Abstract: When a sewing machine is stopped, it is electrically indicated whether a needle is positioned at a range where the thread passing is available so that intermittent indication during stitching operation is not made. The sewing machine is provided with a device including a neon lamp and a light interrupting member. The sewing machine has on its exposed front surface lighting indicators which are illuminated by the light of the neon lamp if this light is transmitted by the interrupting member which is mounted on or in the vicinity of the movable device for vertical movement of the needle so that the indicators indicate in the inoperative position of the sewing machine that the thread may be passed in a thread passing device.

Abstract: A safety control device for a speed control circuit of a commutator motor having an ignition circuit includes a circuit for detecting an armature current and defining whether the commutator motor is energized with a supply current or with a free wheel current, a maintaining circuit for discriminating if the detected result is a continuously energized condition or an intermittent condition, and a bistable circuit. The maintaining circuit smoothes the detected voltage of the energized motor and maintains the voltage over at least one cycle of the power source and functions temporarily in an equivalent manner of continuous energization of the motor when power source is applied to the motor.

Abstract: A speed-control circuit for a motor of the sewing machine is disclosed. The circuit includes a controller, thyristors for controlling the rotation of the motor in dependence upon the operation of the controller, operational amplifiers activated in dependence upon the operation of the controller to produce a speed designating voltage, a frequency - voltage converter which receives a frequency representing the speed of rotation of the main shaft of the sewing machine, a comparator for comparing that speed designating voltage with the voltage of the frequency-voltage converter to detect a motor drive phase, and a light emitting diode and a phototransistor activated by the motor drive phase and operative for controlling the ignition phase of those thyristors.

Abstract: A braking and speed-control circuit is disclosed which prevents excessive current from being dissipated in a DC series motor. The circuit includes two free-wheeling diodes, a forward-biased diode, and a thyristor. The forward-biased diode is placed between the field coil and the armature winding of the motor. One free-wheeling diode is placed across the entire motor circuit, including the forward-biased diode. The other free-wheeling diode is placed across the forward-biased diode and the field coil. The thyristor can short across the forward-biased diode and the armature winding.