is located close to the axis of rotation 86 and on the side of the axis 86 adjacent the photocells 88 and 90 a pair of leads 106 extend from the base of the diode 96 and are coupled to an appropriate power source the cylindrical member 82 not only mounts the diode 96 but also serves as a mask therefor so as to constrain light from the diode 96 into a beam of predetermined shape or pattern and of sufficient width to render the sensor useful within a desired angular range of operation in the example of fig4 the forward portion of the diode 96 resides within a generally cylindrical aperture 108 which is defined by the member 82 and which in this instance comprises an extension of the aperture 98 in which the base of the diode 96 is mounted as will be seen from the discussion to follow the width of the resulting beam which emanates from the point source 104 of the diode 96 is generally defined by the mask formed by the aperture 108 this beam defines an elongated path with rotation of the element 84 the photocells 88 and 90 are thus arranged side by side within the elongated path of the beam so as to be circumferentially spaced apart relative to the axis of rotation 86 the light emitting.