A close-range motion detector has at least one transmitter, at least one receiver, and at least one more transmitter or receiver. The transmitter(s) transmit, and the receiver(s) receive signals in one of the ultrasonic or mm-wave ranges. Multiple transmitters or receivers are spaced apart from one-another along a plane, and transmission of a signal takes place at a known time. Echos of the signal that bounce of a scatterer are received and digitized during a receive window, and the time-of-flight is determined using CAF. Time scaling may be determined as well, and may be determined using CAF. The determined time-of-flight is used to determine an X-Y-coordinate for the scatterer, and its motion (e.g., velocity) can be determined, which are output. In an embodiment, a such a close-range motion detector can be implemented on the side of a smart-watch, making a virtual writing surface on the back of a user's hand.