Patent ID: 7798081

Claim:
A bottom thread supply device for a sewing machine having a remaining thread detection function, comprising: a rotary hook coupled to the sewing machine, and comprising a rotary hook body, in which a receiving seat having a rear slot is formed, and a rotating shaft protruding from an end of the rotary hook body; a shuttle hook inserted into the rotary hook, and comprising a receiving seat having a through slot in a rear surface thereof, and a shaft protruding from a central portion of the receiving seat of the shuttle hook; a bobbin case coupled to the shuttle hook, and comprising a receiving seat, and a coupling hollow shaft fitted over the shaft of the shuttle hook; and a bobbin fitted over the coupling hollow shaft of the bobbin case, with the bottom thread wound around the bobbin, further comprising: an electrode plate coupled to a rear end of the rotating shaft of the rotary hook, and a light receiving sensor, having a first pole connected to the rotary hook body and a second pole connected to the electrode plate, and inserted in the receiving seat of the rotary hook through both the rotary hook body and a central portion of the rotating shaft; a light source mounted in the sewing machine at a position adjacent to the rotary hook, so that the light source emits light into the rear slot of the rotary hook; a first through hole formed through a surface of the shaft of the shuttle hook so as to pass the light, which enters the through slot of the shuttle hook after passing through the rear slot of the rotary hook, and a reflecting plate disposed in the shaft of the shuttle hook at a predetermined angle so as to reflect the light, which passes through the first through hole of the shaft, onto the light receiving sensor of the rotary hook; a second through hole formed through a surface of the coupling hollow shaft of the bobbin case and corresponding to the first through hole formed in the shaft of the shuttle hook; and a sensor circuit unit, comprising: a power supply unit, two poles of which are respectively connected to the electrode plate and the rotary hook body of the rotary hook; and a signal generating unit generating a signal when light is sensed by the light receiving sensor, wherein the bobbin is constructed such that the light, which sequentially passes through the rear slot of the rotary hook and the through slot of the shuttle hook, is transmitted both into the second through hole formed in the coupling hollow shaft of the bobbin case and into the first through hole formed in the shaft of the shuttle hook, so that, when the bottom thread, which has been wound around the bobbin, has been completely used, the signal is generated, thus letting a user know that no more thread remains.