the path of the light beam 330 although it could alternatively be in the path of light beam 320 or even light beam 305 a schematic of the driven element 120 is shown in fig1 this system uses two idlers 115 and 125 maintaining drive to a pulley by means of a belt but also keeping the belt out of the heat a motor 100 drives a pulley 102 that in turn drives the belt 150 the driven element 120 intersects the light gate 130 thereby altering the light in some way e g altering the color or shape of the light the driven element 120 is mounted on a pulley 110 according to this system two idlers shown as 115 and 125 are used the belt is wrapped around the pulley 102 the outside of idler 125 around a portion of pulley 110 which is typically on the opposite side of the pulley to the light gate 130 and around the outside of idler 115 in the disclosed embodiment both sides of the belt are required to transmit drive therefore a double sided belt is used i e one having teeth on both sides alternative systems may use a single sided belt for example a second pair of idlers may be used to wrap the belt around a portion of pulley 102 in the same manner as for pulley 110 note that if the belt were connected around the pulley 110 in the conventional manner it.