of ceramic which define its trajectory in entering said device such as to prevent the yarn f from coming into contact with the casing 2 hence undergoing damage or creating overtensions deleterious for the proper operation of the device 1 and for correct yarn feed to the textile machine the feed device 1 preferably presents an entry yarn brake 11 and a tension sensor 12 of known type and therefore not described the thread guide 10 and the yarn brake 11 project from the casing 2 the feeder 1 presents an optical sensor 13 to measure the quantity of yarn f on which the feeder operates the sensor 13 comprises a first part 15 and a second part 16 surrounding the first the first part is defined by a part 17 totally or partly for example in a lateral surface 22 thereof of any known light transparent material disposed coaxially to the rotary drum 5 and containing a plurality of light emitting members or transmitting photodiodes 18 the part 17 is supported by the casing 2 via a tube 19 positioned within the hollow shaft 6 a and fixed at one end 18 a to this casing the cable for handling the necessary signals sent and received by the sensor 13 passes within the tube the photodiodes 18 are associated with an electronic circuit or electronic card 21 contained in the part 17 which is present in a stationary position at one end of the drum 5 from which the yarn f leaves to reach the textile machine the second part 16 of the sensor 13 also stationary is defined by a hollow annular.