Quadrant is a human-computer interface based on an array of distance sensors. The hardware consists of 4 time-of-flight sensors and is specifically designed to detect the position, velocity, and orientation of the user’s hand in free space. Signal processing techniques are used to recognize gestures and other events, which can be mapped to a variety of musical parameters in software. We have developed Quadrant as an open-hardware circuit board which acts as a USB controller to a host computer.