a monitor is provided for a bobbin thread on double thread lockstitch sewing machines . this is preferably a sewing machine with a rotating hook with a bobbin housing accommodating the lower thread reserve . the monitor includes a revolution sensor which sends a pulse per revolution of the main shaft of the machine to a microcomputer . an infrared laser light transmitter sends a detection beam directed toward a bobbin . the beam is reflected by the bobbin or by the roll of a residual thread length to a receiver and is sent by the latter to the microcomputer in the form of signals for processing , evaluation and / or forwarding . the microcomputer is adapted to compare the different signal patterns with a detection threshold to generate signals for controlling the sewing machine . a controllable power source is connected to the microcomputer for the occasional , stepwise reduction of the output of the infrared laser light transmitter . a bobbin can be inserted into the bobbin housing with at least two chambers separated from one another by a radially directed web . one chamber accommodates a residual thread length and the other chamber accommodates the main thread length . the front surface of the web faces the infrared laser light transmitter as well as the outer surface of the flange of the bobbin . this outer surface is adjacent to the front surface , reflecting light , wherein the flange has openings for the detection beam .