Which motion detector works best to catch intruders who walk toward it?

• A. Infrared
• B. Microwave
• C. Ultrasonic
• D. Video

Answer: (B)

Movement toward the sensor creates the strongest signal with detectors that use Doppler radar. A microwave motion detector emits microwaves and listens for the Doppler shift caused by a moving object. When someone walks straight toward the detector, the radial component of their velocity is largest, producing a noticeable frequency shift and a clear change in the detected signal. That makes approaching intruders easy to pick up.

Infrared sensors rely on heat changes in the detected zone, and they’re most responsive to motion across the field of view, not directly toward it. Ultrasonic detectors use sound waves and can be affected by room acoustics and obstacles. Video requires visible light and image processing, which can be less immediate and more dependent on lighting. So, for someone coming straight at the detector, the Doppler-based microwave approach offers the most reliable cue, making it the best choice in this scenario.