Method of controlling a function of a device and system for detecting the presence of a living being

System for detecting the presence of a living being, including:

• a data processing device (3) having a processor (5) configured to obtain a sequence (19;34;48) of digital images taken at consecutive points in time by a camera, the data processing device configured to:
• select at least one measurement zone (25) including a plurality of image points;
• for at least one measurement zone (25),
• to obtain a signal (30;41;55) representative of at least variations in a time-varying value of a combination of pixel values at at least a number of the image points, and
• to determine at least one characteristic of the signal (30;41;55) within at least a range of interest of its spectrum relative to comparison data, the determination comprising at least one of:
• (i) determining whether the signal (30;41;55) has a spectrum with a local maximum at a frequency matching a comparison frequency to a certain accuracy;
• and
• (ii) determining whether at least a certain frequency component of the signal (30;41;55) is in phase with a comparison signal to a certain accuracy;
• and to adapt its output in dependence on whether a determination is positive, wherein the system is unobtrusive to a living being present in a field of view of the camera, and wherein the living being can be in motion.