirm any motion.

How the motion sensor differs from other sensors

While different technologies are employed in motion sensors, the property they all share is their ability to detect a change in a designated area or field of vision caused by the intrusion of a moving object, whereupon they can be wired electrically to activate alarms, lighting or machine shutdown if a moving part is in danger of colliding with an obstruction.