A trimmer head for use in a flexible line rotary trimmer including a housing adapted to be operatively connected to the trimmer to effect rotation of the head and a spool operatively connected to the housing for rotation therewith. Two dichroic mirrors for both red-reflecting and blue-reflecting lights are utilised for dividing illumination light in the optical projection system. If the coded signal matches the stored signal, a switch is closed for a predetermined amount of time.