a coil spring 4 for biasing the rotor 3 to its initial position a contact member consisting of a movable contact 5 and a stationary contact 6 that is closed when the rotor 3 rotates by a predetermined rotational angle from its initial position a printed circuit board 7 having an electrical circuit for generating an on signal when the contact member is closed and a casing 8 for containing a rotor assembly u201c a u201d shown in fig5 therein a rotor shaft 10 connected to the rotor 3 is rotatably supported by a pair of upright walls 9 a of a housing 9 as shown in fig2 a weight 11 is connected to the rotor 3 so that a gravity center of the rotor 3 is eccentrically positioned with respect to the rotor shaft 10 the cam 2 as shown in fig1 includes a first cam surface 2 a and a second cam surface 2 b both surfaces being formed continuously to each other the first cam surface 2 a is formed at a tip end of the cam 2 with a slant angle so that it pushes the movable contact 5 toward the stationary contact 6 to close both.