Abstract: A pull chain switch circuit device for a stepless control of brightness of lighting fixtures of a ceiling fan is composed of a central control unit, a voltage stabilizing and rectifying unit, a pull chain switch, a protective unit, a zero crossing loop, and a gate unit. The input end of the voltage stabilizing and rectifying unit is connected with an alternating current source, whereas the output end of the voltage stabilizing and rectifying unit is connected with the central control unit. The pull chain switch has one end connected with the output end of the voltage stabilizing and rectifying unit. The protective unit is connected with the alternating current source and the central control unit. The zero crossing loop is connected with the alternating current source and the central control unit for detecting a time point at which the alternating current source becomes 0V.