A method for detecting a characteristic of a textile or metal thread fed to an operating machine, by the generation of a light signal impacting the thread to create a shadow on an optical sensor device connected to means for monitoring a characteristic of the thread on the basis of an electrical signal emitted by such sensor device as a function of the shadow generated on said sensor device by the thread itself, the characteristic being a physical characteristic of the thread such as its diameter or a characteristic of the feed of the thread when in movement, such as its speed. The signal may be detected in analog mode or a digital mode, the signal detected in the digital mode providing real time calibration of the signal detected in analog mode to generate an electrical signal used by the monitoring means to monitor the characteristics of the thread.