Combined alarm sensor and occupant detector

A motion detector comprising a transmitter (3), arranged to transmit ultrasonic waves into a defined space, where a receiver (9) is arranged to receive said waves after reflection. The motion detector comprises a detector arrangement (10, 11, 12, 13, 14, 15, 17, 18, 19), connected to said receiver (3), arranged to detect said waves after reflection. The detector arrangement (10, 11, 12, 13, 14, 15, 17, 18, 19) has in turn an arrangement (13, 14, 15, 17, 18, 19) for filtering at least two signals (16, 20) based on changes in said waves after reflection, where each detected signal (16, 20) is representative of a specific motion type in said defined space, where said motions are preferably generated by an occupant and/or an intruder.