Abstract: An optical switching arrangement for a sewing machine when used as a low bobbin thread detection and indicating system utilizes an infra-red light emitting diode and a phototransistor arranged on opposite sides of the bobbin so that an amount of bobbin thread greater than a predetermined threshold interrupts the light path from the light emitting diode to the phototransistor. A single integrated circuit chip is utilized to modulate the emission from the light emitting diode and to demodulate the output of the phototransistor to differentiate the detected emission from ambient radiation. The same basic configuration is also applied to an optical buttonhole switching arrangement.