A heart beat detection device comprises at least one optical reflection sensor to be positioned on the skin of a person. The sensor unit is provided with a light emitter and a corresponding light receiver which converts the light reflected by the skin into an electric signal and comprises electrically adjustable opt cal filters connected to the emitter, to the receiver or to both of them in order to select, upon operation, a desired light wavelength and perform processing of the signals thus obtained in order to reinforce the heart beat signal. A system with this device and a detection method are also described.