A reed sensor mounted on the side of the cable reel and including four magnetic blocks on the circumference of the cable reel, respectively; and a reed probe arranged adjacent to the circumference of the cable reel and having one end connected to the outer circumference of the cable reel, A control circuit; a power supply, a voltage stabilizing module and a display module are respectively connected in series with the reed switch probe to form a control loop; the display module is a light-emitting diode. The magnetic blocks of the four permanent magnets are evenly distributed on the circumference of the cable reel. When the cable reels are rotated, the magnetic blocks are in turn close to the reed switches and the closed circuits are separated again.