Editorial Feature

Motion Detectors - Working Principles and Applications

Motion can be detected through various means such as infrared, sound, and vibration. Motion detectors are primarily used for detecting moving objects (especially people), and gathering data regarding position, acceleration, and velocity.

Motion detectors most commonly use an infrared detection sensor, which can be incorporated into various devices found around us. This sensor provides data about motion, using which the device can alert the user of motion in the area or perform certain tasks. For example, a motion detector attached to a burglar alarm detects motion and enables the alarm to alert the home owners, or to activate a camera.

Inexpensive motion detectors are capable of detection up to 5 metres, and more advanced systems can have much longer ranges.

Working Principles of Two Types of Motion Detectors

Motion detectors are classified based on how they detect motion. The two divisions are listed below with a brief explanation of their operation:

  • Active detectors – Also referred to as radar-based motion detectors, activated sensors emit waves, i.e. radio waves, which reflect off nearby objects and reflect back to a detector. The sensor looks for a Doppler (frequency) shift in the wave when it returns to detector, which would indicate that the wave has hit a moving object. If it detects a Doppler shift, it activates to show it has detected motion.
  • Passive detectors – Infrared (heat) energy levels are analysed by passive detectors. The sensor can detect objects that vary from the ambient temperature of the area being scanned; for example, a person will emit between 9 and 10 µm of infrared energy. When this signal is received and shown to be moving across the area, motion is detected.

Active motion detectors using microwaves can cause false alarms, as the energy can penetrate most building materials. Hence, manufacturers have begun using the microwave technology in conjunction with passive infrared sensor technology.

This type of combined detector, known as dual technology motion detectors, will activate only when both technologies detect motion.

Motion Sensor Applications

Some of the key applications of motion detectors include:

  • Intruder alarms
  • Automatic ticket gates
  • Entry way lighting
  • Security lighting
  • Automated sinks/toilet flusher
  • Hand dryers
  • Automatic doors


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