Apparatus for detecing spare thread amount of sewing machine

An apparatus for sensing the residual quantity of bottom thread in a sewing machine is provided to sense the residual of the bottom thread in real-time, thereby allowing a user to estimate the consumption speed of the bottom thread and the replacement time of the bottom thread. A bobbin(1) is installed on a rotary hook of a sewing machine. Bottom thread is wound around the bobbin. A light source applies light to the bobbin. A plurality of light sensors(20,30,40) are disposed on a rotary shaft of the bobbin. One or more RFID(Radio Frequency IDentification) tags(50) are connected to the light sensors to transmit information indicating that the light from the light sensor reaches one of the light sensors. An RFID reader(60) receives a signal transmitted from the RFID tag to identify a positional section of the light sensor, thereby reading out the residual quantity of bottom thread wound on the bobbin.