Makers assembled a compact handheld unit that listens to standard wireless network signals and converts subtle shifts in those signals into a real-time radar display of nearby human activity. The entire system fits comfortably in one hand and comes together for roughly fifty dollars in parts. Movement registers even when walls or other barriers stand between the device and the person. No camera, no infrared sensor, and no dedicated motion detector appears anywhere in the build. Instead the unit taps into Channel State Information (CSI) carried by ordinary Wi-Fi traffic in most homes and offices.
